How to reference static method from class variable

Question

This question already has answers here:

Accessing class variables from a list comprehension in the class definition (5 answers)

Closed 3 months ago.

Given the class

from __future__ import annotations
from typing import ClassVar, Dict, Final
import abc

class Cipher(abc.ABC):
    @abc.abstractmethod
    def encrypt(self, plaintext: str) -> str:
        pass

    @abc.abstractmethod
    def decrypt(self, ciphertext: str) -> str:
        pass

class VigenereCipher(Cipher):
    @staticmethod
    def rotate(n: int) -> str:
        return string.ascii_uppercase[n:] + string.ascii_uppercase[:n]

    _TABLE: Final[ClassVar[Dict[str, str]]] = dict({(chr(i + ord("A")), rotate(i)) for i in range(26)})

Compilation fails (using 3.8.0)

../cipher.py:19: in <module>
    class VigenereCipher(Cipher):
../cipher.py:24: in VigenereCipher
    _TABLE: Final[ClassVar[Dict[str, str]]] = dict({(chr(i + ord("A")), rotate(i)) for i in range(26)})
../cipher.py:24: in <setcomp>
    _TABLE: Final[ClassVar[Dict[str, str]]] = dict({(chr(i + ord("A")), rotate(i)) for i in range(26)})
E   NameError: name 'rotate' is not defined

However, according to this post, rotate should be resolvable. Note that qualifying with class name VigenereCipher doesn't work either since it can't find VigenereCipher (makes sense, since we are in the process of defining it).

I can make rotate a module-level method, and that works, but I don't really want to since it only is needed in VigenereCipher.

Also tried this answer with no success.

Actual code is here. Unit test is here.

Answer 1

The error is raised from here:

_TABLE: Final[ClassVar[Dict[str, str]]] = dict({(chr(i + ord("A")), rotate(i)) for i in range(26)})

You are trying to refer the variable rotate that is located in class namespace. However python comprehensions have their own scope and there is no simple way to connect it with class namespace. There is no closure or global variable rotate at the moment of comprehension evaluation - thus NameError is invoked. The code above is equal to your code:

def _create_TABLE():
    d = {}
    for i in range(26):
        d[chr(i + ord("A"))] = rotate(i) # -> NameError('rotate')
    return d
_TABLE: Final[ClassVar[Dict[str, str]]] = dict(_create_TABLE())
del _create_TABLE

How to reference static method from class variable

A class variable in python is some object, so it can refer to any objects in your program. Here some idioms you can follow:

Approach 1:

class VigenereCipher(Cipher):
    @staticmethod
    def rotate(n: int) -> str:
        return string.ascii_uppercase[n:] + string.ascii_uppercase[:n]

    _TABLE: Final[ClassVar[Dict[str, str]]]

VigenereCipher._TABLE = {chr(i + ord("A")): VigenereCipher.rotate(i) for i in range(26)}

Approach 2:

class VigenereCipher(Cipher):
    @staticmethod
    def rotate(n: int) -> str:
        return string.ascii_uppercase[n:] + string.ascii_uppercase[:n]

    _TABLE: Final[ClassVar[Dict[str, str]]] = (
        lambda r=rotate.__func__: {chr(i + ord("A")): r(i) for i in range(26)})()

Approach 3:

class VigenereCipher(Cipher):
    @staticmethod
    def rotate(n: int) -> str:
        return string.ascii_uppercase[n:] + string.ascii_uppercase[:n]

    _TABLE: Final[ClassVar[Dict[str, str]]] = dict(zip(
        (chr(i + ord("A")) for i in range(26)),
        map(rotate.__func__, range(26)),
    ))

Approach 4:

class VigenereCipher(Cipher):
    @staticmethod
    def rotate(n: int) -> str:
        return string.ascii_uppercase[n:] + string.ascii_uppercase[:n]

    _TABLE: Final[ClassVar[Dict[str, str]]] = {
        chr(i + ord("A")): r(i) for r in (rotate.__func__,) for i in range(26)}

There are also approaches based on:

• locals function;
• metaclasses;
• __init__subclass__ method;
• descriptor's __set_name__;
• stack frames;
• using global keyword.

You can find a more detailed answers in the related topic