Call me maybe - an explanations of Python callables

If you have some experience with Python, you must have already seen a TypeError: 'T' object is not callable.

In this article we will try to demystify this kind of errors.

What is a callable?

A callable is anything that can be called, from a function, to a class constructor.

For example, len is a callable because we can write len(object).

Creating a callable

The easiest way is to write a function or a lambda:

def foo(*args):
    pass

bar = lambda *args: 0

But here we won't cover those things as they are basic Python knowledge.

class Foo:
    def __call__(self, *args):
        print("hello world!")

bar = Foo()  # instanciating a Foo object, creating a callable
bar()  # calling it!

We created a basic callable by giving it a __call__ method, and we can check if an object is callable (useful when debugging) by using:

# homemade method to explain EAFP
def is_callable(obj, *args, **kwargs, ***okwargs):
    try:
        obj(*args, **kwargs, ***okwargs)
    except TypeError as e:
        return 'object is not callable' not in str(e)
    return True

# built-in way to check if an object is callable (thanks rhymes for pointing it out): using callable(obj)
>>> callable(foo), callable(bar), callable(Foo), callable(Foo()), callable(3)
(True, True, True, True, False)

Because we can't use hasattr(obj, '__call__'), since Python is skipping __getattribute__ and __getattr__ when calling something, thus we can only do what's called EAFP: Easier to ask for forgiveness than permission.

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Hopefully this article has helped you, if you have anything to add feel free to leave a comment!

Comments

[rhymes]

Hi Alexandre, nice intro to callable.

Python definitely leans on EAFP over LBYL as you know :)

A tip: there's an easier way to check if something is a callable that requires zero lines of custom code. The builtin callable() function.

>>> def foo(*args):
...     pass
...
>>> bar = lambda *args: 0
>>> class Foo:
...     def __call__(self, *args):
...         print("hello world!")
...
>>> callable(foo), callable(bar), callable(Foo), callable(Foo()), callable(3)
(True, True, True, True, False)

hope this helps! :-)

[Alexandre Plt]

Hi!
Thanks for the tip, I had forgotten that this existed :/

I'll add that :)