Source code for str.replace()

NickC · July 27, 2026, 12:10pm

Can anyone tell me where I might find the underlying C source code for Python’s builtin str.replace() method?

Not that I know much about C but wanted to have a go at creating a .swap() method for searching an incoming string and swapping over two sub-strings. A bit like replace but swapping two strings instead.

mfile_bay · July 27, 2026, 1:07pm

I think it’s here:

github.com/python/cpython

Objects/unicodeobject.c

58d9c78d6


      
                  Maximum number of occurrences to replace.
                  -1 (the default value) means replace all occurrences.
          
          Return a copy with all occurrences of substring old replaced by new.
          
          If count is given, only the first count occurrences are replaced.
          If count is not specified or -1, then all occurrences are replaced.
          [clinic start generated code]*/
          
          static PyObject *
          unicode_replace_impl(PyObject *self, PyObject *old, PyObject *new,
                               Py_ssize_t count)
          /*[clinic end generated code: output=b63f1a8b5eebf448 input=d15a6886b05e2edc]*/
          {
              return replace(self, old, new, count);
          }
          
          /*[clinic input]
          str.removeprefix as unicode_removeprefix
          
              prefix: unicode

By the way I usually do this by help(str.replace) then search a part of the docstring in the codebase. However this result happens to be out of GitHub code search for unknown reasons

chris-eibl · July 27, 2026, 4:00pm

See About GitHub Code Search - GitHub Docs . The file is too big: 446 KB vs limit of 350 KiB.

tjreedy · July 27, 2026, 10:51pm

In my local clone, main branch, with Python built, I ran IDLE (python -m idlelib), the help command, selected most of the first line of of the docstring, hit Find-in-files, reduced the search patch to /*.c, checked recurse into subdirs, and go, and in a few seconds had unicode.c, bytes.c, and bytearray.c.

mfile_bay · July 28, 2026, 12:54am

Yeah, local search is faster and better especially if your Internet connection is slow

Rosuav · July 28, 2026, 1:07am

Yep, and you can plan for this by taking a git clone of the repository (a shallow one if you don’t want all the history), and then you can use either “search across files” as Terry did, or the “git grep” command if you prefer the terminal. It’s all there, just a matter of finding it!