TheAlgorithms/Python · error · Exception

'{i}' not in list of symbols

Error message

'{i}' not in list of symbols

What it means

Raised by _plugboard() when a symbol in the plugboard string is not in the Enigma alphabet (uppercase A-Z). Lowercase letters, digits, punctuation, or spaces all fail because abc contains only the 26 uppercase letters.

Source

Thrown at ciphers/enigma_machine2.py:150

    # a) is type string
    # b) has even length (so pairs can be made)
    if not isinstance(pbstring, str):
        msg = f"Plugboard setting isn't type string ({type(pbstring)})"
        raise TypeError(msg)
    elif len(pbstring) % 2 != 0:
        msg = f"Odd number of symbols ({len(pbstring)})"
        raise Exception(msg)
    elif pbstring == "":
        return {}

    pbstring.replace(" ", "")

    # Checks if all characters are unique
    tmppbl = set()
    for i in pbstring:
        if i not in abc:
            msg = f"'{i}' not in list of symbols"
            raise Exception(msg)
        elif i in tmppbl:
            msg = f"Duplicate symbol ({i})"
            raise Exception(msg)
        else:
            tmppbl.add(i)
    del tmppbl

    # Created the dictionary
    pb = {}
    for j in range(0, len(pbstring) - 1, 2):
        pb[pbstring[j]] = pbstring[j + 1]
        pb[pbstring[j + 1]] = pbstring[j]

    return pb


def enigma(
    text: str,

View on GitHub (pinned to f5988cc097)

Solutions

1. Uppercase and strip spaces before calling: _plugboard(pb.upper().replace(' ', ''))
2. Use only A-Z symbols in pairs
3. Pre-validate every char with all(c in string.ascii_uppercase for c in pb)

Example fix

# before
_plugboard('pictures')  # lowercase fails

# after
_plugboard('PICTURES')

import string
pb = pb.upper().replace(' ', '')
assert all(c in string.ascii_uppercase for c in pb)

def is_uppercase_alpha(pb: str) -> bool:
    return all(c.isalpha() and c.isupper() for c in pb.replace(' ', ''))

try:
    pbdict = _plugboard(pb)
except Exception as exc:
    if "not in list of symbols" in str(exc):
        pbdict = _plugboard(pb.upper().replace(' ', ''))
    else:
        raise

When it happens

Trigger: _plugboard('pictures') in lowercase; 'PICTUR3S' with a digit; a space inside pairs like 'PI CT' — spaces are not actually stripped and are not in abc.

Common situations: Accepting user input without uppercasing; copy-pasted settings with whitespace; settings from another alphabet convention.

Related errors

• Plugboard setting isn't type string ({type(pbstring)})
• Odd number of symbols ({len(pbstring)})
• Duplicate symbol ({i})
• Please use 3 unique rotors (not {unique_rotsel})
• First rotor position is not within range of 1..26 ({rotorpos

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/1d26d056b7745746. Report an issue: GitHub.