C407 increases laziness which should be noted since it can change behaviour

The following issues the C407 warning (as desired)

results = all([m.delete() for m in q.where_in(self._other_key, ids).get()])

however if you appease it and switch the code to

results = all((m.delete() for m in q.where_in(self._other_key, ids).get()))

or

results = all(m.delete() for m in q.where_in(self._other_key, ids).get())

It causes a subtle bug due to the new laziness gained by the generator expression

This can be evidenced here

>>> all((print(i) for i in range(1, 10)))
1
False
>>> all((print(i) or True for i in range(1, 10)))
1
2
3
4
5
6
7
8
9
True

This warning should probably remind/warn (haha) the user that there is a risk in changing it to follow the warning if the generate has a side effecting function in it

[merc1031]

Yah, its tough. It seems like a decent rule, but it can bite you pretty hard. Maybe if it defaults to being ignored, and has a more strongly worded warning about its implications? That could be a good compromise.

in the haskell linter, hlint, there are a few lints that warn you that they either increase or decrease laziness.

[adamchainz]

Rule C407 has been a constant source of problems - 5 or 6 bugs in the changelog: https://github.com/adamchainz/flake8-comprehensions/blob/master/HISTORY.rst . I’m tempted to just remove it. I’d prefer this library gave 100% actionable advice rather than “sometimes this is a good idea” tweaks. What do you think?