TheAlgorithms/Python · error · TypeError

argument should be a bytes-like object or ASCII string, not

Error message

argument should be a bytes-like object or ASCII string, not '{encoded_data.__class__.__name__}'

What it means

Raised by base64_decode() in ciphers/base64_cipher.py when encoded_data is neither bytes nor str. The decoder only accepts those two types; anything else (int, list, None) triggers this TypeError before any Base64 work happens.

Source

Thrown at ciphers/base64_cipher.py:94

    >>> c = "QQ=="
    >>> base64_decode(a) == b64decode(a)
    True
    >>> base64_decode(b) == b64decode(b)
    True
    >>> base64_decode(c) == b64decode(c)
    True
    >>> base64_decode("abc")
    Traceback (most recent call last):
      ...
    AssertionError: Incorrect padding
    """
    # Make sure encoded_data is either a string or a bytes-like object
    if not isinstance(encoded_data, bytes) and not isinstance(encoded_data, str):
        msg = (
            "argument should be a bytes-like object or ASCII string, "
            f"not '{encoded_data.__class__.__name__}'"
        )
        raise TypeError(msg)

    # In case encoded_data is a bytes-like object, make sure it contains only
    # ASCII characters so we convert it to a string object
    if isinstance(encoded_data, bytes):
        try:
            encoded_data = encoded_data.decode("utf-8")
        except UnicodeDecodeError:
            raise ValueError("base64 encoded data should only contain ASCII characters")

    padding = encoded_data.count("=")

    # Check if the encoded string contains non base64 characters
    if padding:
        assert all(char in B64_CHARSET for char in encoded_data[:-padding]), (
            "Invalid base64 character(s) found."
        )
    else:
        assert all(char in B64_CHARSET for char in encoded_data), (

View on GitHub (pinned to f5988cc097)

Solutions

Ensure the value is str or bytes before calling: base64_decode(encoded if isinstance(encoded, (str, bytes)) else str(encoded).encode())
Fix the upstream producer so it really yields str/bytes
Guard optionals: base64_decode(encoded or b'')

Example fix

# before
base64_decode(payload.get("data"))  # payload.get returns None when key missing

# after
base64_decode(payload.get("data", ""))

Defensive patterns

Strategy: validation

Validate before calling

if not isinstance(encoded_data, (str, bytes)):
    encoded_data = str(encoded_data).encode("ascii")

Type guard

def is_base64_input(value) -> bool:
    return isinstance(value, (str, bytes))

Try / catch

try:
    decoded = base64_decode(encoded_data)
except TypeError:
    decoded = base64_decode(str(encoded_data))

Prevention

Check isinstance(value, (str, bytes)) before decoding
Guard optional values: value or '' before the call
Log the offending type when the check fails to find upstream producers

When it happens

Trigger: Calling base64_decode(None) when a variable was never assigned; passing a list of code points or an int; chained calls where a previous step returned a non-str/bytes value.

Common situations: Optional values that default to None flowing into the decoder; deserialized JSON data that turned the encoded string into another type; API responses parsed into unexpected shapes.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/0bd319ae99717e81. Report an issue: GitHub.