Spurious warning "Unused entry 'genexpr'"

Cython version 0.25.2

l = [1,2,3]

def f():
    print(x for x in l)

$ cython -Wextra -Werror foo.pyx

Error compiling Cython file:
------------------------------------------------------------
...
l = [1,2,3]

def f():
    print(x for x in l)
           ^
------------------------------------------------------------

foo.pyx:4:12: Unused entry 'genexpr'

This only happens when the generator expression is in a function, rather than at the module level. Surrounding the expression with parentheses doesn’t help. Converting it to a list comprehension does remove the warning, but will make performance worse, especially if the generator expression would not have run to completion.

I like to use -Wextra -Werror because the “unused” warnings often indicate a genuine error in my code.

[mhsmith]

@scoder: I’m sorry, but this isn’t fixed at all. My simple 3-line example above still fails, with almost the same message:

$ cython --version
Cython version 0.29.24
$ cython -Wextra -Werror foo.pyx
/home/smith/.virtualenvs/pypi/lib/python3.8/site-packages/Cython/Compiler/Main.py:369: FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2). This will change in a later release! File: /home/smith/tmp/foo.pyx
  tree = Parsing.p_module(s, pxd, full_module_name)

Error compiling Cython file:
------------------------------------------------------------
...
l = [1,2,3]

def f():
    print(x for x in l)
         ^
------------------------------------------------------------

foo.pyx:4:10: Unused entry 'genexpr'

Please reopen the issue.

[LordAro]

You can use lists explicitly instead of generator expressions. Obviously this doesn’t work in all cases, and can significantly increase memory usage, but it will work some of the time…

print(x for x in foo)

->

print([x for x in foo])