SyntaxWarning for "assertion is always true, perhaps remove parentheses?" does not work with constants

[tsukasa-au]

BPO	43497
Nosy	@markshannon, @serhiy-storchaka, @tsukasa-au
PRs	bpo-43497: Emit SyntaxWarnings for assertions with tuple constants. #24867

^{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 2021-07-28.22:58:30.385>
created_at = <Date 2021-03-15.07:31:24.963>
labels = ['interpreter-core', 'type-bug', '3.8', '3.9', '3.10', '3.7']
title = 'SyntaxWarning for "assertion is always true, perhaps remove parentheses?" does not work with constants'
updated_at = <Date 2021-07-28.22:58:30.385>
user = 'https://github.com/tsukasa-au'

bugs.python.org fields:

activity = <Date 2021-07-28.22:58:30.385>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-07-28.22:58:30.385>
closer = 'iritkatriel'
components = ['Interpreter Core']
creation = <Date 2021-03-15.07:31:24.963>
creator = 'darke'
dependencies = []
files = []
hgrepos = []
issue_num = 43497
keywords = ['patch']
message_count = 5.0
messages = ['388711', '388716', '388718', '388723', '388835']
nosy_count = 3.0
nosy_names = ['Mark.Shannon', 'serhiy.storchaka', 'darke']
pr_nums = ['24867']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43497'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[tsukasa-au]

The following block of code does not produce a SyntaxWarning in python 3.7 and above (it does produce a warning in python 3.6 and below):

assert(False, 'msg')

If the tuple is not a constant (for example (x, 'msg')), then a warning is still produced.

[serhiy-storchaka]

What is the use case for assert with a constant?

[tsukasa-au]

I would argue that there is none (especially if it is tuple/something that is always true) -- thus why I would assume that Python would provide a warning.

This bug comes from a discussion I was having with someone earlier today where they mentioned that it would be nice if the linter complained about the following::

assert 'not reachable'

That code is incorrect (the assertion will never fire, due to the string evaluating to True). We remembered that python did complain about tuples, and tried to see what the warning looked like using a trivial example::

  assert(False, 'msg')

We noticed that this code did not produce a warning on python3.8, but did produce a warning on python2.7.

After much digging we found that the following did work on python3.8::

  assert(False, str())

This allowed us to deduce that something special was happening with constants (well, it also required us to look at the disassembly of the ops codes for the above two statements).

Going back to your original question, I have used assert False in code to "test" if a piece of code was being reached. I have also used it for things similar to the piece of code that started all of this::

  if some_expression:
    # ...
  elif some_other_expression:
    #...
  else:
    assert False, 'This code is unreachable'

[serhiy-storchaka]

In such case I prefer to write

assert not 'reachable'

It is shorter than writing False.

If I want to keep an exception even with -O, I write

    raise AssertionError

[markshannon]

New changeset a8ef457 by tsukasa-au in branch 'master':
bpo-43497: Emit SyntaxWarnings for assertions with tuple constants. (GH-24867)
a8ef457