When is a function in a standard library module called a built-in function?

Question

I am confused about the term "built-in" function. I thought it meant only those functions built into the interpreter and documented in - 2. Built-in Functions
But it appears that functions defined in standard library modules are also built-in functions (in some cases). For example -

>>> import os >>> os.chdir <built-in function chdir> >>> import warnings >>> warnings.warn <built-in function warn> >>> import json >>> json.dumps <function dumps at 0x7f3643a240d0> # not built-in >>> dir <built-in function dir> >>>  

So when is a function in a standard library module called a built-in function and when is it not?

Answer 1

There are two meanings of “built-in” here, although they both mean “part of the interpreter”. The library reference uses it to indicate that a function is available without an import (it is “not part of a module”, although see builtins). The interpreter itself uses it to indicate that a function is implemented natively rather than in Python (in CPython, it is at least nominally implemented in C).

There is yet another meaning on the C side: an extension module (i.e., one written in C) is built-in if the Python binary incorporates it rather than loading it if and when needed.