dynamic python class with partial function

Question

print(I'm puzzled by what appears to be a bug in python3.

I want to create a class dynamically with some class methods. I seem to be able to do it like this:

import types

def class_body(ns):
    ns.update({"a": lambda self, x: x - 1})
    return ns

newc = types.new_class("foo", exec_body=class_body)

print(newc().a(3))

# prints 2 as expected

But I also want to create the class methods dynamically. I seem to be able to do something like:

import types
def funcs_gen(k=1):
    def fn(self, a):
        return a + k + self.i
    return fn

def class_body(ns):
    ns.update({"a": funcs_gen(k=2), "i": 5})
    return ns

newc = types.new_class("foo", exec_body=class_body)

print(newc().a(1))

# prints 8 as expected

But something weird seems to happen if I try to do the same with partial functions:

import types
from functools import partial

def fn(self, a, k=1):
    return a + k + self.i

def class_body(ns):
    ns.update({"a": partial(fn, k=2), "i": 5})
    return ns

newc = types.new_class("foo", exec_body=class_body)

print(newc().a(1))

# Unexpectedly produces: `TypeError: fn() missing 1 required positional argument: 'a'`

Sure, but in my real implementation I what if I want to access some class data. — Al Hoo
– Al Hoo, Commented Oct 26, 2020 at 19:03
Because partial objects do not implement the descriptor protocol to bind the instance as the first argument to itself. As an aside, note that types.new_class is a just a convienience function, you can also just use type directly, which takes type(name, bases, namespace) — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Oct 26, 2020 at 19:20

Tibebes. M · Accepted Answer · 2020-10-26 19:08:31Z

1

Use partialmethod for methods.

import types
from functools import partialmethod

def fn(self, a, k=1):
    return a + k + self.i

def class_body(ns):
    ns.update({"a": partialmethod(fn, k=2), "i": 5})
    return ns

newc = types.new_class("foo", exec_body=class_body)

print(newc().a(1))

output:

answered Oct 26, 2020 at 19:08

Tibebes. M

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