pypa/pip · error · ClassNotFound

no lexer for filename

Error message

no lexer for filename {_fn!r} found

What it means

Raised by get_lexer_for_filename() as a ClassNotFound when find_lexer_class_for_filename(_fn, code) returns None (line 225-227), meaning no builtin lexer and no plugin lexer has a filename glob matching basename(_fn). It is the strict counterpart that throws instead of returning None.

Solutions

1. Call find_lexer_class_for_filename(fn) first — it returns None instead of raising, so you can branch on a miss.
2. Map the extension to a known lexer alias manually and use get_lexer_by_name() instead (e.g., get_lexer_by_name('text') or 'ini').
3. Install or register a Pygments plugin / entry point that declares a filename glob covering your extension.

Example fix

// before
lexer = get_lexer_for_filename('config.xyz')

// after
from pygments.lexers import find_lexer_class_for_filename, get_lexer_by_name
lexer_cls = find_lexer_class_for_filename('config.xyz')
lexer = (lexer_cls or get_lexer_by_name('ini'))()

from pygments.lexers import find_lexer_class_for_filename
def has_filename_lexer(fn):
    return find_lexer_class_for_filename(fn) is not None

from pygments.lexers import find_lexer_class_for_filename, get_lexer_by_name
from pygments.util import ClassNotFound
cls = find_lexer_class_for_filename(fn)
lexer = cls() if cls else get_lexer_by_name('text')

When it happens

Trigger: Calling get_lexer_for_filename('file.unknownext') where '.unknownext' matches no registered filename pattern in LEXERS or any plugin; passing a filename whose basename matches no glob in any lexer's filenames tuple.

Common situations: An unsupported or uncommon file extension (e.g., a proprietary config format); a custom file type with no registered lexer; the plugins providing the lexer are not installed in the environment; case-sensitivity issues where the pattern expects lowercase.

Related errors

• no lexer for filename
• no lexer for mimetype
• no lexer matching the text found
• no valid class found in
• cannot read

AI-assisted analysis of pypa/pip@f399c37189 (2026-08-08). Data as JSON: /api/errors/f9ef33be42480b75. Report an issue: GitHub.

Appendix: source

Thrown at src/pip/_vendor/pygments/lexers/__init__.py:227

        return matches[-1][0]


def get_lexer_for_filename(_fn, code=None, **options):
    """Get a lexer for a filename.

    Return a `Lexer` subclass instance that has a filename pattern
    matching `fn`. The lexer is given the `options` at its
    instantiation.

    Raise :exc:`pygments.util.ClassNotFound` if no lexer for that filename
    is found.

    If multiple lexers match the filename pattern, use their ``analyse_text()``
    methods to figure out which one is more appropriate.
    """
    res = find_lexer_class_for_filename(_fn, code)
    if not res:
        raise ClassNotFound(f'no lexer for filename {_fn!r} found')
    return res(**options)


def get_lexer_for_mimetype(_mime, **options):
    """
    Return a `Lexer` subclass instance that has `mime` in its mimetype
    list. The lexer is given the `options` at its instantiation.

    Will raise :exc:`pygments.util.ClassNotFound` if not lexer for that mimetype
    is found.
    """
    for modname, name, _, _, mimetypes in LEXERS.values():
        if _mime in mimetypes:
            if name not in _lexer_cache:
                _load_lexers(modname)
            return _lexer_cache[name](**options)
    for cls in find_plugin_lexers():
        if _mime in cls.mimetypes:

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