Finding the source code for built-in Python functions?

Question

Is there a way to see how built in functions work in python? I don't mean just how to use them, but also how were they built, what is the code behind sorted or enumerate etc...?

Answer 1

Since Python is open source you can read the source code.

To find out what file a particular module or function is implemented in you can usually print the __file__ attribute. Alternatively, you may use the inspect module, see the section Retrieving Source Code in the documentation of inspect.

For built-in classes and methods this is not so straightforward since inspect.getfile and inspect.getsource will return a type error stating that the object is built-in. However, many of the built-in types can be found in the Objects sub-directory of the Python source trunk. For example, see here for the implementation of the enumerate class or here for the implementation of the list type.

• Benjamin: Can you give an example with enumerate?
• user3064538: @Quetzalcoatl the source code for sorted() is in /Python/bltinmodule.c although it just calls list.sort() so the real source is in /Objects/listobject.c
• stackoverflowpro: would have been helpful if you had given an example of how to use __file__
• peter.slizik: As a note to self, and for the future googlers: the open() functions is defined in Modules/_io/_iomodule.c in Python 3 (and not among the other builtins).
• Jitin: I tried input.__file__ but it says 'builtin_function_or_method' object has no attribute '_ _ file _ _'. :-\

Answer 2

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I had to dig a little to find the source of the following Built-in Functions as the search would yield thousands of results. (Good luck searching for any of those to find where it's source is)

Anyway, all those functions are defined in bltinmodule.c Functions start with builtin_{functionname}

Built-in Source: https://github.com/python/cpython/blob/master/Python/bltinmodule.c

For Built-in Types: https://github.com/python/cpython/tree/master/Objects

• user1767754: A list is an object/type, not a builtin function. You can find the implementation details for that in listobject.c github.com/python/cpython/tree/master/Objects
• user3064538: dir is not implemented in C so it's not in that file.
• Stef: Looking for the implementation of builtin pow in bltinmodule.c, I only find an unhelpful static PyObject * builtin_pow_impl(PyObject *module, PyObject *base, PyObject *exp, PyObject *mod) { return PyNumber_Power(base, exp, mod); }. Is there an easy way to find where the actual algorithm implementation is hidden?

Answer 3

Here is a cookbook answer to supplement @Chris' answer, CPython has moved to GitHub and the Mercurial repository will no longer be updated:

1. Install Git if necessary.

2. git clone https://github.com/python/cpython.git

3. Code will checkout to a subdirectory called cpython -> cd cpython

4. Let's say we are looking for the definition of print()...
5. egrep --color=always -R 'print' | less -R
6. Aha! See Python/bltinmodule.c -> builtin_print()

Enjoy.

• Pod: bltinmodule. Arrrrrrrrrgh. Why did they have to spell it so badly? I tried a quick filesystem search for builtin and came up with nothing!

Answer 4

The iPython shell makes this easy: function? will give you the documentation. function?? shows also the code. BUT this only works for pure python functions.

Then you can always download the source code for the (c)Python.

If you're interested in pythonic implementations of core functionality have a look at PyPy source.

• user395760: PyPy uses RPython for most built-in stuff, which can be nearly as low-level as C to almost as high-level as Python. Usually it's in between. In either case it's statically typed, so it isn't really Python.
• Thomas: See an early project for viewing source code of a builtin function: github.com/punchagan/cinspect

Answer 5

2 methods,

1. You can check usage about snippet using help()
2. you can check hidden code for those modules using inspect

1) inspect:

use inpsect module to explore code you want... NOTE: you can able to explore code only for modules (aka) packages you have imported

for eg:

 >>> import randint 
 >>> from inspect import getsource
 >>> getsource(randint) # here i am going to explore code for package called `randint`

2) help():

you can simply use help() command to get help about builtin functions as well its code.

for eg: if you want to see the code for str() , simply type - help(str)

it will return like this,

>>> help(str)
Help on class str in module __builtin__:
class str(basestring)
 | str(object='') -> string
 |
 | Return a nice string representation of the object.
 | If the argument is a string, the return value is the same object.
 |
 | Method resolution order:
 | str
 | basestring
 | object
 |
 | Methods defined here:
 |
 | __add__(...)
 | x.__add__(y) <==> x+y
 |
 | __contains__(...)
 | x.__contains__(y) <==> y in x
 |
 | __eq__(...)
 | x.__eq__(y) <==> x==y
 |
 | __format__(...)
 | S.__format__(format_spec) -> string
 |
 | Return a formatted version of S as described by format_spec.
 |
 | __ge__(...)
 | x.__ge__(y) <==> x>=y
 |
 | __getattribute__(...)
-- More --

• 0xc0de: The OP specifically wants to look at the code, help gives only documentation.

Answer 6

Let's go straight to your question.

Finding the source code for built-in Python functions?

The source code is located at cpython/Python/bltinmodule.c

To find the source code in the GitHub repository go here. You can see that all in-built functions start with builtin_<name_of_function>, for instance, sorted() is implemented in builtin_sorted.

For your pleasure I'll post the implementation of sorted():

builtin_sorted(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
{
 PyObject *newlist, *v, *seq, *callable;
 /* Keyword arguments are passed through list.sort() which will check
 them. */
 if (!_PyArg_UnpackStack(args, nargs, "sorted", 1, 1, &seq))
 return NULL;
 newlist = PySequence_List(seq);
 if (newlist == NULL)
 return NULL;
 callable = _PyObject_GetAttrId(newlist, &PyId_sort);
 if (callable == NULL) {
 Py_DECREF(newlist);
 return NULL;
 }
 assert(nargs >= 1);
 v = _PyObject_FastCallKeywords(callable, args + 1, nargs - 1, kwnames);
 Py_DECREF(callable);
 if (v == NULL) {
 Py_DECREF(newlist);
 return NULL;
 }
 Py_DECREF(v);
 return newlist;
}

As you may have noticed, that's not Python code, but C code.

Answer 7

Quite an unknown resource is the Python Developer Guide.

In a (somewhat) recent GH issue, a new chapter was added for to address the question you're asking: CPython Source Code Layout. If something should change, that resource will also get updated.

Answer 8

As mentioned by @Jim, the file organization is described here. Reproduced for ease of discovery:

For Python modules, the typical layout is:

Lib/<module>.py
Modules/_<module>.c (if there’s also a C accelerator module)
Lib/test/test_<module>.py
Doc/library/<module>.rst

For extension-only modules, the typical layout is:

Modules/<module>module.c
Lib/test/test_<module>.py
Doc/library/<module>.rst

For builtin types, the typical layout is:

Objects/<builtin>object.c
Lib/test/test_<builtin>.py
Doc/library/stdtypes.rst

For builtin functions, the typical layout is:

Python/bltinmodule.c
Lib/test/test_builtin.py
Doc/library/functions.rst

Some exceptions:

builtin type int is at Objects/longobject.c
builtin type str is at Objects/unicodeobject.c
builtin module sys is at Python/sysmodule.c
builtin module marshal is at Python/marshal.c
Windows-only module winreg is at PC/winreg.c

Answer 9

Here is the directory layout quoted from the "CPython Internals Book"(Python 3.9 at the time of writing the book):

cpython
├── Doc Source for the documentation
├── Grammar The computer-readable language definition
├── Include The C header files
├── Lib Standard library modules written in Python
├── Mac macOS support files
├── Misc Miscellaneous files
├── Modules Standard library modules written in C
├── Objects Core types and the object model
├── Parser The Python parser source code
├── PC Windows build support files for older versions of Windows
├── PCbuild Windows build support files
├── Programs Source code for the ‘python’ executable and other binaries
├── Python The CPython interpreter source code
├── Tools Standalone tools useful for building or extending CPython
├── m4 Custom scripts to automate configuration of the makefile

_* the builtin functions are inside the bltinmodule.c file inside the /Python directory.