Add test classes for custom __index__, __int__, __float__ and __complex__

[serhiy-storchaka]

BPO	40129
Nosy	@rhettinger, @mdickinson, @serhiy-storchaka
PRs	bpo-40129: Add fake number classes in test.support. #19262

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GitHub fields:

assignee = None
closed_at = <Date 2020-04-11.08:00:46.286>
created_at = <Date 2020-03-31.20:37:44.559>
labels = ['type-feature', 'tests', '3.9']
title = 'Add test classes for custom __index__, __int__, __float__ and __complex__'
updated_at = <Date 2020-04-11.08:00:46.285>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-04-11.08:00:46.285>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-11.08:00:46.286>
closer = 'serhiy.storchaka'
components = ['Tests']
creation = <Date 2020-03-31.20:37:44.559>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40129
keywords = ['patch']
message_count = 2.0
messages = ['365422', '365432']
nosy_count = 3.0
nosy_names = ['rhettinger', 'mark.dickinson', 'serhiy.storchaka']
pr_nums = ['19262']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40129'
versions = ['Python 3.9']

[serhiy-storchaka]

There are many tests for int-like objects (which implement custom __index__ or __int__ methods) in different files. There are less tests for float-like objects (with the __float__ method). There are even tests for complex-like (with the __complex__ method) in different files. To simplify maintaining old tests and adding new tests I propose to add general test classes with custom methods __index__, __int__, __float__ or __complex__ which return the specified value or raise an exception. There is already one similar general class: FakePath.

It could be done using unittest.mock, but Mock objects are much more complex and has too much magic, so they can have different behavior than simpler classes with a single special method.

[rhettinger]

I don't think this makes the world better. It just makes it more intertwined. Currently, I can read the test code in isolation and understand it. But with the proposed PR, a person now needs to know more of what is in test_support. I really like seeing the classes in-line so I know exactly what they are doing and exactly what a test is doing.

Also, I don't like reinventing another alternative to mocks. Ideally, we want to minimize the number of things a person has to know in order to read and write our tests.

I understand your desire to factor everything you see, but in so doing you're creating tighter coupling and increasing complexity for the everyday contributor.