Increasing verbosity turns off the assertion rewriting

[JBKahn]

Given a file:

a = [1, 2, 3]
b = [1, 2]

def test_a():
    assert len(a) == len(b)

def test_bigger_assertion():
    first_dict = {a: a for a in range(100)}
    second_dict = {b: b for b in range(1, 101)}
    assert first_dict == second_dict

pytest will output

 ✔  pytest test_file.py
============================================================================================== test session starts ==============================================================================================
platform darwin -- Python 3.7.4, pytest-5.2.1, py-1.8.0, pluggy-0.13.0
rootdir: /Users/jkahn/wave/src/external/pytest-clarity
collected 2 items

test_file.py FF                                                                                                                                                                                           [100%]

=================================================================================================== FAILURES ====================================================================================================
____________________________________________________________________________________________________ test_a _____________________________________________________________________________________________________

    def test_a():
>       assert len(a) == len(b)
E       assert 3 == 2
E        +  where 3 = len([1, 2, 3])
E        +  and   2 = len([1, 2])

test_file.py:5: AssertionError
_____________________________________________________________________________________________ test_bigger_assertion _____________________________________________________________________________________________

    def test_bigger_assertion():
        first_dict = {a: a for a in range(100)}
        second_dict = {b: b for b in range(1, 101)}
>       assert first_dict == second_dict
E       assert {0: 0, 1: 1, 2: 2, 3: 3, ...} == {1: 1, 2: 2, 3: 3, 4: 4, ...}
E         Omitting 99 identical items, use -vv to show
E         Left contains 1 more item:
E         {0: 0}
E         Right contains 1 more item:
E         {100: 100}
E         Use -v to get the full diff

test_file.py:10: AssertionError
=============================================================================================== 2 failed in 0.04s ===============================================================================================

However if I increase the verbosity, so that I can see more of the diff (either -v or -vv) like so, then it removed the helpful assertion rewriting from the first test

 ↵ 1  pytest test_file.py -vv
============================================================================================== test session starts ==============================================================================================
platform darwin -- Python 3.7.4, pytest-5.2.1, py-1.8.0, pluggy-0.13.0 -- /Users/jkahn/.local/share/virtualenvs/pytest-clarity-JGV2Xd-w/bin/python3.7
cachedir: .pytest_cache
rootdir: /Users/jkahn/wave/src/external/pytest-clarity
collected 2 items

test_file.py::test_a FAILED                                                                                                                                                                               [ 50%]
test_file.py::test_bigger_assertion FAILED                                                                                                                                                                [100%]

=================================================================================================== FAILURES ====================================================================================================
____________________________________________________________________________________________________ test_a _____________________________________________________________________________________________________

    def test_a():
>       assert len(a) == len(b)
E       assert 3 == 2
E         -3
E         +2

test_file.py:5: AssertionError
_____________________________________________________________________________________________ test_bigger_assertion _____________________________________________________________________________________________

    def test_bigger_assertion():
        first_dict = {a: a for a in range(100)}
        second_dict = {b: b for b in range(1, 101)}
>       assert first_dict == second_dict
E       assert {0: 0, 1: 1, 2: 2, 3: 3, ...} == {1: 1, 2: 2, 3: 3, 4: 4, ...}
E         Common items:
E         {1: 1,
E          2: 2,
E          3: 3,
E          4: 4,
E          5: 5,
E          6: 6,
E          7: 7,
E          8: 8,
E          9: 9,
E          10: 10,
E          11: 11,
E          12: 12,
E          13: 13,
E          14: 14,
E          15: 15,
E          16: 16,
E          17: 17,
E          18: 18,
E          19: 19,
E          20: 20,
E          21: 21,
E          22: 22,
E          23: 23,
E          24: 24,
E          25: 25,
E          26: 26,
E          27: 27,
E          28: 28,
E          29: 29,
E          30: 30,
E          31: 31,
E          32: 32,
E          33: 33,
E          34: 34,
E          35: 35,
E          36: 36,
E          37: 37,
E          38: 38,
E          39: 39,
E          40: 40,
E          41: 41,
E          42: 42,
E          43: 43,
E          44: 44,
E          45: 45,
E          46: 46,
E          47: 47,
E          48: 48,
E          49: 49,
E          50: 50,
E          51: 51,
E          52: 52,
E          53: 53,
E          54: 54,
E          55: 55,
E          56: 56,
E          57: 57,
E          58: 58,
E          59: 59,
E          60: 60,
E          61: 61,
E          62: 62,
E          63: 63,
E          64: 64,
E          65: 65,
E          66: 66,
E          67: 67,
E          68: 68,
E          69: 69,
E          70: 70,
E          71: 71,
E          72: 72,
E          73: 73,
E          74: 74,
E          75: 75,
E          76: 76,
E          77: 77,
E          78: 78,
E          79: 79,
E          80: 80,
E          81: 81,
E          82: 82,
E          83: 83,
E          84: 84,
E          85: 85,
E          86: 86,
E          87: 87,
E          88: 88,
E          89: 89,
E          90: 90,
E          91: 91,
E          92: 92,
E          93: 93,
E          94: 94,
E          95: 95,
E          96: 96,
E          97: 97,
E          98: 98,
E          99: 99}
E         Left contains 1 more item:
E         {0: 0}
E         Right contains 1 more item:
E         {100: 100}
E         Full diff:
E         - {0: 0,
E         -  1: 1,
E         ? ^
E         + {1: 1,
E         ? ^
E         2: 2,
E         3: 3,
E         4: 4,
E         5: 5,
E         6: 6,
E         7: 7,
E         8: 8,
E         9: 9,
E         10: 10,
E         11: 11,
E         12: 12,
E         13: 13,
E         14: 14,
E         15: 15,
E         16: 16,
E         17: 17,
E         18: 18,
E         19: 19,
E         20: 20,
E         21: 21,
E         22: 22,
E         23: 23,
E         24: 24,
E         25: 25,
E         26: 26,
E         27: 27,
E         28: 28,
E         29: 29,
E         30: 30,
E         31: 31,
E         32: 32,
E         33: 33,
E         34: 34,
E         35: 35,
E         36: 36,
E         37: 37,
E         38: 38,
E         39: 39,
E         40: 40,
E         41: 41,
E         42: 42,
E         43: 43,
E         44: 44,
E         45: 45,
E         46: 46,
E         47: 47,
E         48: 48,
E         49: 49,
E         50: 50,
E         51: 51,
E         52: 52,
E         53: 53,
E         54: 54,
E         55: 55,
E         56: 56,
E         57: 57,
E         58: 58,
E         59: 59,
E         60: 60,
E         61: 61,
E         62: 62,
E         63: 63,
E         64: 64,
E         65: 65,
E         66: 66,
E         67: 67,
E         68: 68,
E         69: 69,
E         70: 70,
E         71: 71,
E         72: 72,
E         73: 73,
E         74: 74,
E         75: 75,
E         76: 76,
E         77: 77,
E         78: 78,
E         79: 79,
E         80: 80,
E         81: 81,
E         82: 82,
E         83: 83,
E         84: 84,
E         85: 85,
E         86: 86,
E         87: 87,
E         88: 88,
E         89: 89,
E         90: 90,
E         91: 91,
E         92: 92,
E         93: 93,
E         94: 94,
E         95: 95,
E         96: 96,
E         97: 97,
E         98: 98,
E         -  99: 99}
E         ?        ^
E         +  99: 99,
E         ?        ^
E         +  100: 100}

test_file.py:10: AssertionError

This is caused by these two lines here:

pytest/src/_pytest/assertion/util.py

Lines 148 to 149 in c58b0fb

	elif verbose > 0:
	explanation = _compare_eq_verbose(left, right)

Which was introduced here
#4607

I came across this when working on plugin stuff (I opened this to get help: #5931). Which is related in a way, in that the Rewriter isn't being used when assertrepr_compare (either builtin or as a plugin) returns something.

[blueyed]

Thanks for the details report. It's a duplicate of #5192 AFAICS.

[JBKahn]

Very correct. Feel free to close it, and merge them if you'd like. My other issue is related to both in that the cause of this is the interaction between this hook and the rewriting. Is it intentional that this method returning a value doesn't do the rewrite?

[blueyed]

See #5933.

If the hook returns something the default is not used anymore.

[blueyed]

Closing as duplicate - please follow up in the PR (give it a try?).