Is `list()` considered a function?

Question

list is obviously a built-in type in Python. I saw a comment under this question which calls list() a built-in function. And when we check the documentation, it is, indeed, included in Built-in functions list but the documentation again states:

Rather than being a function, list is actually a mutable sequence type

Which brings me to my question: Is list() considered a function? Can we refer to it as a built-in function?

If we were talking about C++, I'd say we are just calling the constructor, but I am not sure if the term constructor applies to Python (never encountered its use in this context).

I'd say "constructor" is close enough. After all, if you have any other custom class, say `Foo`, then `Foo` is also a `type`, but also a callable function (the constructor). — tobias_k, Dec 06 '18 at 11:04
https://stackoverflow.com/questions/40173300/why-are-many-python-built-in-standard-library-functions-actually-classes — Chris_Rands, Dec 06 '18 at 11:06

DeepSpace · Accepted Answer · 2018-12-06T11:14:33.963

list is a type, which means it is defined somewhere as a class, just like int and float.

>> type(list)
<class 'type'>

If you check its definition in builtins.py (the actual code is implemented in C):

class list(object):
    """
    Built-in mutable sequence.

    If no argument is given, the constructor creates a new empty list.
    The argument must be an iterable if specified.
    """

    ...

    def __init__(self, seq=()): # known special case of list.__init__
        """
        Built-in mutable sequence.

        If no argument is given, the constructor creates a new empty list.
        The argument must be an iterable if specified.
        # (copied from class doc)
        """
        pass

So, list() is not a function. It is just calling list.__init__() (with some arguments which are irrelevant for this discussion) just like any call to CustomClass() is doing.

Thanks for @jpg for adding in the comments: classes and functions in Python have a common property: they are both considered as callables, which means they are allowed to be invoked with (). There is a built-in function callable that checks if the given argument is callable:

>> callable(1)
False
>> callable(int)
True
>> callable(list)
True
>> callable(callable)
True

callable is also defined in builtins.py:

def callable(i_e_, some_kind_of_function): # real signature unknown; restored from __doc__
    """
    Return whether the object is callable (i.e., some kind of function).

    Note that classes are callable, as are instances of classes with a
    __call__() method.
    """
    pass

Conceptually in Python, it's worth noting classes with `__init__` and functions are both *callable* objects. They just share a common feature. The built-in [`callable`](https://docs.python.org/3/library/functions.html#callable) lets you test this. — jpp, Dec 06 '18 at 11:10
Now my understanding is: No, it is not a function despite being in the built in functions list, but it's rather a `type` object which just happens to be *callable*, and yes `__init__` is in fact referred as *constructor* in Python. Is that correct? — Aykhan Hagverdili, Dec 06 '18 at 11:21
@Ayxan Not *per se*. I personally don't like to refer to `__init__` as a constructor (because then how would you refer to `__new__`?). As the [docs](https://docs.python.org/3/reference/datamodel.html#object.__init__) suggests, `__init__` is called only after the object is created, and *initializes* its attributes, it does not create it. — DeepSpace, Dec 06 '18 at 11:24
The signature for `callable()` is `callable(obj)` - it expects an object of any type. — bruno desthuilliers, Dec 06 '18 at 12:31

score 2 · Answer 2 · answered Dec 06 '18 at 11:08

When you call list(), you're invoking the constructor of the list class (list.__init__).

If you have any doubt about the use of the term "constructor" in Python, this is the exact word that the implementers of list chose to refer to __init__:

https://github.com/python/cpython/blob/master/Objects/listobject.c#L2695

Is `list()` considered a function?

2 Answers2

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