stubtest.rst

.. program:: stubtest

Automatic stub testing (stubtest)

Stub files are files containing type annotations. See PEP 484 for more motivation and details.

A common problem with stub files is that they tend to diverge from the actual implementation. Mypy includes the stubtest tool that can automatically check for discrepancies between the stubs and the implementation at runtime.

What stubtest does and does not do

Stubtest will import your code and introspect your code objects at runtime, for example, by using the capabilities of the :py:mod:`inspect` module. Stubtest will then analyse the stub files, and compare the two, pointing out things that differ between stubs and the implementation at runtime.

It's important to be aware of the limitations of this comparison. Stubtest will not make any attempt to statically analyse your actual code and relies only on dynamic runtime introspection (in particular, this approach means stubtest works well with extension modules). However, this means that stubtest has limited visibility; for instance, it cannot tell if a return type of a function is accurately typed in the stubs.

For clarity, here are some additional things stubtest can't do:

• Type check your code -- use mypy instead
• Generate stubs -- use stubgen or pyright --createstub instead
• Generate stubs based on running your application or test suite -- use monkeytype instead
• Apply stubs to code to produce inline types -- use retype or libcst instead

In summary, stubtest works very well for ensuring basic consistency between stubs and implementation or to check for stub completeness. It's used to test Python's official collection of library stubs, typeshed.

Warning

stubtest will import and execute Python code from the packages it checks.

Example

Here's a quick example of what stubtest can do:

$ python3 -m pip install mypy

$ cat library.py
x = "hello, stubtest"

def foo(x=None):
    print(x)

$ cat library.pyi
x: int

def foo(x: int) -> None: ...

$ python3 -m mypy.stubtest library
error: library.foo is inconsistent, runtime argument "x" has a default value but stub argument does not
Stub: at line 3
def (x: builtins.int)
Runtime: in file ~/library.py:3
def (x=None)

error: library.x variable differs from runtime type Literal['hello, stubtest']
Stub: at line 1
builtins.int
Runtime:
'hello, stubtest'

Usage

Running stubtest can be as simple as stubtest module_to_check. Run :option:`stubtest --help` for a quick summary of options.

Stubtest must be able to import the code to be checked, so make sure that mypy is installed in the same environment as the library to be tested. In some cases, setting PYTHONPATH can help stubtest find the code to import.

Similarly, stubtest must be able to find the stubs to be checked. Stubtest respects the MYPYPATH environment variable -- consider using this if you receive a complaint along the lines of "failed to find stubs".

Note that stubtest requires mypy to be able to analyse stubs. If mypy is unable to analyse stubs, you may get an error on the lines of "not checking stubs due to mypy build errors". In this case, you will need to mitigate those errors before stubtest will run. Despite potential overlap in errors here, stubtest is not intended as a substitute for running mypy directly.

If you wish to ignore some of stubtest's complaints, stubtest supports a pretty handy allowlist system.

The rest of this section documents the command line interface of stubtest.

.. option:: --concise

    Makes stubtest's output more concise, one line per error

.. option:: --ignore-missing-stub

    Ignore errors for stub missing things that are present at runtime

.. option:: --ignore-positional-only

    Ignore errors for whether an argument should or shouldn't be positional-only

.. option:: --allowlist FILE

    Use file as an allowlist. Can be passed multiple times to combine multiple
    allowlists. Allowlists can be created with --generate-allowlist. Allowlists
    support regular expressions.

    The presence of an entry in the allowlist means stubtest will not generate
    any errors for the corresponding definition.

.. option:: --generate-allowlist

    Print an allowlist (to stdout) to be used with --allowlist

    When introducing stubtest to an existing project, this is an easy way to
    silence all existing errors.

.. option:: --ignore-unused-allowlist

    Ignore unused allowlist entries

    Without this option enabled, the default is for stubtest to complain if an
    allowlist entry is not necessary for stubtest to pass successfully.

    Note if an allowlist entry is a regex that matches the empty string,
    stubtest will never consider it unused. For example, to get
    `--ignore-unused-allowlist` behaviour for a single allowlist entry like
    ``foo.bar`` you could add an allowlist entry ``(foo\.bar)?``.
    This can be useful when an error only occurs on a specific platform.

.. option:: --mypy-config-file FILE

    Use specified mypy config file to determine mypy plugins and mypy path

.. option:: --custom-typeshed-dir DIR

    Use the custom typeshed in DIR

.. option:: --check-typeshed

    Check all stdlib modules in typeshed

.. option:: --help

    Show a help message :-)