TheAlgorithms/Python · error · TypeError

key must be a string

Error message

key must be a string

What it means

Raised by autokey encrypt() when the key argument is not a str. Alongside the plaintext check, the function requires both operands to be strings because it concatenates key += plaintext and indexes both with ord() arithmetic.

Source

Thrown at ciphers/autokey.py:41

        ...
    TypeError: key must be a string
    >>> encrypt("", "TheAlgorithms")
    Traceback (most recent call last):
        ...
    ValueError: plaintext is empty
    >>> encrypt("coffee is good as python", "")
    Traceback (most recent call last):
        ...
    ValueError: key is empty
    >>> encrypt(527.26, "TheAlgorithms")
    Traceback (most recent call last):
        ...
    TypeError: plaintext must be a string
    """
    if not isinstance(plaintext, str):
        raise TypeError("plaintext must be a string")
    if not isinstance(key, str):
        raise TypeError("key must be a string")

    if not plaintext:
        raise ValueError("plaintext is empty")
    if not key:
        raise ValueError("key is empty")

    key += plaintext
    plaintext = plaintext.lower()
    key = key.lower()
    plaintext_iterator = 0
    key_iterator = 0
    ciphertext = ""
    while plaintext_iterator < len(plaintext):
        if (
            ord(plaintext[plaintext_iterator]) < 97
            or ord(plaintext[plaintext_iterator]) > 122
        ):
            ciphertext += plaintext[plaintext_iterator]

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass a string key: encrypt(text, str(2)) or better, an actual keyword like 'TheAlgorithms'.
2. If key arrives as bytes, decode it: key.decode('utf-8').
3. Remember the order: encrypt(plaintext, key) — plaintext first.

Example fix

# before
encrypt('hello', 2)  # TypeError: key must be a string

# after
encrypt('hello', 'coffee')

if not isinstance(key, str) or not key:
    raise ValueError('a non-empty string key is required')

def is_nonempty_str(v: object) -> bool:
    return isinstance(v, str) and len(v) > 0

When it happens

Trigger: Calling encrypt('coffee is good as python', 2), or passing None/bytes/list as the key.

Common situations: Using a numeric key out of habit from Caesar/Vigenère examples, or swapping the argument order so a number lands in the key slot.

Related errors

• plaintext must be a string
• ciphertext must be a string
• operation can not be conducted on an object of type {type(nu
• Input value must be an 'int' type
• plain must contain only lowercase letters (a-z)

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/49e6ad2df8572a4d. Report an issue: GitHub.