pytest-dev/pytest · error · TypeError

ext must not be None

Error message

ext must not be None

What it means

Raised by Pytester._makefile (the backend of makefile/makepyfile) when the ext argument is None. makefile requires a concrete file extension string to construct the output filename via Path.with_suffix(ext), and None is not a valid suffix. This is a hard precondition check before any file is written.

Source

Thrown at src/_pytest/pytester.py:769

    def chdir(self) -> None:
        """Cd into the temporary directory.

        This is done automatically upon instantiation.
        """
        self._monkeypatch.chdir(self.path)

    def _makefile(
        self,
        ext: str,
        lines: Sequence[Any | bytes],
        files: Mapping[str, _FileContent],
        encoding: str = "utf-8",
    ) -> Path:
        items = list(files.items())

        if ext is None:
            raise TypeError("ext must not be None")

        if ext and not ext.startswith("."):
            raise ValueError(
                f"pytester.makefile expects a file extension, try .{ext} instead of {ext}"
            )

        def to_text(s: Any | bytes) -> str:
            return s.decode(encoding) if isinstance(s, bytes) else str(s)

        if lines:
            source = "\n".join(to_text(x) for x in lines)
            basename = self._name
            items.insert(0, (basename, source))

        ret = None
        for basename, value in items:
            p = self.path.joinpath(basename).with_suffix(ext)
            p.parent.mkdir(parents=True, exist_ok=True)

View on GitHub (pinned to 98b357f69e)

Solutions

1. Pass a concrete extension including the dot, e.g. '.py' or '.txt'.
2. Default ext to '.py' in your own helper when the source value is None.
3. Use makepyfile(...) instead of makefile when you always want Python files, since it supplies the extension for you.

Example fix

// before
pytester.makefile(ext, foo=source)  # ext may be None
// after
pytester.makefile(ext or ".py", foo=source)

def safe_makefile(pytester, ext, **kw):
    if ext is None:
        ext = ".py"
    return pytester.makefile(ext, **kw)

def is_valid_ext(ext: object) -> bool:
    return isinstance(ext, str)

When it happens

Trigger: Calling pytester.makefile(None, ...) explicitly, or a wrapper/helper that forwards a None ext, or calling makepyfile internals that default ext to None for a package-less layout.

Common situations: Programmatically building the ext argument from a config value that resolved to None; copy-pasting a makepyfile call pattern into a custom makefile invocation; a refactor that changed the default of a helper to None.

Related errors

• pytester.makefile expects a file extension, try .{ext} inste
• name is not allowed to contain path separators
• relative tolerance for a scalar value must be an int, float
• expected value must support abs(...) when relative tolerance
• absolute tolerance for a scalar value must be an int, float

AI-assisted analysis of pytest-dev/pytest@98b357f69e (2026-08-04). Data as JSON: /data/errors/ee011038c8a7e7a7.json. Report an issue: GitHub.