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…?

Asked By: user1073865

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Answers:

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.

Answered By: Chris

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.

Answered By: tback

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.

Answered By: kevinarpe

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  --

enter image description here

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

Answered By: user1767754

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.

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
Answered By: Mateen Ulhaq

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.

Answered By: Guzman Ojero

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.

Answered By: S.B
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