Updated files to run under python 2 and 3

git-svn-id: http://svn.stsci.edu/svn/ssb/stsci_python/stwcs/trunk@39775 fe389314-cf27-0410-b35b-8c050e845b92

26 files changed, 277 insertions, 255 deletions

diff --git a/lib/stwcs/__init__.py b/lib/stwcs/__init__.py
index d48931a..208644f 100644
--- a/lib/stwcs/__init__.py
+++ b/lib/stwcs/__init__.py
@@ -14,10 +14,10 @@ transformation. wcsutil also provides functions for manipulating alternate WCS
 descriptions in the headers.
 
 """
-from __future__ import division # confidence high
+from __future__ import absolute_import, print_function # confidence high
 import os
 
-import distortion
+from . import distortion
 from stsci.tools import fileutil
 from stsci.tools import teal
 
@@ -28,6 +28,6 @@ from .version import *
 try:
     import gui
     teal.print_tasknames(gui.__name__, os.path.dirname(gui.__file__))
-    print '\n'
+    print('\n')
 except:
-    print 'No TEAL-based tasks available for this package!'
+    print('No TEAL-based tasks available for this package!')
diff --git a/lib/stwcs/distortion/coeff_converter.py b/lib/stwcs/distortion/coeff_converter.py
index aace72f..415b512 100644
--- a/lib/stwcs/distortion/coeff_converter.py
+++ b/lib/stwcs/distortion/coeff_converter.py
@@ -1,4 +1,4 @@
-from __future__ import division # confidence high
+from __future__ import division, print_function # confidence high
 
 import numpy as np
 from astropy.io import fits
@@ -20,7 +20,7 @@ def sip2idc(wcs):
         order = wcs.get('A_ORDER', None)
         sipa, sipb = _read_sip_kw(wcs)
         if None in [ocx10, ocx11, ocy10, ocy11, sipa, sipb]:
-            print 'Cannot convert SIP to IDC coefficients.\n'
+            print('Cannot convert SIP to IDC coefficients.\n')
             return None, None
     elif isinstance(wcs, pywcs.WCS):
         try:
@@ -29,29 +29,29 @@ def sip2idc(wcs):
             ocy10 = wcs.ocy10
             ocy11 = wcs.ocy11
         except AttributeError:
-            print 'First order IDCTAB coefficients are not available.\n'
-            print 'Cannot convert SIP to IDC coefficients.\n'
+            print('First order IDCTAB coefficients are not available.\n')
+            print('Cannot convert SIP to IDC coefficients.\n')
             return None, None
         try:
             sipa = wcs.sip.a
             sipb = wcs.sip.b
         except AttributeError:
-            print 'SIP coefficients are not available.'
-            print 'Cannot convert SIP to IDC coefficients.\n'
+            print('SIP coefficients are not available.')
+            print('Cannot convert SIP to IDC coefficients.\n')
             return None, None
         try:
             order = wcs.sip.a_order
         except AttributeError:
-            print 'SIP model order unknown, exiting ...\n'
+            print('SIP model order unknown, exiting ...\n')
             return None, None
 
     else:
-        print 'Input to sip2idc must be a PyFITS header or a wcsutil.HSTWCS object\n'
+        print('Input to sip2idc must be a PyFITS header or a wcsutil.HSTWCS object\n')
         return
 
 
     if None in [ocx10, ocx11, ocy10, ocy11]:
-        print 'First order IDC coefficients not found, exiting ...\n'
+        print('First order IDC coefficients not found, exiting ...\n')
         return None, None
     idc_coeff = np.array([[ocx11, ocx10], [ocy11, ocy10]])
     cx = np.zeros((order+1,order+1), dtype=np.double)
diff --git a/lib/stwcs/distortion/models.py b/lib/stwcs/distortion/models.py
index 3e7d2b4..231a9f1 100644
--- a/lib/stwcs/distortion/models.py
+++ b/lib/stwcs/distortion/models.py
@@ -1,11 +1,11 @@
-from __future__ import division # confidence high
+from __future__ import absolute_import, division, print_function # confidence high
+
 
-import types
-# Import PyDrizzle utility modules
-import mutil
 import numpy as np
-import mutil
-from mutil import combin
+
+# Import PyDrizzle utility modules
+from . import mutil
+from .mutil import combin
 
 yes = True
 no = False
@@ -76,14 +76,14 @@ class GeometryModel:
         _cys = np.zeros(shape=self.cy.shape,dtype=self.cy.dtype)
         _k = self.norder + 1
         # loop over each input coefficient
-        for m in xrange(_k):
-            for n in xrange(_k):
+        for m in range(_k):
+            for n in range(_k):
                 if m >= n:
                     # For this coefficient, shift by xs/ys.
-                    _ilist = range(m, _k)
+                    _ilist = list(range(m, _k))
                     # sum from m to k
                     for i in _ilist:
-                        _jlist = range(n, i - (m-n)+1)
+                        _jlist = list(range(n, i - (m-n)+1))
                         # sum from n to i-(m-n)
                         for j in _jlist:
                             _cxs[m,n] += self.cx[i,j]*combin(j,n)*combin((i-j),(m-n))*pow(xs,(j-n))*pow(ys,((i-j)-(m-n)))
@@ -156,7 +156,7 @@ class GeometryModel:
         elif self.norder==5:
             lines.append('quintic\n')
         else:
-            raise ValueError, "Drizzle cannot handle poly distortions of order %d"%self.norder
+            raise ValueError("Drizzle cannot handle poly distortions of order %d" % self.norder)
 
         str = '%16.8f %16.8g %16.8g %16.8g %16.8g \n'% (x0,cx[1,1],cx[1,0],cx[2,2],cx[2,1])
         lines.append(str)
@@ -219,7 +219,7 @@ class GeometryModel:
         _cx[0,0] = 0.
         _cy[0,0] = 0.
 
-        if isinstance(_p,types.ListType) or isinstance(_p,types.TupleType):
+        if isinstance(_p, list) or isinstance(_p, tuple):
             _p = np.array(_p,dtype=np.float64)
             _convert = yes
 
diff --git a/lib/stwcs/distortion/mutil.py b/lib/stwcs/distortion/mutil.py
index d116bbf..aa9d167 100644
--- a/lib/stwcs/distortion/mutil.py
+++ b/lib/stwcs/distortion/mutil.py
@@ -1,4 +1,4 @@
-from __future__ import division # confidence high
+from __future__ import division, print_function # confidence high
 
 from stsci.tools import fileutil
 import numpy as np
@@ -34,7 +34,7 @@ def readIDCtab (tabname, chip=1, date=None, direction='forward',
  # Return a default geometry model if no IDCTAB filename
     # is given.  This model will not distort the data in any way.
     if tabname == None:
-        print 'Warning: No IDCTAB specified! No distortion correction will be applied.'
+        print('Warning: No IDCTAB specified! No distortion correction will be applied.')
         return defaultModel()
 
     # Implement default values for filters here to avoid the default
@@ -62,7 +62,7 @@ def readIDCtab (tabname, chip=1, date=None, direction='forward',
         err_str += "present run will continue using the old coefficients provided in         \n"
         err_str += "the Dither Package (ca. 1995-1998).                                      \n"
         err_str += "------------------------------------------------------------------------ \n"
-        raise IOError,err_str
+        raise IOError(err_str)
 
     #First thing we need, is to read in the coefficients from the IDC
     # table and populate the Fx and Fy matrices.
@@ -108,7 +108,7 @@ def readIDCtab (tabname, chip=1, date=None, direction='forward',
 
     # Loop over all the rows looking for the one which corresponds
     # to the value of CCDCHIP we are working on...
-    for i in xrange(fshape[0]):
+    for i in range(fshape[0]):
 
         try:
             # Match FILTER combo to appropriate row,
@@ -178,9 +178,9 @@ def readIDCtab (tabname, chip=1, date=None, direction='forward',
         err_str += '     FILTERS: '+filtstr+'\n'
         ftab.close()
         del ftab
-        raise LookupError,err_str
+        raise LookupError(err_str)
     else:
-        print '- IDCTAB: Distortion model from row',str(row+1),'for chip',detchip,':',filtstr
+        print('- IDCTAB: Distortion model from row',str(row+1),'for chip',detchip,':',filtstr)
 
     # Read in V2REF and V3REF: this can either come from current table,
     # or from an OFFTAB if time-dependent (i.e., for WFPC2)
@@ -227,9 +227,9 @@ def readIDCtab (tabname, chip=1, date=None, direction='forward',
         cxstr = 'A'
         cystr = 'B'
 
-    for i in xrange(norder+1):
+    for i in range(norder+1):
         if i > 0:
-            for j in xrange(i+1):
+            for j in range(i+1):
                 xcname = cxstr+str(i)+str(j)
                 ycname = cystr+str(i)+str(j)
                 fx[i,j] = ftab[1].data.field(xcname)[row]
@@ -307,7 +307,7 @@ def read_tdd_coeffs(phdr, chip=1):
         if "TDDORDER" in phdr:
             n = int(phdr["TDDORDER"])
         else:
-            print ('TDDORDER kw not present, using default TDD correction')
+            print('TDDORDER kw not present, using default TDD correction')
             return None
 
         a = np.zeros((n+1,), np.float64)
@@ -316,7 +316,7 @@ def read_tdd_coeffs(phdr, chip=1):
             a[i] = phdr.get(("TDD_A%d" % i), 0.0)
             b[i] = phdr.get(("TDD_B%d" % i), 0.0)
         if (a==0).all() and (b==0).all():
-            print ('Warning: TDD_A and TDD_B coeffiecients have values of 0, \n \
+            print('Warning: TDD_A and TDD_B coeffiecients have values of 0, \n \
                    but TDDORDER is %d.' % TDDORDER)
 
         skew_coeffs['TDDORDER'] = n
@@ -346,7 +346,7 @@ def readOfftab(offtab, date, chip=None):
     try:
         ftab = fileutil.openImage(offtab)
     except:
-        raise IOError,"Offset table '%s' not valid as specified!" % offtab
+        raise IOError("Offset table '%s' not valid as specified!" % offtab)
 
     #Determine row from which to get the coefficients.
     # How many rows do we have in the table...
@@ -365,7 +365,7 @@ def readOfftab(offtab, date, chip=None):
     num_date = convertDate(date)
     # Loop over all the rows looking for the one which corresponds
     # to the value of CCDCHIP we are working on...
-    for ri in xrange(fshape[0]):
+    for ri in range(fshape[0]):
         i = fshape[0] - ri - 1
         if 'DETCHIP' in colnames:
             detchip = ftab[1].data.field('DETCHIP')[i]
@@ -405,12 +405,12 @@ def readOfftab(offtab, date, chip=None):
     del ftab
 
     if row_start == None and row_end == None:
-        print 'Row corresponding to DETCHIP of ',detchip,' was not found!'
+        print('Row corresponding to DETCHIP of ',detchip,' was not found!')
         raise LookupError
     elif row_start == None:
-        print '- OFFTAB: Offset defined by row',str(row_end+1)
+        print('- OFFTAB: Offset defined by row',str(row_end+1))
     else:
-        print '- OFFTAB: Offset interpolated from rows',str(row_start+1),'and',str(row_end+1)
+        print('- OFFTAB: Offset interpolated from rows',str(row_start+1),'and',str(row_end+1))
 
     # Now, do the interpolation for v2ref, v3ref, and theta
     if row_start == None or row_end == row_start:
@@ -476,16 +476,16 @@ def readWCSCoeffs(header):
     cxstr = 'A_'
     cystr = 'B_'
     # Read coeffs into their own matrix
-    for i in xrange(_xorder+1):
-        for j in xrange(i+1):
+    for i in range(_xorder+1):
+        for j in range(i+1):
             xcname = cxstr+str(j)+'_'+str(i-j)
             if xcname in header:
                 fx[i,j] = header[xcname]
 
     # Extract Y coeffs separately as a different order may
     # have been used to fit it.
-    for i in xrange(_yorder+1):
-        for j in xrange(i+1):
+    for i in range(_yorder+1):
+        for j in range(i+1):
             ycname = cystr+str(j)+'_'+str(i-j)
             if ycname in header:
                 fy[i,j] = header[ycname]
diff --git a/lib/stwcs/distortion/utils.py b/lib/stwcs/distortion/utils.py
index 5e0bf79..449bc18 100644
--- a/lib/stwcs/distortion/utils.py
+++ b/lib/stwcs/distortion/utils.py
@@ -1,4 +1,5 @@
-from __future__ import division # confidence high
+from __future__ import division, print_function # confidence high
+
 import os
 import numpy as np
 from astropy import wcs as pywcs
@@ -92,7 +93,7 @@ def  undistortWCS(wcsobj):
     plate scale in the undistorted frame.
     """
     assert isinstance(wcsobj, pywcs.WCS)
-    import coeff_converter
+    from . import coeff_converter
 
     cx, cy = coeff_converter.sip2idc(wcsobj)
     # cx, cy can be None because either there is no model available
@@ -103,9 +104,9 @@ def  undistortWCS(wcsobj):
             Run updatewcs() to update the headers or
             pass 'undistort=False' keyword to output_wcs().\n
             """
-            raise RuntimeError, m
+            raise RuntimeError(m)
         else:
-            print 'Distortion model is not available, using input reference image for output WCS.\n'
+            print('Distortion model is not available, using input reference image for output WCS.\n')
             return wcsobj.copy()
     crpix1 = wcsobj.wcs.crpix[0]
     crpix2 = wcsobj.wcs.crpix[1]
@@ -128,7 +129,7 @@ def  undistortWCS(wcsobj):
     # Check the determinant for singularity
     _det = (am * dm) - (bm * cm)
     if ( _det == 0.0):
-        print 'Singular matrix in updateWCS, aborting ...'
+        print('Singular matrix in updateWCS, aborting ...')
         return
 
     lin_wcsobj = pywcs.WCS()
@@ -160,10 +161,10 @@ def apply_idc(pixpos, cx, cy, pixref, pscale= None, order=None):
         return pixpos
 
     if order is None:
-        print 'Unknown order of distortion model \n'
+        print('Unknown order of distortion model \n')
         return pixpos
     if pscale is None:
-        print 'Unknown model plate scale\n'
+        print('Unknown model plate scale\n')
         return pixpos
 
     # Apply in the same way that 'drizzle' would...
diff --git a/lib/stwcs/gui/__init__.py b/lib/stwcs/gui/__init__.py
index 559d361..cd21bf6 100644
--- a/lib/stwcs/gui/__init__.py
+++ b/lib/stwcs/gui/__init__.py
@@ -4,16 +4,16 @@ This package defines the TEAL interfaces for public, file-based operations
 provided by the STWCS package.
 
 """
-from __future__ import division # confidence high
+from __future__ import absolute_import # confidence high
 __docformat__ = 'restructuredtext'
 
 # import modules which define the TEAL interfaces 
-import write_headerlet
-import extract_headerlet
-import attach_headerlet
-import delete_headerlet
-import headerlet_summary
-import archive_headerlet
-import restore_headerlet
-import apply_headerlet
-import updatewcs
+from . import write_headerlet
+from . import extract_headerlet
+from . import attach_headerlet
+from . import delete_headerlet
+from . import headerlet_summary
+from . import archive_headerlet
+from . import restore_headerlet
+from . import apply_headerlet
+from . import updatewcs
diff --git a/lib/stwcs/gui/archive_headerlet.py b/lib/stwcs/gui/archive_headerlet.py
index 70dbc89..3cc29e3 100644
--- a/lib/stwcs/gui/archive_headerlet.py
+++ b/lib/stwcs/gui/archive_headerlet.py
@@ -1,3 +1,4 @@
+from __future__ import print_function
 import os
 import string
 
@@ -35,11 +36,11 @@ def getHelpAsString(docstring=False):
 def run(configObj=None):
 
     if configObj['hdrname'] in ['',' ','INDEF']:
-        print '='*60
-        print 'ERROR:'
-        print '    No valid "hdrname" parameter value provided!'
-        print '    Please restart this task and provide a value for this parameter.'
-        print '='*60
+        print('='*60)
+        print('ERROR:')
+        print('    No valid "hdrname" parameter value provided!')
+        print('    Please restart this task and provide a value for this parameter.')
+        print('='*60)
         return
 
     str_kw = ['wcsname','destim','sipname','npolfile','d2imfile',
diff --git a/lib/stwcs/gui/delete_headerlet.py b/lib/stwcs/gui/delete_headerlet.py
index 96d3ed1..b3df5a7 100644
--- a/lib/stwcs/gui/delete_headerlet.py
+++ b/lib/stwcs/gui/delete_headerlet.py
@@ -1,3 +1,4 @@
+from __future__ import print_function
 import os
 
 from stsci.tools import teal
@@ -34,11 +35,11 @@ def run(configObj=None):
 
     if configObj['hdrname'] == '' and configObj['hdrext'] is None and \
         configObj['distname'] == '':
-        print '='*60
-        print 'ERROR:'
-        print '    No valid "hdrname", "hdrext" or "distname" parameter value provided!'
-        print '    Please restart this task and provide a value for one of these parameters.'
-        print '='*60
+        print('='*60)
+        print('ERROR:')
+        print('    No valid "hdrname", "hdrext" or "distname" parameter value provided!')
+        print('    Please restart this task and provide a value for one of these parameters.')
+        print('='*60)
         return
     filename = parseinput.parseinput(configObj['filename'])[0]
     # Call function with properly interpreted input parameters
diff --git a/lib/stwcs/gui/extract_headerlet.py b/lib/stwcs/gui/extract_headerlet.py
index 972f5c7..02ecd7a 100644
--- a/lib/stwcs/gui/extract_headerlet.py
+++ b/lib/stwcs/gui/extract_headerlet.py
@@ -1,3 +1,4 @@
+from __future__ import print_function
 import os
 
 from stsci.tools import teal
@@ -33,11 +34,11 @@ def getHelpAsString(docstring=False):
 def run(configObj=None):
 
     if configObj['output'] in ['',' ',None]:
-        print '='*60
-        print 'ERROR:'
-        print '    No valid "output" parameter value provided!'
-        print '    Please restart this task and provide a value for this parameter.'
-        print '='*60
+        print('='*60)
+        print('ERROR:')
+        print('    No valid "output" parameter value provided!')
+        print('    Please restart this task and provide a value for this parameter.')
+        print('='*60)
         return
 
     # create dictionary of remaining parameters, deleting extraneous ones
diff --git a/lib/stwcs/gui/updatewcs.py b/lib/stwcs/gui/updatewcs.py
index f8f10d7..3dacb67 100644
--- a/lib/stwcs/gui/updatewcs.py
+++ b/lib/stwcs/gui/updatewcs.py
@@ -1,3 +1,4 @@
+from __future__ import print_function
 import os
 
 from astropy.io import fits
@@ -68,7 +69,7 @@ def run(configObj=None):
         # Check to insure that there is a valid reference file to be used
         idctab = fits.getval(file, 'idctab')
         if not os.path.exists(fileutil.osfn(idctab)):
-            print 'No valid distortion reference file ',idctab,' found in ',file,'!'
+            print('No valid distortion reference file ',idctab,' found in ',file,'!')
             raise ValueError
 
     # Re-define 'cdict' to only have switches for steps supported by that instrument
diff --git a/lib/stwcs/gui/write_headerlet.py b/lib/stwcs/gui/write_headerlet.py
index 353e32b..e18bed8 100644
--- a/lib/stwcs/gui/write_headerlet.py
+++ b/lib/stwcs/gui/write_headerlet.py
@@ -1,3 +1,4 @@
+from __future__ import print_function
 import os
 
 from stsci.tools import teal
@@ -34,19 +35,19 @@ def getHelpAsString(docstring=False):
 def run(configObj=None):
     flist,oname = parseinput.parseinput(configObj['filename'])
     if len(flist) == 0:
-        print '='*60
-        print 'ERROR:'
-        print '    No valid "filename" parameter value provided!'
-        print '    Please check the working directory and restart this task.'
-        print '='*60
+        print('='*60)
+        print('ERROR:')
+        print('    No valid "filename" parameter value provided!')
+        print('    Please check the working directory and restart this task.')
+        print('='*60)
         return
 
     if configObj['hdrname'] in ['',' ','INDEF']:
-        print '='*60
-        print 'ERROR:'
-        print '    No valid "hdrname" parameter value provided!'
-        print '    Please restart this task and provide a value for this parameter.'
-        print '='*60
+        print('='*60)
+        print('ERROR:')
+        print('    No valid "hdrname" parameter value provided!')
+        print('    Please restart this task and provide a value for this parameter.')
+        print('='*60)
         return
 
     if configObj['output'] in ['',' ','INDEF']:
diff --git a/lib/stwcs/updatewcs/__init__.py b/lib/stwcs/updatewcs/__init__.py
index aca33f7..a7f9a3e 100644
--- a/lib/stwcs/updatewcs/__init__.py
+++ b/lib/stwcs/updatewcs/__init__.py
@@ -1,11 +1,10 @@
-from __future__ import division # confidence high
+from __future__ import division, print_function # confidence high
 
-import os
 from astropy.io import fits
-import numpy as np
 from stwcs import wcsutil
 from stwcs.wcsutil import HSTWCS
 import stwcs
+
 from astropy import wcs as pywcs
 import astropy
 
@@ -84,7 +83,7 @@ def updatewcs(input, vacorr=True, tddcorr=True, npolcorr=True, d2imcorr=True,
     if checkfiles:
         files = checkFiles(files)
         if not files:
-            print 'No valid input, quitting ...\n'
+            print('No valid input, quitting ...\n')
             return
 
     for f in files:
@@ -242,9 +241,9 @@ def getNrefchip(fobj):
                      img.header['EXTNAME'].lower()=='sci']
         fitsext = [i for i in range(len(fobj))[1:] if
                    fobj[i].header['EXTNAME'].lower()=='sci']
-        det2ext=dict(map(None, detectors,extvers))
-        ext2det=dict(map(None, extvers, detectors))
-        ext2fitsext=dict(map(None, extvers, fitsext))
+        det2ext=dict(list(zip(detectors, extvers)))
+        ext2det=dict(list(zip(extvers, detectors)))
+        ext2fitsext=dict(list(zip(extvers, fitsext)))
 
         if 3 not in detectors:
             nrefchip = ext2det.pop(extvers[0])
@@ -263,9 +262,9 @@ def getNrefchip(fobj):
                      img.header['EXTNAME'].lower()=='sci']
         fitsext = [i for i in range(len(fobj))[1:] if
                    fobj[i].header['EXTNAME'].lower()=='sci']
-        det2ext=dict(map(None, detectors,extvers))
-        ext2det=dict(map(None, extvers, detectors))
-        ext2fitsext=dict(map(None, extvers, fitsext))
+        det2ext=dict(list(zip(detectors, extvers)))
+        ext2det=dict(list(zip(extvers, detectors)))
+        ext2fitsext=dict(list(zip(extvers, fitsext)))
 
         if 2 not in detectors:
             nrefchip = ext2det.pop(extvers[0])
@@ -356,7 +355,7 @@ def cleanWCS(fname):
         keys.remove(' ')
     except ValueError:
         pass
-    fext = range(len(f))
+    fext = list(range(len(f)))
     for key in keys:
         wcsutil.deleteWCS(fname, ext=fext, wcskey=key)
 
@@ -369,5 +368,5 @@ def getCorrections(instrument):
     """
     acorr = apply_corrections.allowed_corrections[instrument]
 
-    print "The following corrections will be performed for instrument %s\n" % instrument
-    for c in acorr: print c,': ' ,  apply_corrections.cnames[c]
+    print("The following corrections will be performed for instrument %s\n" % instrument)
+    for c in acorr: print(c,': ' ,  apply_corrections.cnames[c])
diff --git a/lib/stwcs/updatewcs/apply_corrections.py b/lib/stwcs/updatewcs/apply_corrections.py
index b7e41b2..2871a46 100644
--- a/lib/stwcs/updatewcs/apply_corrections.py
+++ b/lib/stwcs/updatewcs/apply_corrections.py
@@ -1,4 +1,4 @@
-from __future__ import division # confidence high
+from __future__ import division, print_function # confidence high
 
 import os
 from astropy.io import fits
@@ -198,7 +198,7 @@ def applyD2ImCorr(fname, d2imcorr):
             msg = """\n\tKw D2IMFILE exists in primary header but file %s not found\n
                      Detector to image correction will not be applied\n""" % fd2im0
             logger.critical(msg)
-            print msg
+            print(msg)
             applyD2IMCorr = False
             return applyD2IMCorr
         try:
@@ -220,6 +220,6 @@ def applyD2ImCorr(fname, d2imcorr):
             # in first extension header
             applyD2IMCorr = True
     except KeyError:
-        print 'D2IMFILE keyword not found in primary header'
+        print('D2IMFILE keyword not found in primary header')
         applyD2IMCorr = False
         return applyD2IMCorr
diff --git a/lib/stwcs/updatewcs/corrections.py b/lib/stwcs/updatewcs/corrections.py
index f44b73b..e36dcd6 100644
--- a/lib/stwcs/updatewcs/corrections.py
+++ b/lib/stwcs/updatewcs/corrections.py
@@ -1,14 +1,16 @@
-from __future__ import division # confidence high
+from __future__ import division, print_function # confidence high
 
 import copy
 import datetime
+import logging, time
 import numpy as np
 from numpy import linalg
 from stsci.tools import fileutil
-from utils import diff_angles
-import makewcs, npol
 
-import logging, time
+from . import npol
+from . import makewcs
+from .utils import diff_angles
+
 logger=logging.getLogger('stwcs.updatewcs.corrections')
 
 MakeWCS = makewcs.MakeWCS
@@ -221,7 +223,7 @@ class TDDCorr(object):
                 err_str += "         The pre-SM4 time-dependent skew solution will be used by default.\n"
                 err_str += "         Please update IDCTAB with new reference file from HST archive.   \n"
                 err_str +=  "------------------------------------------------------------------------  \n"
-                print err_str
+                print(err_str)
             # Using default pre-SM4 coefficients
             skew_coeffs = {'TDD_A':[0.095,0.090/2.5],
                         'TDD_B':[-0.029,-0.030/2.5],
diff --git a/lib/stwcs/updatewcs/det2im.py b/lib/stwcs/updatewcs/det2im.py
index 902deaa..dfda9d1 100644
--- a/lib/stwcs/updatewcs/det2im.py
+++ b/lib/stwcs/updatewcs/det2im.py
@@ -1,9 +1,10 @@
-from __future__ import division # confidence high
+from __future__ import absolute_import, division # confidence high
 
+import numpy as np
 from astropy.io import fits
 from stsci.tools import fileutil
-import utils
-import numpy as np
+
+from . import utils
 
 import logging, time
 logger = logging.getLogger('stwcs.updatewcs.d2im')
@@ -223,7 +224,7 @@ class DET2IMCorr(object):
 
         kw_comm1 = {}
         kw_val1 = {}
-        for key in kw.keys():
+        for key in kw:
             for i in range(1, naxis+1):
                 si = str(i)
                 kw_comm1[key+si] = kw[key]
@@ -254,9 +255,9 @@ class DET2IMCorr(object):
                     'CCDCHIP': ccdchip,
                 }
         cdl = []
-        for key in kw_comm0.keys():
+        for key in kw_comm0:
             cdl.append((key, kw_val0[key], kw_comm0[key]))
-        for key in kw_comm1.keys():
+        for key in kw_comm1:
             cdl.append((key, kw_val1[key], kw_comm1[key]))
         # Now add keywords from NPOLFILE header to document source of calibration
         # include all keywords after and including 'FILENAME' from header
diff --git a/lib/stwcs/updatewcs/makewcs.py b/lib/stwcs/updatewcs/makewcs.py
index 68ceded..06c6f9c 100644
--- a/lib/stwcs/updatewcs/makewcs.py
+++ b/lib/stwcs/updatewcs/makewcs.py
@@ -1,13 +1,14 @@
-from __future__ import division # confidence high
+from __future__ import absolute_import, division # confidence high
 
 import datetime
 
 import numpy as np
+import logging, time
 from math import sin, sqrt, pow, cos, asin, atan2,pi
-import utils
+
 from stsci.tools import fileutil
+from . import utils
 
-import logging, time
 logger = logging.getLogger(__name__)
 
 class MakeWCS(object):
diff --git a/lib/stwcs/updatewcs/npol.py b/lib/stwcs/updatewcs/npol.py
index cee7b9b..4e109ae 100644
--- a/lib/stwcs/updatewcs/npol.py
+++ b/lib/stwcs/updatewcs/npol.py
@@ -1,11 +1,13 @@
-from __future__ import division # confidence high
+from __future__ import absolute_import, division # confidence high
 
+import logging, time
+
+import numpy as np
 from astropy.io import fits
+
 from stsci.tools import fileutil
-import utils
-import numpy as np
+from . import utils
 
-import logging, time
 logger = logging.getLogger('stwcs.updatewcs.npol')
 
 class NPOLCorr(object):
@@ -264,7 +266,7 @@ class NPOLCorr(object):
 
         kw_comm1 = {}
         kw_val1 = {}
-        for key in kw.keys():
+        for key in kw:
             for i in range(1, naxis+1):
                 si = str(i)
                 kw_comm1[key+si] = kw[key]
@@ -297,9 +299,9 @@ class NPOLCorr(object):
                     'CCDCHIP': ccdchip,
                 }
         cdl = []
-        for key in kw_comm0.keys():
+        for key in kw_comm0:
             cdl.append((key, kw_val0[key], kw_comm0[key]))
-        for key in kw_comm1.keys():
+        for key in kw_comm1:
             cdl.append((key, kw_val1[key], kw_comm1[key]))
         # Now add keywords from NPOLFILE header to document source of calibration
         # include all keywords after and including 'FILENAME' from header
diff --git a/lib/stwcs/wcsutil/__init__.py b/lib/stwcs/wcsutil/__init__.py
index b92299e..65280be 100644
--- a/lib/stwcs/wcsutil/__init__.py
+++ b/lib/stwcs/wcsutil/__init__.py
@@ -1,12 +1,11 @@
-from __future__ import division # confidence high
+from __future__ import absolute_import, print_function # confidence high
 
-from altwcs import *
-from hstwcs import HSTWCS
+from .altwcs import *
+from .hstwcs import HSTWCS
 
 
 def help():
-    print 'How to create an HSTWCS object:\n\n'
-    print """ \
+    doc = """ \
     1. Using a `astropy.io.fits.HDUList` object and an extension number \n
     Example:\n
     from astropy.io improt fits
@@ -31,3 +30,5 @@ def help():
     w = wcsutil.HSTWCS(instrument='DEFAULT')\n\n
     """
 
+    print('How to create an HSTWCS object:\n\n')
+    print(doc)
diff --git a/lib/stwcs/wcsutil/altwcs.py b/lib/stwcs/wcsutil/altwcs.py
index 7719ce7..15123b7 100644
--- a/lib/stwcs/wcsutil/altwcs.py
+++ b/lib/stwcs/wcsutil/altwcs.py
@@ -1,4 +1,4 @@
-from __future__ import division # confidence high
+from __future__ import division, print_function # confidence high
 import os
 import string
 
@@ -80,7 +80,7 @@ def archiveWCS(fname, ext, wcskey=" ", wcsname=" ", reusekey=False):
         Alternatively choose another wcskey with altwcs.available_wcskeys()." %wcskey)
     elif wcskey == " ":
         # wcsname exists, overwrite it if reuse is True or get the next key
-        if wcsname.strip() in wcsnames(f[wcsext].header).values():
+        if wcsname.strip() in wcsnames(f[wcsext].header):
             if reusekey:
                 # try getting the key from an existing WCS with WCSNAME
                 wkey = getKeyFromName(f[wcsext].header, wcsname)
@@ -132,8 +132,8 @@ def archiveWCS(fname, ext, wcskey=" ", wcsname=" ", reusekey=False):
             old_wcsname=hwcs.pop('WCSNAME')
         except:
             pass
-        for k in hwcs.keys():
-
+        
+        for k in hwcs:
             key = k[:7] + wkey
             f[e].header[key] = hwcs[k]
     closefobj(fname, f)
@@ -202,7 +202,7 @@ def restore_from_to(f, fromext=None, toext=None, wcskey=" ", wcsname=" "):
                 raise KeyError("Could not get a key from wcsname %s ." % wcsname)
     else:
         if wcskey not in wcskeys(fobj, ext=wcskeyext):
-            print "Could not find alternate WCS with key %s in this file" % wcskey
+            print("Could not find alternate WCS with key %s in this file" % wcskey)
             closefobj(f, fobj)
             return
         wkey = wcskey
@@ -322,7 +322,7 @@ def deleteWCS(fname, ext, wcskey=" ", wcsname=" "):
     ext = _buildExtlist(fobj, ext)
     # Do not allow deleting the original WCS.
     if wcskey == 'O':
-        print "Wcskey 'O' is reserved for the original WCS and should not be deleted."
+        print("Wcskey 'O' is reserved for the original WCS and should not be deleted.")
         closefobj(fname, fobj)
         return
 
@@ -349,14 +349,14 @@ def deleteWCS(fname, ext, wcskey=" ", wcsname=" "):
         hwcs = readAltWCS(fobj,i,wcskey=wkey)
         if hwcs is None:
             continue
-        for k in hwcs.keys():
+        for k in hwcs:
             del hdr[k]
             #del hdr['ORIENT'+wkey]
         prexts.append(i)
     if prexts != []:
-        print 'Deleted all instances of WCS with key %s in extensions' % wkey, prexts
+        print('Deleted all instances of WCS with key %s in extensions' % wkey, prexts)
     else:
-        print "Did not find WCS with key %s in any of the extensions" % wkey
+        print("Did not find WCS with key %s in any of the extensions" % wkey)
     closefobj(fname, fobj)
 
 def _buildExtlist(fobj, ext):
@@ -419,9 +419,9 @@ def _restore(fobj, ukey, fromextnum,
     if hwcs is None:
         return
 
-    for k in hwcs.keys():
+    for k in hwcs:
         key = k[:-1]
-        if key in fobj[toextension].header.keys():
+        if key in fobj[toextension].header:
             #fobj[toextension].header.update(key=key, value = hwcs[k])
             fobj[toextension].header[key] = hwcs[k]
         else:
@@ -484,8 +484,8 @@ def readAltWCS(fobj, ext, wcskey=' ',verbose=False):
         nwcs = pywcs.WCS(hdr, fobj=fobj, key=wcskey)
     except KeyError:
         if verbose:
-            print 'readAltWCS: Could not read WCS with key %s' %wcskey
-            print '            Skipping %s[%s]' % (fobj.filename(), str(ext))
+            print('readAltWCS: Could not read WCS with key %s' %wcskey)
+            print('            Skipping %s[%s]' % (fobj.filename(), str(ext)))
         return None
     hwcs = nwcs.to_header()
 
@@ -693,7 +693,7 @@ def _parpasscheck(fobj, ext, wcskey, fromext=None, toext=None, reusekey=False):
         A flag which indicates whether to reuse a wcskey in the header
     """
     if not isinstance(fobj, fits.HDUList):
-        print "First parameter must be a fits file object or a file name."
+        print("First parameter must be a fits file object or a file name.")
         return False
 
     # first one covers the case of an object created in memory
@@ -703,27 +703,27 @@ def _parpasscheck(fobj, ext, wcskey, fromext=None, toext=None, reusekey=False):
     else:
         # an HDUList object with associated file
         if fobj.fileinfo(0)['filemode'] is not 'update':
-            print "First parameter must be a file name or a file object opened in 'update' mode."
+            print("First parameter must be a file name or a file object opened in 'update' mode.")
             return False
 
     if not isinstance(ext, int) and not isinstance(ext, tuple) \
         and not isinstance(ext,str) \
         and not isinstance(ext, list) and ext is not None:
-        print "Ext must be integer, tuple, string,a list of int extension numbers, \n\
-        or a list of tuples representing a fits extension, for example ('sci', 1)."
+        print("Ext must be integer, tuple, string,a list of int extension numbers, \n\
+        or a list of tuples representing a fits extension, for example ('sci', 1).")
         return False
 
     if not isinstance(fromext, str) and fromext is not None:
-        print "fromext must be a string representing a valid extname"
+        print("fromext must be a string representing a valid extname")
         return False
 
     if not isinstance(toext, list) and not isinstance(toext, str) and \
                      toext is not None :
-        print "toext must be a string or a list of strings representing extname"
+        print("toext must be a string or a list of strings representing extname")
         return False
 
     if len(wcskey) != 1:
-        print 'Parameter wcskey must be a character - one of "A"-"Z" or " "'
+        print('Parameter wcskey must be a character - one of "A"-"Z" or " "')
         return False
 
     return True
diff --git a/lib/stwcs/wcsutil/convertwcs.py b/lib/stwcs/wcsutil/convertwcs.py
index 7ece2a0..a384eb1 100644
--- a/lib/stwcs/wcsutil/convertwcs.py
+++ b/lib/stwcs/wcsutil/convertwcs.py
@@ -1,4 +1,4 @@
-from astropy.io import fits
+from __future__ import print_function
 try:
     import stwcs
     from stwcs import wcsutil
@@ -23,9 +23,9 @@ def archive_prefix_OPUS_WCS(fobj,extname='SCI'):
 
     """
     if stwcs is None:
-        print '====================='
-        print 'The STWCS package is needed to convert an old-style OPUS WCS to an alternate WCS'
-        print '====================='
+        print('=====================')
+        print('The STWCS package is needed to convert an old-style OPUS WCS to an alternate WCS')
+        print('=====================')
         raise ImportError
 
 
@@ -41,7 +41,7 @@ def archive_prefix_OPUS_WCS(fobj,extname='SCI'):
 
     numextn = fileutil.countExtn(fobj)
     extlist = []
-    for e in xrange(1,numextn+1):
+    for e in range(1,numextn+1):
         extlist.append(('sci',e))
 
     # Insure that the 'O' alternate WCS is present
@@ -57,10 +57,10 @@ def archive_prefix_OPUS_WCS(fobj,extname='SCI'):
     # create HSTWCS object from PRIMARY WCS
     wcsobj = wcsutil.HSTWCS(fobj,ext=ext,wcskey='O')
     # get list of WCS keywords
-    wcskeys = wcsobj.wcs2header().keys()
+    wcskeys = list(wcsobj.wcs2header().keys())
 
     # For each SCI extension...
-    for e in xrange(1,numextn+1):
+    for e in range(1,numextn+1):
         # Now, look for any WCS keywords with a prefix of 'O'
         for key in wcskeys:
             okey = 'O'+key[:7]
@@ -98,7 +98,7 @@ def create_prefix_OPUS_WCS(fobj,extname='SCI'):
     else:
         # check to make sure this FITS obj has been opened in update mode
         if fobj.fileinfo(0)['filemode'] != 'update':
-            print 'File not opened with "mode=update". Quitting...'
+            print('File not opened with "mode=update". Quitting...')
             raise IOError
 
     # check for existance of O-prefix WCS
@@ -108,7 +108,7 @@ def create_prefix_OPUS_WCS(fobj,extname='SCI'):
         numextn = fileutil.countExtn(fobj,extname=extname)
         if numextn == 0:
             extname = ''
-        for extn in xrange(1,numextn+1):
+        for extn in range(1,numextn+1):
             hdr = fobj[(extname,extn)].header
             for okey in owcskeys:
                 hdr[okey] = hdr[okey[1:]+'O']
diff --git a/lib/stwcs/wcsutil/getinput.py b/lib/stwcs/wcsutil/getinput.py
index dbd240f..8ee1123 100644
--- a/lib/stwcs/wcsutil/getinput.py
+++ b/lib/stwcs/wcsutil/getinput.py
@@ -24,7 +24,7 @@ def parseSingleInput(f=None, ext=None):
         hdr0 = phdu[0].header
         try:
             ehdr = phdu[extnum].header
-        except (IndexError, KeyError), e:
+        except (IndexError, KeyError) as e:
             raise e.__class__('Unable to get extension %s.' % extnum)
 
     elif isinstance(f, fits.HDUList):
diff --git a/lib/stwcs/wcsutil/headerlet.py b/lib/stwcs/wcsutil/headerlet.py
index aaca6b9..6b3e752 100644
--- a/lib/stwcs/wcsutil/headerlet.py
+++ b/lib/stwcs/wcsutil/headerlet.py
@@ -10,10 +10,11 @@ when only the WCS information has been updated.
 
 """
 
-from __future__ import division
+from __future__ import absolute_import, division, print_function
+import os
+import sys
 import functools
 import logging
-import os
 import textwrap
 import copy
 import time
@@ -24,16 +25,16 @@ from astropy.io import fits
 from astropy import wcs as pywcs
 from astropy.utils import lazyproperty
 
-import altwcs
-import wcscorr
-from hstwcs import HSTWCS
-from mappings import basic_wcs
-from stwcs.updatewcs import utils
-
 from stsci.tools.fileutil import countExtn
 from stsci.tools import fileutil as fu
 from stsci.tools import parseinput
 
+from stwcs.updatewcs import utils
+from . import altwcs
+from . import wcscorr
+from .hstwcs import HSTWCS
+from .mappings import basic_wcs
+
 #### Logging support functions
 class FuncNameLoggingFormatter(logging.Formatter):
     def __init__(self, fmt=None, datefmt=None):
@@ -108,8 +109,12 @@ def with_logging(func):
         level = kw.get('logging', 100)
         mode = kw.get('logmode', 'w')
         func_args = kw.copy()
-        for argname, arg in zip(func.func_code.co_varnames, args):
-            func_args[argname] = arg
+        if sys.version_info[0] < 3:
+            for argname, arg in zip(func.func_code.co_varnames, args):
+                func_args[argname] = arg
+        else:
+            for argname, arg in zip(func.__code__.co_varnames, args):
+                func_args[argname] = arg
         init_logging(func.__name__, level, mode, **func_args)
         return func(*args, **kw)
     return wrapped
@@ -152,7 +157,11 @@ def parse_filename(fname, mode='readonly'):
     """
     close_fobj = False
     if not isinstance(fname, list):
-        if isinstance(fname, basestring):
+        if sys.version > '3':
+            is_string = isinstance(fname, str)
+        else:
+            is_string = isinstance(fname, basestring)
+        if is_string:
             fname = fu.osfn(fname)
         fobj = fits.open(fname, mode=mode)
         close_fobj = True
@@ -400,7 +409,7 @@ def print_summary(summary_cols, summary_dict, pad=2, maxwidth=None, idcol=None,
             outstr += COLUMN_FMT.format(val, width=column_widths[kw])
         outstr += '\n'
     if not quiet:
-        print outstr
+        print(outstr)
 
     # If specified, write info to separate text file
     write_file = False
@@ -411,9 +420,9 @@ def print_summary(summary_cols, summary_dict, pad=2, maxwidth=None, idcol=None,
             if clobber:
                 os.remove(output)
             else:
-                print 'WARNING: Not writing results to file!'
-                print '         Output text file ', output, ' already exists.'
-                print '         Set "clobber" to True or move file before trying again.'
+                print('WARNING: Not writing results to file!')
+                print('         Output text file ', output, ' already exists.')
+                print('         Set "clobber" to True or move file before trying again.')
                 write_file = False
         if write_file:
             fout = open(output, mode='w')
@@ -1139,7 +1148,7 @@ def apply_headerlet_as_primary(filename, hdrlet, attach=True, archive=True,
                 "{0:d} filenames and {1:d} headerlets specified".format(len(filename),len(hdrlet)))
 
     for fname,h in zip(filename,hdrlet):
-        print "Applying {0} as Primary WCS to {1}".format(h,fname)
+        print("Applying {0} as Primary WCS to {1}".format(h,fname))
         hlet = Headerlet.fromfile(h, logging=logging, logmode=logmode)
         hlet.apply_as_primary(fname, attach=attach, archive=archive,
                           force=force)
@@ -1181,7 +1190,7 @@ def apply_headerlet_as_alternate(filename, hdrlet, attach=True, wcskey=None,
                 "{0:d} filenames and {1:d} headerlets specified".format(len(filename),len(hdrlet)))
 
     for fname,h in zip(filename,hdrlet):
-        print 'Applying {0} as an alternate WCS to {1}'.format(h,fname)
+        print('Applying {0} as an alternate WCS to {1}'.format(h,fname))
         hlet = Headerlet.fromfile(h, logging=logging, logmode=logmode)
         hlet.apply_as_alternate(fname, attach=attach,
                             wcsname=wcsname, wcskey=wcskey)
@@ -1211,7 +1220,7 @@ def attach_headerlet(filename, hdrlet, logging=False, logmode='a'):
                 "{0:d} filenames and {1:d} headerlets specified".format(len(filename),len(hdrlet)))
 
     for fname,h in zip(filename,hdrlet):
-        print 'Attaching {0} as Headerlet extension to {1}'.format(h,fname)
+        print('Attaching {0} as Headerlet extension to {1}'.format(h,fname))
         hlet = Headerlet.fromfile(h, logging=logging, logmode=logmode)
         hlet.attach_to_file(fname,archive=True)
 
@@ -1252,7 +1261,7 @@ def delete_headerlet(filename, hdrname=None, hdrext=None, distname=None,
         filename = [filename]
 
     for f in filename:
-        print "Deleting Headerlet from ",f
+        print("Deleting Headerlet from ",f)
         _delete_single_headerlet(f, hdrname=hdrname, hdrext=hdrext,
                             distname=distname, logging=logging, logmode='a')
 
@@ -1386,8 +1395,8 @@ def headerlet_summary(filename, columns=None, pad=2, maxwidth=None,
                     for key in COLUMN_DICT:
                         summary_dict[kw][key].extend(ext_summary[kw][key])
             except:
-                print "Skipping headerlet"
-                print "Could not read Headerlet from extension ", hdrlet_indx
+                print("Skipping headerlet")
+                print("Could not read Headerlet from extension ", hdrlet_indx)
 
     if close_fobj:
         fobj.close()
@@ -1484,7 +1493,7 @@ def restore_from_headerlet(filename, hdrname=None, hdrext=None, archive=True,
     priwcs_name = None
 
     scihdr = extlist[0].header
-    sci_wcsnames = altwcs.wcsnames(scihdr).values()
+    #sci_wcsnames = altwcs.wcsnames(scihdr).values()
     if 'hdrname' in scihdr:
         priwcs_hdrname = scihdr['hdrname']
     else:
diff --git a/lib/stwcs/wcsutil/hstwcs.py b/lib/stwcs/wcsutil/hstwcs.py
index 7618412..cb61f1c 100644
--- a/lib/stwcs/wcsutil/hstwcs.py
+++ b/lib/stwcs/wcsutil/hstwcs.py
@@ -1,19 +1,18 @@
-from __future__ import division # confidence high
+from __future__ import absolute_import, division, print_function # confidence high
 
 import os
 from astropy.wcs import WCS
 from astropy.io import fits
-import instruments
 from stwcs.distortion import models, coeff_converter
-import altwcs
 import numpy as np
 from stsci.tools import fileutil
 
-import getinput
-import mappings
-from mappings import inst_mappings, ins_spec_kw
-from mappings import basic_wcs
-
+from . import altwcs
+from . import getinput
+from . import mappings
+from . import instruments
+from .mappings import inst_mappings, ins_spec_kw
+from .mappings import basic_wcs
 
 __docformat__ = 'restructuredtext'
 
@@ -138,7 +137,7 @@ class HSTWCS(WCS):
                                                                    ext=ext)
             self.filename = filename
             instrument_name = hdr0.get('INSTRUME', 'DEFAULT')
-            if instrument_name == 'DEFAULT' or instrument_name not in inst_mappings.keys():
+            if instrument_name == 'DEFAULT' or instrument_name not in inst_mappings:
                 #['IRAF/ARTDATA','',' ','N/A']:
                 self.instrument = 'DEFAULT'
             else:
@@ -204,7 +203,7 @@ class HSTWCS(WCS):
             extension header
 
         """
-        if self.instrument in inst_mappings.keys():
+        if self.instrument in inst_mappings:
             inst_kl = inst_mappings[self.instrument]
             inst_kl = instruments.__dict__[inst_kl]
             insobj = inst_kl(prim_hdr, ext_hdr)
@@ -221,7 +220,7 @@ class HSTWCS(WCS):
                         pass
 
         else:
-            raise KeyError, "Unsupported instrument - %s" %self.instrument
+            raise KeyError("Unsupported instrument - %s" %self.instrument)
 
     def setPscale(self):
         """
@@ -233,9 +232,9 @@ class HSTWCS(WCS):
             self.pscale = np.sqrt(np.power(cd11,2)+np.power(cd21,2)) * 3600.
         except AttributeError:
             if self.wcs.has_cd():
-                print "This file has a PC matrix. You may want to convert it \n \
+                print("This file has a PC matrix. You may want to convert it \n \
                 to a CD matrix, if reasonable, by running pc2.cd() method.\n \
-                The plate scale can be set then by calling setPscale() method.\n"
+                The plate scale can be set then by calling setPscale() method.\n")
             self.pscale = None
 
     def setOrient(self):
@@ -248,9 +247,9 @@ class HSTWCS(WCS):
             self.orientat = np.rad2deg(np.arctan2(cd12,cd22))
         except AttributeError:
             if self.wcs.has_cd():
-                print "This file has a PC matrix. You may want to convert it \n \
+                print("This file has a PC matrix. You may want to convert it \n \
                 to a CD matrix, if reasonable, by running pc2.cd() method.\n \
-                The orientation can be set then by calling setOrient() method.\n"
+                The orientation can be set then by calling setOrient() method.\n")
             self.pscale = None
 
     def updatePscale(self, scale):
@@ -283,20 +282,20 @@ class HSTWCS(WCS):
             if header is not None and 'IDCSCALE' in header:
                 self._readModelFromHeader(header)
             else:
-                print "Distortion model is not available: IDCTAB=None\n"
+                print("Distortion model is not available: IDCTAB=None\n")
                 self.idcmodel = None
         elif not os.path.exists(fileutil.osfn(self.idctab)):
             if header is not None and 'IDCSCALE' in header:
                 self._readModelFromHeader(header)
             else:
-                print 'Distortion model is not available: IDCTAB file %s not found\n' % self.idctab
+                print('Distortion model is not available: IDCTAB file %s not found\n' % self.idctab)
                 self.idcmodel = None
         else:
             self.readModelFromIDCTAB(header=header, update=update)
 
     def _readModelFromHeader(self, header):
         # Recreate idc model from SIP coefficients and header kw
-        print 'Restoring IDC model from SIP coefficients\n'
+        print('Restoring IDC model from SIP coefficients\n')
         model = models.GeometryModel()
         cx, cy = coeff_converter.sip2idc(self)
         model.cx = cx
@@ -331,7 +330,7 @@ class HSTWCS(WCS):
 
         """
         if self.date_obs == None:
-            print 'date_obs not available\n'
+            print('date_obs not available\n')
             self.idcmodel = None
             return
         if self.filter1 == None and self.filter2 == None:
@@ -349,7 +348,7 @@ class HSTWCS(WCS):
 
         if update:
             if header==None:
-                print 'Update header with IDC model kw requested but header was not provided\n.'
+                print('Update header with IDC model kw requested but header was not provided\n.')
             else:
                 self._updatehdr(header)
 
@@ -776,7 +775,7 @@ adaptive=False, detect_divergence=False, quiet=False)
         dn2       = dn2prev
 
         # prepare for iterative process
-        iterlist  = range(1, maxiter+1)
+        iterlist  = list(range(1, maxiter+1))
         accuracy2 = accuracy**2
         ind       = None
         inddiv    = None
@@ -839,7 +838,7 @@ adaptive=False, detect_divergence=False, quiet=False)
         #####################################################################
         if adaptive:
             if ind is None:
-                ind = np.asarray(range(npts), dtype=np.int64)
+                ind = np.asarray(list(range(npts)), dtype=np.int64)
 
             for k in iterlist:
                 # check convergence:
@@ -953,11 +952,11 @@ adaptive=False, detect_divergence=False, quiet=False)
         """
         Print the basic WCS keywords.
         """
-        print 'WCS Keywords\n'
-        print 'CD_11  CD_12: %r %r' % (self.wcs.cd[0,0],  self.wcs.cd[0,1])
-        print 'CD_21  CD_22: %r %r' % (self.wcs.cd[1,0],  self.wcs.cd[1,1])
-        print 'CRVAL    : %r %r' % (self.wcs.crval[0], self.wcs.crval[1])
-        print 'CRPIX    : %r %r' % (self.wcs.crpix[0], self.wcs.crpix[1])
-        print 'NAXIS    : %d %d' % (self.naxis1, self.naxis2)
-        print 'Plate Scale : %r' % self.pscale
-        print 'ORIENTAT : %r' % self.orientat
+        print('WCS Keywords\n')
+        print('CD_11  CD_12: %r %r' % (self.wcs.cd[0,0],  self.wcs.cd[0,1]))
+        print('CD_21  CD_22: %r %r' % (self.wcs.cd[1,0],  self.wcs.cd[1,1]))
+        print('CRVAL    : %r %r' % (self.wcs.crval[0], self.wcs.crval[1]))
+        print('CRPIX    : %r %r' % (self.wcs.crpix[0], self.wcs.crpix[1]))
+        print('NAXIS    : %d %d' % (self.naxis1, self.naxis2))
+        print('Plate Scale : %r' % self.pscale)
+        print('ORIENTAT : %r' % self.orientat)
diff --git a/lib/stwcs/wcsutil/instruments.py b/lib/stwcs/wcsutil/instruments.py
index 8641e51..45da046 100644
--- a/lib/stwcs/wcsutil/instruments.py
+++ b/lib/stwcs/wcsutil/instruments.py
@@ -1,6 +1,6 @@
-from __future__ import division # confidence high
+from __future__ import absolute_import, division, print_function # confidence high
 
-from mappings import ins_spec_kw
+from .mappings import ins_spec_kw
 
 class InstrWCS(object):
     """
@@ -157,7 +157,7 @@ class ACSWCS(InstrWCS):
         try:
             self.detector = self.primhdr['DETECTOR']
         except KeyError:
-            print 'ERROR: Detector kw not found.\n'
+            print('ERROR: Detector kw not found.\n')
             raise
 
     def set_parity(self):
@@ -165,7 +165,7 @@ class ACSWCS(InstrWCS):
                 'HRC':[[-1.0,0.0],[0.0,1.0]],
                 'SBC':[[-1.0,0.0],[0.0,1.0]]}
 
-        if self.detector not in parity.keys():
+        if self.detector not in parity:
             parity = InstrWCS.set_parity(self)
         else:
             self.parity = parity[self.detector]
@@ -208,7 +208,7 @@ class WFPC2WCS(InstrWCS):
         try:
             self.detector = self.exthdr['DETECTOR']
         except KeyError:
-            print 'ERROR: Detector kw not found.\n'
+            print('ERROR: Detector kw not found.\n')
             raise
 
 
@@ -227,7 +227,7 @@ class WFC3WCS(InstrWCS):
         try:
             self.detector = self.primhdr['DETECTOR']
         except KeyError:
-            print 'ERROR: Detector kw not found.\n'
+            print('ERROR: Detector kw not found.\n')
             raise
 
     def set_filter1(self):
@@ -243,7 +243,7 @@ class WFC3WCS(InstrWCS):
         parity = {'UVIS':[[-1.0,0.0],[0.0,1.0]],
           'IR':[[-1.0,0.0],[0.0,1.0]]}
 
-        if self.detector not in parity.keys():
+        if self.detector not in parity:
             parity = InstrWCS.set_parity(self)
         else:
             self.parity = parity[self.detector]
@@ -278,7 +278,7 @@ class NICMOSWCS(InstrWCS):
         try:
             self.detector = self.primhdr['CAMERA']
         except KeyError:
-            print 'ERROR: Detector kw not found.\n'
+            print('ERROR: Detector kw not found.\n')
             raise
 
 class STISWCS(InstrWCS):
@@ -309,7 +309,7 @@ class STISWCS(InstrWCS):
         try:
             self.detector = self.primhdr['DETECTOR']
         except KeyError:
-            print 'ERROR: Detector kw not found.\n'
+            print('ERROR: Detector kw not found.\n')
             raise
 
     def set_date_obs(self):
diff --git a/lib/stwcs/wcsutil/mosaic.py b/lib/stwcs/wcsutil/mosaic.py
index 7efefc3..9d2d0a3 100644
--- a/lib/stwcs/wcsutil/mosaic.py
+++ b/lib/stwcs/wcsutil/mosaic.py
@@ -1,4 +1,4 @@
-from __future__ import division
+from __future__ import division, print_function
 import numpy as np
 from matplotlib import pyplot as plt
 from astropy.io import fits
@@ -86,10 +86,10 @@ def updatehdr(fname, wcsobj, wkey, wcsname, ext=1, clobber=False):
     hdr = fits.getheader(fname, ext=ext)
     all_keys = list(string.ascii_uppercase)
     if wkey.upper() not in all_keys:
-        raise KeyError, "wkey must be one character: A-Z"
+        raise KeyError("wkey must be one character: A-Z")
     if wkey not in altwcs.available_wcskeys(hdr):
         if not clobber:
-            raise ValueError, "wkey %s is already in use. Use clobber=True to overwrite it or specify a different key." %wkey
+            raise ValueError("wkey %s is already in use. Use clobber=True to overwrite it or specify a different key." %wkey)
         else:
             altwcs.deleteWCS(fname, ext=ext, wcskey='V')
     f = fits.open(fname, mode='update')
@@ -97,7 +97,7 @@ def updatehdr(fname, wcsobj, wkey, wcsname, ext=1, clobber=False):
     hwcs = wcs2header(wcsobj)
     wcsnamekey = 'WCSNAME' + wkey
     f[ext].header[wcsnamekey] = wcsname
-    for k in hwcs.keys():
+    for k in hwcs:
         f[ext].header[k[:7]+wkey] = hwcs[k]
 
     f.close()
@@ -164,17 +164,17 @@ def readWCS(input, exts=None, extname=None):
                     continue
             fobj.close()
     if fomited != []:
-        print "These files were skipped:"
+        print("These files were skipped:")
         for f in fomited:
-            print f
+            print(f)
     return wcso
 
 
 def validateExt(ext):
     if not isinstance(ext, int) and not isinstance(ext, tuple) \
        and not isinstance(ext, list):
-        print "Ext must be integer, tuple, a list of int extension numbers, \
-        or a list of tuples representing a fits extension, for example ('sci', 1)."
+        print("Ext must be integer, tuple, a list of int extension numbers, \
+        or a list of tuples representing a fits extension, for example ('sci', 1).")
         return False
     else:
         return True
diff --git a/lib/stwcs/wcsutil/wcscorr.py b/lib/stwcs/wcsutil/wcscorr.py
index fedc2b2..3f9b7d5 100644
--- a/lib/stwcs/wcsutil/wcscorr.py
+++ b/lib/stwcs/wcsutil/wcscorr.py
@@ -1,12 +1,14 @@
+from __future__ import absolute_import, division, print_function
+
 import os,copy
-from astropy.io import fits
 import numpy as np
+from astropy.io import fits
 
-from stsci.tools import fileutil
 import stwcs
 from stwcs.wcsutil import altwcs
 from stwcs.updatewcs import utils
-import convertwcs
+from stsci.tools import fileutil
+from . import convertwcs
 
 DEFAULT_WCS_KEYS = ['CRVAL1','CRVAL2','CRPIX1','CRPIX2',
                     'CD1_1','CD1_2','CD2_1','CD2_2',
@@ -51,7 +53,7 @@ def init_wcscorr(input, force=False):
             return
         else:
             del fimg['wcscorr']
-    print 'Initializing new WCSCORR table for ',fimg.filename()
+    print('Initializing new WCSCORR table for ',fimg.filename())
 
     used_wcskeys = altwcs.wcskeys(fimg['SCI', 1].header)
 
@@ -73,7 +75,7 @@ def init_wcscorr(input, force=False):
     idc2header = True
     if wcs1.idcscale is None:
         idc2header = False
-    wcs_keywords = wcs1.wcs2header(idc2hdr=idc2header).keys()
+    wcs_keywords = list(wcs1.wcs2header(idc2hdr=idc2header).keys())
 
     prihdr = fimg[0].header
     prihdr_keys = DEFAULT_PRI_KEYS
@@ -82,7 +84,7 @@ def init_wcscorr(input, force=False):
                  'D2IMNAME':stwcs.updatewcs.utils.build_d2imname}
 
     # Now copy original OPUS values into table
-    for extver in xrange(1, numsci + 1):
+    for extver in range(1, numsci + 1):
         rowind = find_wcscorr_row(wcsext.data,
                                   {'WCS_ID': 'OPUS', 'EXTVER': extver,
                                    'WCS_key':'O'})
@@ -107,7 +109,7 @@ def init_wcscorr(input, force=False):
         if wcsext.data.field('CRVAL1')[rownum] != 0:
             # If we find values for these keywords already in the table, do not
             # overwrite them again
-            print 'WCS keywords already updated...'
+            print('WCS keywords already updated...')
             break
         for key in wcs_keywords:
             if key in wcsext.data.names:
@@ -129,7 +131,7 @@ def init_wcscorr(input, force=False):
     # TODO: Much of this appears to be redundant with update_wcscorr; consider
     # merging them...
     for uwkey in used_wcskeys:
-        for extver in xrange(1, numsci + 1):
+        for extver in range(1, numsci + 1):
             hdr = fimg['SCI', extver].header
             wcs = stwcs.wcsutil.HSTWCS(fimg, ext=('SCI', extver),
                                        wcskey=uwkey)
@@ -146,7 +148,7 @@ def init_wcscorr(input, force=False):
             if len(rows[0]) > 0:
                 rownum = np.where(rowind)[0][0]
             else:
-                print 'No available rows found for updating. '
+                print('No available rows found for updating. ')
 
             # Update selection columns for this row with relevant values
             wcsext.data.field('WCS_ID')[rownum] = wcsid
@@ -294,7 +296,7 @@ def update_wcscorr(dest, source=None, extname='SCI', wcs_id=None, active=True):
 
     for colname in wcscorr_cols:
         if colname not in old_table.data.columns.names:
-            print "WARNING:    Replacing outdated WCSCORR table..."
+            print("WARNING:    Replacing outdated WCSCORR table...")
             outdated_table = old_table.copy()
             del dest['WCSCORR']
             init_wcscorr(dest)
@@ -305,7 +307,7 @@ def update_wcscorr(dest, source=None, extname='SCI', wcs_id=None, active=True):
     # extension version; if this should not be assumed then this can be
     # modified...
     wcs_keys = altwcs.wcskeys(source[(extname, 1)].header)
-    wcs_keys = filter(None, wcs_keys)
+    wcs_keys = [kk for kk in wcs_keys if kk]
     if ' ' not in wcs_keys: wcs_keys.append(' ') # Insure that primary WCS gets used
     # apply logic for only updating WCSCORR table with specified keywords
     # corresponding to the WCS with WCSNAME=wcs_id
@@ -319,7 +321,7 @@ def update_wcscorr(dest, source=None, extname='SCI', wcs_id=None, active=True):
             wkeys.remove(' ')
         wcs_keys = wkeys
     wcshdr = stwcs.wcsutil.HSTWCS(source, ext=(extname, 1)).wcs2header()
-    wcs_keywords = wcshdr.keys()
+    wcs_keywords = list(wcshdr.keys())
 
     if 'O' in wcs_keys:
         wcs_keys.remove('O') # 'O' is reserved for original OPUS WCS
@@ -368,7 +370,7 @@ def update_wcscorr(dest, source=None, extname='SCI', wcs_id=None, active=True):
             wcshdr = wcs.wcs2header()
 
             # Update selection column values
-            for key, val in selection.iteritems():
+            for key, val in selection.items():
                 if key in new_table.data.names:
                     new_table.data.field(key)[idx] = val
 
@@ -521,7 +523,7 @@ def create_wcscorr(descrip=False, numrows=1, padding=0):
                          array=np.array(['OPUS'] * numrows + [''] * padding,
                                         dtype='S24'))
     extver_col = fits.Column(name='EXTVER', format='I',
-                             array=np.array(range(1, numrows + 1),
+                             array=np.array(list(range(1, numrows + 1)),
                                             dtype=np.int16))
     wcskey_col = fits.Column(name='WCS_key', format='A',
                              array=np.array(['O'] * numrows + [''] * padding,
@@ -572,16 +574,16 @@ def delete_wcscorr_row(wcstab,selections=None,rows=None):
     """
 
     if selections is None and rows is None:
-        print 'ERROR: Some row selection information must be provided!'
-        print '       Either a row numbers or "selections" must be provided.'
+        print('ERROR: Some row selection information must be provided!')
+        print('       Either a row numbers or "selections" must be provided.')
         raise ValueError
 
     delete_rows = None
     if rows is None:
         if 'wcs_id' in selections and selections['wcs_id'] == 'OPUS':
             delete_rows = None
-            print 'WARNING: OPUS WCS information can not be deleted from WCSCORR table.'
-            print '         This row will not be deleted!'
+            print('WARNING: OPUS WCS information can not be deleted from WCSCORR table.')
+            print('         This row will not be deleted!')
         else:
             rowind = find_wcscorr_row(wcstab, selections=selections)
             delete_rows = np.where(rowind)[0].tolist()
@@ -626,8 +628,8 @@ def update_wcscorr_column(wcstab, column, values, selections=None, rows=None):
         of the value of 'selections'
     """
     if selections is None and rows is None:
-        print 'ERROR: Some row selection information must be provided!'
-        print '       Either a row numbers or "selections" must be provided.'
+        print('ERROR: Some row selection information must be provided!')
+        print('       Either a row numbers or "selections" must be provided.')
         raise ValueError
 
     if not isinstance(values, list):
@@ -637,8 +639,8 @@ def update_wcscorr_column(wcstab, column, values, selections=None, rows=None):
     if rows is None:
         if 'wcs_id' in selections and selections['wcs_id'] == 'OPUS':
             update_rows = None
-            print 'WARNING: OPUS WCS information can not be deleted from WCSCORR table.'
-            print '         This row will not be deleted!'
+            print('WARNING: OPUS WCS information can not be deleted from WCSCORR table.')
+            print('         This row will not be deleted!')
         else:
             rowind = find_wcscorr_row(wcstab, selections=selections)
             update_rows = np.where(rowind)[0].tolist()
@@ -652,11 +654,11 @@ def update_wcscorr_column(wcstab, column, values, selections=None, rows=None):
 
     # Expand single input value to apply to all selected rows
     if len(values) > 1 and len(values) < len(update_rows):
-        print 'ERROR: Number of new values',len(values)
-        print '       does not match number of rows',len(update_rows),' to be updated!'
-        print '       Please enter either 1 value or the same number of values'
-        print '       as there are rows to be updated.'
-        print '    Table will not be updated...'
+        print('ERROR: Number of new values',len(values))
+        print('       does not match number of rows',len(update_rows),' to be updated!')
+        print('       Please enter either 1 value or the same number of values')
+        print('       as there are rows to be updated.')
+        print('    Table will not be updated...')
         raise ValueError
 
     if len(values) == 1 and len(values) < len(update_rows):