[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Vitaly]

Vitaly added the comment:

... and an unnecessary incompatibility between 2.7 and 3.x

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Vitaly]

Vitaly added the comment:

Yes, this caused me to examine Counter, which I had not used before, and now I 
understand it, too. However, the new name by itself does not sufficiently 
reflect the subtlety of element-by-element counts, which does a disservice to 
the readability of test code.

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Terry J. Reedy]

Terry J. Reedy added the comment:

It compares element by element counts, as indicated by "Equivalent to: 
assertEqual(Counter(list(first)), Counter(list(second))) ..." and the failure 
messages.  Since I understand Counter equality, I ignore the rest.

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resolution:  -> not a bug
stage: test needed -> resolved
status: open -> closed

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Vitaly]

Vitaly added the comment:

I think that we can close this as not a bug, as concerning 2.7. However, I 
think that 3.x made the wrong decision when it renamed assertItemsEqual with 
assertCountEqual, which now misleads users into thinking that it compares 
counts.

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Vitaly]

Vitaly added the comment:

Thanks Terry, your assessment is correct. The maintainers appear to have drawn 
the wrong conclusion in #17866, which mislead me about the existing 
assertItemsEqual in 2.7, which appears to work correctly after all!

import unittest as u
class Test(u.TestCase):
def test_equal_count_of_same_elements(self):
self.assertItemsEqual([1,2,3], [1,2,3])

def test_equal_count_of_different_elements(self):
self.assertItemsEqual([1,2,3], [1,2,4])

def test_different_count(self):
self.assertItemsEqual([1,2,3], [1,2,3,4])
u.main()


$ python assert_items_equal_test.py --verbose
test_different_count (__main__.Test) ... FAIL
test_equal_count_of_different_elements (__main__.Test) ... FAIL
test_equal_count_of_same_elements (__main__.Test) ... ok

==
FAIL: test_different_count (__main__.Test)
--
Traceback (most recent call last):
  File "assert_items_equal_test.py", line 12, in test_different_count
self.assertItemsEqual([1,2,3], [1,2,3,4])
AssertionError: Element counts were not equal:
First has 0, Second has 1:  4

==
FAIL: test_equal_count_of_different_elements (__main__.Test)
--
Traceback (most recent call last):
  File "assert_items_equal_test.py", line 8, in 
test_equal_count_of_different_elements
self.assertItemsEqual([1,2,3], [1,2,4])
AssertionError: Element counts were not equal:
First has 1, Second has 0:  3
First has 0, Second has 1:  4

--
Ran 3 tests in 0.001s

FAILED (failures=2)

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Vitaly]

Vitaly added the comment:

According to comment http://bugs.python.org/issue17866#msg188052 in #17866, the 
maintainers renamed `assertItemsEqual` to `assertCountEqual` somewhere in the 
3.x development timeframe. They apparently latched on to the implementation of 
`assertItemsEqual` in 2.7.x, which compares counts only, rather than its 
documentation.

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Terry J. Reedy]

Terry J. Reedy added the comment:

Here is the specific test code so you can replace my lists with examples that 
you think malfunction.

import unittest as u
class Test(u.TestCase):
def test_Count(self):
self.assertItemsEqual([1,2,3], [1,2,4])
u.main()

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Terry J. Reedy]

Terry J. Reedy added the comment:

Note: 2.7 doc says that this is renamed assertCountEqual in 3.x.  It was added 
in 2.7 and 3.2.  The 3.x doc is essentially the same, but with this instead: 
"Equivalent to: assertEqual(Counter(list(first)), Counter(list(second))) ...".

If the method misfunctions, then it should be fixed, not have the bug 
documented.  But you need to provide specific evidence and example.  The 
unittest in 3.x has this test with same number, different elements.
self.assertRaises(self.failureException, self.assertCountEqual,
  [1, 2] + [3] * 100, [1] * 100 + [2, 3])

I ran following test with same number, different items.
FAIL: test_Count (__main__.Test)
--
Traceback (most recent call last):
  File "F:\Python\mypy\tem.py", line 4, in test_Count
self.assertItemsEqual([1,2,3], [1,2,4])
AssertionError: Element counts were not equal:
First has 1, Second has 0:  3
First has 0, Second has 1:  4
 
This also fails in 3.5 with name change.

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components: +Library (Lib) -Documentation
nosy: +terry.reedy
stage:  -> test needed
type:  -> behavior

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Georg Brandl]

Changes by Georg Brandl :


--
nosy: +michael.foord

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Vitaly]

Changes by Vitaly :


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assignee:  -> docs@python
components: +Documentation
nosy: +docs@python

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Vitaly]

Changes by Vitaly :


--
versions: +Python 2.7

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[issue27060] Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

[Vitaly]

New submission from Vitaly:

Python 2.7 documentation is VERY misleading about the functionality of 
assertItemsEqual. The implementation only compares item counts, but the 
documentation actually claims to compare not only the counts, but the actual 
sorted elements themselves. This documentation mislead my group to use this 
method for comparing the elements. See 
https://hg.python.org/cpython/file/d9921cb6e3cd/Doc/library/unittest.rst, which 
is what appears on 
https://docs.python.org/2.7/library/unittest.html#unittest.TestCase.assertItemsEqual:

   .. method:: assertItemsEqual(actual, expected, msg=None)

  Test that sequence *expected* contains the same elements as *actual*,
  regardless of their order. When they don't, an error message listing the
  differences between the sequences will be generated.

  Duplicate elements are *not* ignored when comparing *actual* and
  *expected*. It verifies if each element has the same count in both
  sequences. It is the equivalent of ``assertEqual(sorted(expected),
  sorted(actual))`` but it works with sequences of unhashable objects as
  well.

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messages: 265889
nosy: vitaly
priority: normal
severity: normal
status: open
title: Documentation of assertItemsEqual in unittest is VERY misleading in 2.7

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