Multi version support for Python doctests

Question

I am writing my doctests like this:

>>> some_function(a=1, b=2)
{u'id': u'123', u'name': u'abc'}

This works fine for Python version 2.5, 2.6 & 2.7 but fails for Python 3 with following error:

Expected:
    {u'id': u'123', u'name': u'abc'}
Got:
    {'id': '123', 'name': 'abc'}

Problem is that if I write my doctests like this:

>>> some_function(a=1, b=2)
{'id': '123', 'name': 'abc'}

They will work only for Python3 and fail on Python2 version. My question is how do I make it cross version compatible?

Answer 1

I ran into the same problem with doctests in IPython. There's no neat solution, but I wrapped all of the u' prefixes in {}, i.e. {u}', and made a little function that would include or exclude them as appropriate.

You can see the u_format() function and a doctest using it.

But that's rather messy, so I've moved many tests away from doctests.

Alternatively, you can test it like this:

>>> some_function(a=1, b=2) == {'id': '123', 'name': 'abc'}
True

If you need some unicode strings in the keys, you can use u'abþ', and use distribute to run 2to3 on the doctests. But that only works on input code, not output reprs.

Answer 2

If you use pytest, you can just do:

>>> some_function(a=1, b=2) # doctest: +ALLOW_UNICODE
{u'id': u'123', u'name': u'abc'}

And the u will be stripped if you are running Python 3, and kept in Python 2.

Answer 3

I run into the same issue with doctests in NLTK; it was solved by using a custom doctest output checker (that treats u'foo' and 'foo' the same) which is installed by a custom nose plugin: https://github.com/nltk/nltk/blob/develop/nltk/test/doctest_nose_plugin.py

This solution is not pretty, but it works quite well (there are about 0.5 megabytes of doctests in NLTK) and it doesn't make doctests less readable.

EDIT: found a simplified standalone version of this nose plugin: https://github.com/gnublade/doctest-ignore-unicode