Refactor str tests to reflect that str and unicode are merged in Python 3

[asqui]

BPO	36824
Nosy	@ncoghlan, @serhiy-storchaka, @asqui, @tirkarthi
PRs	gh-81005: Refactor str tests to reflect that str and unicode are merged in Python 3 #13172

^{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 = None
created_at = <Date 2019-05-07.06:37:53.164>
labels = ['3.8', 'type-feature', 'tests']
title = 'Refactor str tests to reflect that str and unicode are merged in Python 3'
updated_at = <Date 2019-05-07.20:05:56.998>
user = 'https://github.com/asqui'

bugs.python.org fields:

activity = <Date 2019-05-07.20:05:56.998>
actor = 'dfortunov'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2019-05-07.06:37:53.164>
creator = 'dfortunov'
dependencies = []
files = []
hgrepos = []
issue_num = 36824
keywords = ['patch']
message_count = 4.0
messages = ['341685', '341687', '341689', '341809']
nosy_count = 4.0
nosy_names = ['ncoghlan', 'serhiy.storchaka', 'dfortunov', 'xtreak']
pr_nums = ['13172']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36824'
versions = ['Python 3.8']

[asqui]

Unit tests of str and related types (e.g. UserString) contain nomenclature and structure that dates back to the Python 2 distinction between str and unicode.

Previously it was undesirable to disturb the structure of these tests too much as this would complicate the merging of bug fixes back to Python 2, however with Python 2 drawing near to end-of-life this is perhaps less of a concern.

I would propose the following changes as a start:

• Rename test_unicode.py to test_str.py, and UnicodeTest class to StrTest
  (to reflect the type that is now being tested)
• Remove MixinStrUnicodeUserStringTest class and move its tests into CommonTest
  (The comment for this class says

  additional tests that only work for

  stringlike objects, i.e. str, UserString

  but in the absence of unicode the CommonTest
  class is also only used for these two types now.)
• Promote tests from the current UnicodeTest class to CommonTest
  where it makes sense to do so; remove checks that no longer make sense.
  (e.g. checks around mixed str/unicode arguments that are all just str now)
  Maybe the duplicative checkequalnofix() method can also go away now?
• The BadSeq1 helper class is not used because the corresponding check was
  commented out in 2007. Either reinstate the check, or remove this class.

I'm happy to submit a PR for this, but just wanted to get some feedback before making the changes.

[serhiy-storchaka]

There is still the functionality common to str, bytes and bytearray. So it make sense to have the common class for testing str, bytes, bytearray and UserList and other common class (subclass of the former one) for testing str and UserList.

[asqui]

Agreed. This functionality is in BaseTest (which is the base for CommonTest) and I don't propose to change this.

[asqui]

PS opened here: #13172
I've tried to break down the changes into individual steps, with justification in commit messages. Happy to collapse these down into fewer commits before merge if preferred.

I haven't done the "Promote tests from the current UnicodeTest class to CommonTest" portion yet, but I'm running out of sprint time so I wanted to submit what I have.

I believe these changes are suitable for merge to master, but keen to hear feedback :-)