Python Version 2 versus 3¶

ParaView is working towards full Python 3 support. To that end, python code should be written so it is compatible with Python 2.7 and Python 3.5.

Here are the changes we have needed making the transition to Python 3.

`print` is a function¶

print "helpful message"

should be replaced by:

print("helpful message")

Re-directing to stderr is slightly more complicated. Python 2 did:

print >> stderr, "Error: Failed to evaluate '%s'. " % query

To maintain python-2 compatibility, you need the future module:

from __future__ import print_function
...
print("Error: Failed to evaluate '%s'. " % query, file=sys.stderr)

raise RuntimeError, "failed"

is replaced by:

raise RuntimeError("failed")

Handling exceptions as objects must use the as keyword:

except AttributeError as attrErr:

To create an iterable, Python 3 needs a __next__() method. Assign the Python 2 next() method to it:

__next__ = next # Python 3.x compatibility

Several methods now return an iterable object, not a list. Examples include:

If you need a list, just wrap the return in a list:

list(array_colors.keys())

Dictionaries also lost the iterkeys(), iteritems() and itervalues() methods. Simply use keys or items() to get values instead.

for key, value in kwargs.iteritems():
    self.analysis[key] = value

becomes:

for key in kwargs:
    self.analysis[key] = kwargs[key]

or:

for key, value in kwargs.items():
    self.analysis[key] = value

The only valid syntax in Python 3 for relative submodule import is:

from .some_module import *

Be explicit when importing submodules whenever possible.

Any new modules or significant work on old code should add:

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals

to the top, to ensure forward compatibility with Python 3.