Why is repr(int) faster than str(int)?

Because using str(obj) must first go through type.__call__ then str.__new__ (create a new string) then PyObject_Str (make a string out of the object) which invokes int.__str__ and, finally, uses the function you linked.

repr(obj), which corresponds to builtin_repr, directly calls PyObject_Repr (get the object repr) which then calls int.__repr__ which uses the same function as int.__str__.

Additionally, the path they take through call_function (the function that handles the CALL_FUNCTION opcode that’s generated for calls) is slightly different.

From the master branch on GitHub (CPython 3.7):

• str goes through _PyObject_FastCallKeywords (which is the one that calls type.__call__). Apart from performing more checks, this also needs to create a tuple to hold the positional arguments (see _PyStack_AsTuple).
• repr goes through _PyCFunction_FastCallKeywords which calls _PyMethodDef_RawFastCallKeywords. repr is also lucky because, since it only accepts a single argument (the switch leads it to the METH_0 case in _PyMethodDef_RawFastCallKeywords) there’s no need to create a tuple, just indexing of the args.

As your update states, this isn’t about int.__repr__ vs int.__str__, they are the same function after all; it’s all about how repr and str reach them. str just needs to work a bit harder.