How do I test dictionary-equality with Python'

Question

I'm writing a doctest for a function that outputs a dictionary. The doctest looks like

>>> my_function()
{'this': 'is', 'a': 'dictionary'}

When I run it, it fails with

Expected:
    {'this': 'is', 'a': 'dictionary'}
Got:
    {'a': 'dictionary', 'this': 'is'}

My best guess as to the cause of this failure is that doctest isn't checking dictionary equality, but __repr__ equality. This post indicates that there's some way to trick doctest into checking dictionary equality. How can I do this?

Answer 1

Doctest doesn't check __repr__ equality, per se, it just checks that the output is exactly the same. You have to ensure that whatever is printed will be the same for the same dictionary. You can do that with this one-liner:

>>> sorted(my_function().items())
[('a', 'dictionary'), ('this', 'is')]

Although this variation on your solution might be cleaner:

>>> my_function() == {'this': 'is', 'a': 'dictionary'}
True

Answer 2

turn it into a list via dict.items() and then sort it ...

>>> l = my_function().items()
>>> l.sort()
>>> l
[('a', 'dictionary'), ('this', 'is')]

Answer 3

You can create an instance of unittest.TestCase class inside your doctests, and use it to compare dictionaries:

def my_function(x):
    """
    >>> from unittest import TestCase
    >>> t = TestCase()

    >>> t.assertDictEqual(
    ...     my_function('a'),
    ...     {'this': 'is', 'a': 'dictionary'}
    ... )

    >>> t.assertDictEqual(
    ...     my_function('b'),
    ...     {'this': 'is', 'b': 'dictionary'}
    ... )

    """
    return {'this': 'is', x: 'dictionary'}

Note: this approach is better than simply checking if dictionaries are equal, because it will show diff between the two dictionaries.

Answer 4

Another good way is to use pprint (in the standard library).

>>> import pprint
>>> pprint.pprint({"second": 1, "first": 0})
{'first': 0, 'second': 1}

According to its source code, it's sorting dicts for you:

http://hg.python.org/cpython/file/2.7/Lib/pprint.py#l158

items = _sorted(object.items())

Answer 5

I ended up using this. Hacky, but it works.

>>> p = my_function()
>>> {'this': 'is', 'a': 'dictionary'} == p
True

Answer 6

Most of it has been already said here.. anyway JSYK: there is a dedicated section in doctest documentation:

https://docs.python.org/3.5/library/doctest.html#warnings