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).
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
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
