Why aren't builtin functions classified as callable in python and how can I make them appear callable in my program?

Question

print(callable(chr(97)))
print(callable(ord("a")))

I was surprised to see that these lines of code both printed false. It doesn't make intuitive sense that builtin functions, such as chr or ord are technically not callable, since I can literally call them anywhere in a program. How can I make these builtin functions appear callable in my program without redefining them? I'm receiving the errors below in my college's autograder:

exercises/ex03/cipher.py:9: error: Function "builtins.chr" is not valid as a type exercises/ex03/cipher.py:9: note: Perhaps you need "Callable[...]" or a callback protocol? exercises/ex03/cipher.py:11: error: Function "builtins.chr" is not valid as a type exercises/ex03/cipher.py:11: note: Perhaps you need "Callable[...]" or a callback protocol? exercises/ex03/cipher.py:12: error: chr? has no attribute "islower" exercises/ex03/cipher.py:32: error: Function "builtins.chr" is not valid as a type exercises/ex03/cipher.py:32: note: Perhaps you need "Callable[...]" or a callback protocol?

"""Caesar Cipher encoding and decoding program."""

UPPER_A: int = ord("A")
UPPER_Z: int = ord("Z")
LOWER_A: int = ord("a")
LOWER_Z: int = ord("z")


def encode_char(input: chr) -> chr:
    """Encode character function."""
    output: chr = ""
    if input.islower() is True:
        output = chr((ord(input) - LOWER_A + 1) % 26 + LOWER_A)
    else:
        output = chr((ord(input) - UPPER_A + 1) % 26 + UPPER_A)
    return output


def encode_str(input: str) -> str:
    """Encode string function."""
    output: str = ""
    c: int = 0
    while (c < len(input)):
        if input[c].islower() is True:
            output += chr((ord(input[c]) - LOWER_A + 1) % 26 + LOWER_A)
        else:
            output += chr((ord(input[c]) - UPPER_A + 1) % 26 + UPPER_A)
        c += 1
    return output


def decode_char(input: chr) -> chr:
    """Decode character function."""
    output: str = ""
    if input.islower() is True:
        output = chr(LOWER_Z - (LOWER_Z - ord(input) + 1) % 26)
    else:
        output = chr(UPPER_Z - (UPPER_Z - ord(input) + 1) % 26)
    return output


def decode_str(input: str) -> str:
    """Decode string function."""
    output: str = ""
    c: int = 0
    while (c < len(input)):
        if input[c].islower() is True:
            output += chr(LOWER_Z - (LOWER_Z - ord(input[c]) + 1) % 26)
        else:
            output += chr(UPPER_Z - (UPPER_Z - ord(input[c]) + 1) % 26)
        c += 1
    return output

This is my actual Caesar Cipher program.

Answer 1

Your line:

print(callable(chr(97)))

is actually calling the built-in chr() with the parameter 97 . This call returns a string instance, which is being passed to callable() . So an instance of a string is NOT callable.

Answer 2

chr(97) evaluates to 'a' . So you are effectively writing callable('a') , which is false. If you had tried callable(chr) , it would be true since chr is a callable function.

You are getting an error because you are trying to use chr as a type when there is no such type in Python. Replace all instances of chr in your code where it is used as a type to str and it'll be fine.