python函数可以知道它何时被列表推导调用？

Question

I want to make a python function that behaves differently when it's being called from a list comprehension:

def f():
    # this function returns False when called normally,
    # and True when called from a list comprehension
    pass

>>> f()
False
>>> [f() for _ in range(3)]
[True, True, True]

I tried looking at the inspect module, the dis module, and lib2to3's parser for something to make this trick work, but haven't found anything. There also might be a simple reason why this cannot exist, that I haven't thought of.

Answer 1

You can determine this by inspecting the stack frame in the following sort of way:

def f():
    try:
        raise ValueError
    except Exception as e:
        if e.__traceback__.tb_frame.f_back.f_code.co_name == '<listcomp>':
            return True

Then:

>>> print(f())
None
>>> print([f() for x in range(10)])
[True, True, True, True, True, True, True, True, True, True]

Its not to be recommended though. Really, its not.

NOTE

As it stands this only detects list comprehensions as requested. It will not detect the use of a generator. For example:

>>> print(list(f() for x in range(10)))
[None, None, None, None, None, None, None, None, None, None]

Answer 2

Addon to my comment:

def f(lst=False):
    return True if lst else False

f() #False
[f(True) for _ in range(3)] # [True True True]

This would work, but is this the real problem that you're trying to solve? It seems a really unintuitive use-case which can be solved better by other means.