How to create an inline function that modifies a variable in Python?

Question

Suppose I have a class that looks like this:

class Foo:
    def __init__(self, method):
        self.method = method

    def do_action(self):
        self.method()

and I want to instantiate it as follows:

some_var = False

def bar():
    # Modifies existing variable's value
    global some_var
    some_var = True
foo = Foo(bar)

how do I do that without having to define the bar() method? I've tried the following and it doesn't work.

foo = Foo(lambda: (some_var := True))

When I do this the IDE tells me there's an identifier expected. Thanks in advance!

EDIT: Thank you to those who answered, however I didn't really find exactly what I needed. Not sure if it's the best practice, but I ended up using python's exec and it works as intended:

foo = Foo(lambda: exec("some_var = True"))

You might need to include more code. What's some_var actually supposed to be? Your example bar is a no-op since some_var is a local variable. — Samwise
– Samwise, Commented Aug 1, 2021 at 19:58
Assuming some_var is a local/global variable, and if you really, really want to do something like this, you can add nonlocal some_var or global some_var (which every is appropriate) to bar. But this question really sounds like an XY problem. Could you elaborate on why you want to add a method to an object that modifies a local/global variable? — Brian61354270
– Brian61354270, Commented Aug 1, 2021 at 20:11
@Samwise It's supposed to change the value of an existing global variable. I thought the comment in the bar would suffice but I'll add in a declaration. — kguzek
– kguzek, Commented Aug 1, 2021 at 20:11
oh that's extremely gross, you'd have to use globals() to be able to modify that inside a lambda. Don't do it IMO. Global variables are bad enough without doing unnecessarily weird shenanigans like this. — Samwise
– Samwise, Commented Aug 1, 2021 at 20:12

Samwise · Accepted Answer · 2021-08-01 20:11:22Z

1

If the idea is to have bar be able to modify an instance attribute, have bar take the self parameter so do_action can tell it which instance it's operating on. You can't do a variable assignment inside a lambda, so use __setattr__:

class Foo:
    def __init__(self, method):
        self.method = method
        self.some_var = False

    def do_action(self):
        self.method(self)


foo = Foo(lambda self: self.__setattr__("some_var", True))
foo.do_action()
print(foo.some_var)  # True

answered Aug 1, 2021 at 20:11

Samwise

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