Supporting Multiple Python Versions In Your Code?

Today I tried using pyPdf 1.12 in a script I was writing that targets Python 2.6. When running my script, and even importing pyPdf, I get complaints about deprecated functionality (md5->hashsum, sets). I'd like to contribute a patch to make th开发者_高级运维is work cleanly in 2.6, but I imagine the author does not want to break compatibility for older versions (2.5 and earlier).

Searching Google and Stack Overflow have so far turned up nothing. I feel like I have seen try/except blocks around import statements before that accomplish something similar, but can't find any examples. Is there a generally accepted best practice for supporting multiple Python versions?

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There are two ways to do this:

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(1) Just like you described: Try something and work around the exception for old versions. For example, you could try to import the json module and import a userland implementation if this fails:

try:
    import json
except ImportError:
    import myutils.myjson as json

This is an example from Django (they use this technique often):

try:
    reversed
except NameError:
    from django.utils.itercompat import reversed     # Python 2.3 fallback

If the iterator reversed is available, they use it. Otherwise, they import their own implementation from the utils package.

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(2) Explicitely compare the version of the Python interpreter:

import sys
if sys.version_info < (2, 6, 0):
    # Do stuff for old version...
else:
    # Do 2.6+ stuff

sys.version_info is a tuple that can easily be compared with similar version tuples.

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You can certainly do

try:
  import v26
except ImportError:
  import v25

Dive Into Python—Using Exceptions for Other Purposes

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Multiple versions of Python are supported here. You can a) conditionally use the newer version, which takes a little work, or b) turn off the warnings, which should really be the default (and is on newer Pythons).