Better error messages in unittest.mock

[PetterS]

BPO	35047
Nosy	@vstinner, @voidspace, @eamanu, @PetterS, @tirkarthi
PRs	bpo-35047: Better error messages in unittest.mock #10090 bpo-35047: Update whatsnew/3.8. #13746

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-10-28.20:42:15.416>
created_at = <Date 2018-10-23.08:35:35.824>
labels = ['3.8', 'library']
title = 'Better error messages in unittest.mock'
updated_at = <Date 2019-06-04.19:35:08.173>
user = 'https://github.com/PetterS'

bugs.python.org fields:

activity = <Date 2019-06-04.19:35:08.173>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-10-28.20:42:15.416>
closer = 'vstinner'
components = ['Library (Lib)']
creation = <Date 2018-10-23.08:35:35.824>
creator = 'Petter S'
dependencies = []
files = []
hgrepos = []
issue_num = 35047
keywords = ['patch']
message_count = 8.0
messages = ['328292', '328344', '328378', '328379', '328386', '328743', '328745', '344636']
nosy_count = 5.0
nosy_names = ['vstinner', 'michael.foord', 'eamanu', 'Petter S', 'xtreak']
pr_nums = ['10090', '13746']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue35047'
versions = ['Python 3.8']

[PetterS]

When developing unit tests with unittest.mock, it is common to see error messages like this:

AssertionError: Expected 'info' to not have been called. Called 3 times.

It would be really helpful if those 3 calls were listed in the assertion error.

I am happy to add this if people agree it is a good thing.

[eamanu]

It seems to be a good idea.

If you attach a sample, it would be great.

[PetterS]

Sure, consider the following code:

from unittest import mock

m = mock.Mock()
m(1, 3)
m("Test", data=[42])

When I call

m.assert_not_called()

I get the following error message:

AssertionError: Expected 'mock' to not have been called. Called 2 times.

This is what I would like to improve. On the other hand, if I call

m.assert_has_calls(mock.call(3))

I get the following error:

AssertionError: Calls not found.
Expected: ['', (3,), {}]
Actual: [call(1, 3), call('Test', data=[42])]

This is great and may serve as a template for what I want to introduce.

[PetterS]

(The example above should have been "m.assert_has_calls([mock.call(3)])" but it does not affect my point.)

[eamanu]

I think that is a good change.

Maybe you can apply the change on 3.6, 3.7 and 3.8

[vstinner]

New changeset 47d9424 by Victor Stinner (Petter Strandmark) in branch 'master':
bpo-35047, unittest.mock: Better error messages on assert_called_xxx failures (GH-10090)
47d9424

[vstinner]

Thanks Petter Strandmark!

[vstinner]

New changeset 001d63c by Victor Stinner (Petter Strandmark) in branch 'master':
bpo-35047: Update whatsnew/3.8 for better mock error message (GH-13746)
001d63c