How do I get PyCharm to show entire error diffs from pytest?

Question

I am using Pycharm to run my pytest unit tests. I am testing a REST API, so I often have to validate blocks of JSON. When a test fails, I'll see something like this:

FAILED test_document_api.py:0 (test_create_documents) {'items': [{'i...ages': 1, ...} != {'items': [{'...ages': 1, ...}  Expected :{'items': [{'...ages': 1, ...} Actual   :{'items': [{'i...ages': 1, ...}  <Click to see difference> 

When I click on the "Click to see difference" link, most of the difference is converted to points of ellipses, like so

This is useless since it doesn't show me what is different. I get this behavior for any difference larger than a single string or number.

I assume Pycharm and/or pytest tries to elide uninformative parts of differences for large outputs. However, it's being too aggressive here and eliding everything.

How do I get Pycharm and/or pytest to show me the entire difference?

I've tried adding -vvv to pytest's Additional Arguments, but that has no effect.

---

Since the original post I verified that I see the same behavior when I run unit tests from the command line. So this is an issue with pytest and not Pycharm.

After looking at the answers I've got so far I guess what I'm really asking is "in pytest is it possible to set maxDiff=None without changing the source code of your tests?" The impression I've gotten from reading about pytest is that the -vv switch is what controls this setting, but this does not appear to be the case.

Answer 1

If you look closely into PyCharm sources, from the whole pytest output, PyCharm uses a single line the to parse the data for displaying in the Click to see difference dialog. This is the AssertionError: <message> line:

def test_spam(): >       assert v1 == v2 E       AssertionError: assert {'foo': 'bar'} == {'foo': 'baz'} E         Differing items: E         {'foo': 'bar'} != {'foo': 'baz'} E         Use -v to get the full diff 

If you want to see the full diff line without truncation, you need to customize this line in the output. For a single test, this can be done by adding a custom message to the assert statement:

def test_eggs():     assert a == b, '{0} != {1}'.format(a, b) 

If you want to apply this behaviour to all tests, define custom pytest_assertrepr_compare hook. In the conftest.py file:

# conftest.py def pytest_assertrepr_compare(config, op, left, right):     if op in ('==', '!='):         return ['{0} {1} {2}'.format(left, op, right)] 

The equality comparison of the values will now still be stripped when too long; to show the complete line, you still need to increase the verbosity with -vv flag.

Now the equality comparison of the values in the AssertionError line will not be stripped and the full diff is displayed in the Click to see difference dialog, highlighting the diff parts: