TheAlgorithms/Python · error · ValueError

plaintext is empty

Error message

plaintext is empty

What it means

Raised by autokey encrypt() when the plaintext is a str but empty (''). An empty plaintext would leave the autokey stream (key + plaintext) with nothing to encrypt and produce a meaningless empty ciphertext, so the function rejects it explicitly.

Source

Thrown at ciphers/autokey.py:44

    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]
            plaintext_iterator += 1
        elif ord(key[key_iterator]) < 97 or ord(key[key_iterator]) > 122:
            key_iterator += 1

View on GitHub (pinned to f5988cc097)

Solutions

1. Skip empty inputs upstream: if not plaintext: continue / return '' early in your loop.
2. Treat empty text as a no-op in your API: return '' without calling encrypt.
3. Validate combined conditions once: if not isinstance(p, str) or not p: raise ValueError(...).

Example fix

# before
encrypt('', 'TheAlgorithms')  # ValueError: plaintext is empty

# after
if plaintext:
    ciphertext = encrypt(plaintext, 'TheAlgorithms')
else:
    ciphertext = ''

if not plaintext:
    return ''  # nothing to encrypt
# only now call encrypt(plaintext, key)

try:
    encrypt(p, k)
except ValueError as e:
    if 'empty' in str(e):
        ciphertext = ''
    else:
        raise

When it happens

Trigger: Calling encrypt('', 'TheAlgorithms'). The isinstance checks pass because '' is a str; only the emptiness triggers this.

Common situations: Empty form fields, blank lines read from a file, or filtered data that reduced to zero characters reaching the cipher.

Related errors

• plain must contain only lowercase letters (a-z)
• key is empty
• ciphertext is empty
• encode() accepts only letters of the alphabet and spaces
• decode() accepts only 'A', 'B' and spaces

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/dce8ef68b9c63eea. Report an issue: GitHub.