[Python-ideas] PEP proposal: unifying function/method classes

Jeroen Demeyer J.Demeyer at UGent.be
Thu Mar 22 16:34:44 EDT 2018

Dear python-ideas,

I would like to draft a PEP to change the implementation of functions 
and methods in CPython. The idea for this PEP come from a discussion on 

The core idea of the PEP is to reorganize classes for functions and 
methods with the goal of removing the difference between 
functions/methods implemented in C and functions/methods implemented in 

Currently, CPython has two different function classes: the first is 
Python functions, which is what you get when defining a function with 
def or lambda. The second is built-in functions such as len, isinstance 
or numpy.dot. These are implemented in C.

These two classes are completely independent with different 
functionality. For example, built-in functions do not have __get__ and 
therefore cannot be used as methods. And only Python functions support 
introspection like inspect.getargspec or inspect.getsourcefile.

This is a problem for projects like Cython that want to implement 
functions in C but still want to offer functionality close to Python 
functions. In Cython, this was solved by inventing a new function class 
called cyfunction. Unfortunately, this new function class creates 
problems, for example the inspect module does not recognize such 
functions as being functions (this is the bug mentioned above) and the 
performance is worse (CPython has specific optimizations for calling 
built-in functions).

When you look at methods, the inconsistency increases: On the one hand, 
the implementation of built-in functions and bound methods is very 
similar but built-in unbound methods are different. On the other hand, 
Python functions and unbound methods are identical but bound methods are 

Do you think that it makes sense to propose a PEP for this?


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