TheAlgorithms/Python · error · TypeError

The parameter bwt_string type must be str.

Error message

The parameter bwt_string type must be str.

What it means

Raised by reverse_bwt() in data_compression/burrows_wheeler.py when bwt_string is not a str. The inverse transform repeatedly prepends characters and sorts, which requires str input. This is the first validation in the function, checked before emptiness and index validation.

Source

Thrown at data_compression/burrows_wheeler.py:140

        ...
    TypeError: The parameter idx_original_string type must be int or passive
    of cast to int.
    >>> reverse_bwt("mnpbnnaaaaaa", -1)
    Traceback (most recent call last):
        ...
    ValueError: The parameter idx_original_string must not be lower than 0.
    >>> reverse_bwt("mnpbnnaaaaaa", 12) # doctest: +NORMALIZE_WHITESPACE
    Traceback (most recent call last):
        ...
    ValueError: The parameter idx_original_string must be lower than
    len(bwt_string).
    >>> reverse_bwt("mnpbnnaaaaaa", 11.0)
    'panamabanana'
    >>> reverse_bwt("mnpbnnaaaaaa", 11.4)
    'panamabanana'
    """
    if not isinstance(bwt_string, str):
        raise TypeError("The parameter bwt_string type must be str.")
    if not bwt_string:
        raise ValueError("The parameter bwt_string must not be empty.")
    try:
        idx_original_string = int(idx_original_string)
    except ValueError:
        raise TypeError(
            "The parameter idx_original_string type must be int or passive"
            " of cast to int."
        )
    if idx_original_string < 0:
        raise ValueError("The parameter idx_original_string must not be lower than 0.")
    if idx_original_string >= len(bwt_string):
        raise ValueError(
            "The parameter idx_original_string must be lower than len(bwt_string)."
        )

    ordered_rotations = [""] * len(bwt_string)
    for _ in range(len(bwt_string)):

View on GitHub (pinned to f5988cc097)

Solutions

1. Decode back to str before inverting: reverse_bwt(data.decode('utf-8'), idx) when data came back as bytes.
2. Pass the exact values returned by bwt_transform() without intermediate conversions.
3. Guard for None explicitly before calling.

Example fix

# before
reverse_bwt(payload['bwt'], payload['idx'])  # bwt is None/bytes

# after
bwt = payload['bwt'] or ''
if isinstance(bwt, bytes):
    bwt = bwt.decode('utf-8')
reverse_bwt(bwt, payload['idx'])

if isinstance(bwt_string, bytes):
    bwt_string = bwt_string.decode('utf-8')
if not isinstance(bwt_string, str):
    raise TypeError('bwt_string must be str')
plain = reverse_bwt(bwt_string, idx)

def is_str(value: object) -> bool:
    return isinstance(value, str)

When it happens

Trigger: Calling reverse_bwt(None, 0), reverse_bwt(123, 0), reverse_bwt(['m','n','p'], 0), or passing bytes produced by a transport layer instead of the str returned by bwt_transform().

Common situations: Round-tripping a BWT result through a medium that changes its type (encoded to bytes for network/file storage, then not decoded), or deserializing from JSON where the field is missing and defaults to None.

Related errors

• The parameter s type must be str.
• The parameter idx_original_string type must be int or passiv
• The parameter s must not be empty.
• The parameter bwt_string must not be empty.
• The parameter idx_original_string must not be lower than 0.

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/363b9290bd114af2. Report an issue: GitHub.