The comment string above the implementation of _PyBytes_FromStringAndSize in Objects/bytesobject.c starts with:

  /*
  For both PyBytes_FromString() and PyBytes_FromStringAndSize(), the
  parameter `size' denotes number of characters to allocate, not 
  counting   any null terminating character.

This is misleading since PyBytes_FromString() has no 'size' parameter. 

The problem also exists for PyString in Python 2.x

Could you write a patch to fix this comment?

Sure, I just wanted confirmation from another dev that it's indeed an error and I'm not missing anything. 

I suppose the fix is just replace "for both PyBytes_FromString() and PyBytes_FromStringAndSize()" with just "for PyBytes_FromStringAndSize()".

I'll commit the change to all relevant branches.

PyObject *
PyBytes_FromString(const char *str)
{
    register size_t size;
    ...
    size = strlen(str);
    ...
}

PyBytes_FromString() does compute the input string size using strlen().

Yes it does, but the comment says something about "parameter 'size'" which is clearly absent from the function signature of PyBytes_FromString.

I propose the attached patch (for the latest default branch). It simply removes the first paragraph of that comment, since it's misleading and redundant. The *last* paragraph explains the same thing just in a clear and correct way (except that it also has a small typo the patch fixes :-)

New changeset a729dfdbd24b by Eli Bendersky in branch 'default':
Issue #11634: Remove misleading paragraph from a comment
http://hg.python.org/cpython/rev/a729dfdbd24b

New changeset 44749e501982 by Eli Bendersky in branch '2.7':
Issue #11634: Remove misleading paragraph from a comment
http://hg.python.org/cpython/rev/44749e501982

Patch reviewed by Nick Coghlan and committed