Running mypy and managing imports

The Getting started page should have already introduced you to the basics of how to run mypy – pass in the files and directories you want to type check via the command line:

$ mypy foo.py bar.py some_directory

This page discusses in more detail how exactly to specify what files you want mypy to type check, how mypy discovers imported modules, and recommendations on how to handle any issues you may encounter along the way.

If you are interested in learning about how to configure the actual way mypy type checks your code, see our The mypy command line guide.

Specifying code to be checked

Mypy lets you specify what files it should type check in several different ways.

  1. First, you can pass in paths to Python files and directories you want to type check. For example:

    $ mypy file_1.py foo/file_2.py file_3.pyi some/directory
    

    The above command tells mypy it should type check all of the provided files together. In addition, mypy will recursively type check the entire contents of any provided directories.

    For more details about how exactly this is done, see Mapping file paths to modules.

  2. Second, you can use the -m flag (long form: --module) to specify a module name to be type checked. The name of a module is identical to the name you would use to import that module within a Python program. For example, running:

    $ mypy -m html.parser
    

    …will type check the module html.parser (this happens to be a library stub).

    Mypy will use an algorithm very similar to the one Python uses to find where modules and imports are located on the file system. For more details, see How imports are found.

  3. Third, you can use the -p (long form: --package) flag to specify a package to be (recursively) type checked. This flag is almost identical to the -m flag except that if you give it a package name, mypy will recursively type check all submodules and subpackages of that package. For example, running:

    $ mypy -p html
    

    …will type check the entire html package (of library stubs). In contrast, if we had used the -m flag, mypy would have type checked just html’s __init__.py file and anything imported from there.

    Note that we can specify multiple packages and modules on the command line. For example:

    $ mypy --package p.a --package p.b --module c
    
  4. Fourth, you can also instruct mypy to directly type check small strings as programs by using the -c (long form: --command) flag. For example:

    $ mypy -c 'x = [1, 2]; print(x())'
    

    …will type check the above string as a mini-program (and in this case, will report that List[int] is not callable).

Reading a list of files from a file

Finally, any command-line argument starting with @ reads additional command-line arguments from the file following the @ character. This is primarily useful if you have a file containing a list of files that you want to be type-checked: instead of using shell syntax like:

$ mypy $(cat file_of_files.txt)

you can use this instead:

$ mypy @file_of_files.txt

This file can technically also contain any command line flag, not just file paths. However, if you want to configure many different flags, the recommended approach is to use a configuration file instead.

How mypy handles imports

When mypy encounters an import statement, it will first attempt to locate that module or type stubs for that module in the file system. Mypy will then type check the imported module. There are three different outcomes of this process:

  1. Mypy is unable to follow the import: the module either does not exist, or is a third party library that does not use type hints.
  2. Mypy is able to follow and type check the import, but you did not want mypy to type check that module at all.
  3. Mypy is able to successfully both follow and type check the module, and you want mypy to type check that module.

The third outcome is what mypy will do in the ideal case. The following sections will discuss what to do in the other two cases.

Missing imports

When you import a module, mypy may report that it is unable to follow the import.

This could happen if the code is importing a non-existent module or if the code is importing a library that does not use type hints. Specifically, the library is neither declared to be a PEP 561 compliant package nor has registered any stubs on typeshed, the repository of stubs for the standard library and popular 3rd party libraries.

This can cause a lot of errors that look like the following:

main.py:1: error: No library stub file for standard library module 'antigravity'
main.py:2: error: No library stub file for module 'flask'
main.py:3: error: Cannot find module named 'this_module_does_not_exist'

If the module genuinely does not exist, you should of course fix the import statement. If the module is a module within your codebase that mypy is somehow unable to discover, we recommend reading the How imports are found section below to help you debug the issue.

If the module is a library that does not use type hints, the easiest fix is to silence the error messages by adding a # type: ignore comment on each respective import statement.

If you have many of these errors from a specific library, it may be more convenient to silence all of those errors at once using the mypy config file. For example, suppose your codebase makes heavy use of an (untyped) library named foobar. You can silence all import errors associated with that library and that library alone by adding the following section to your config file:

[mypy-foobar]
ignore_missing_imports = True

Note: this option is equivalent to adding a # type: ignore to every import of foobar in your codebase. For more information, see the documentation about configuring import discovery in config files.

If you would like to silence all missing import errors in your codebase, you can do so by using the --ignore-missing-imports flag. We recommend using this flag only as a last resort: it’s equivalent to adding a # type: ignore to all unresolved imports in your codebase.

A more involved solution would be to reverse-engineer how the library works, create type hints for the library, and point mypy at those type hints either by passing in in via the command line or by adding the location of your custom stubs to either the MYPYPATH environment variable or the mypy_path config file option.

If you want to share your work, you can try contributing your stubs back to the library – see our documentation on creating PEP 561 compliant packages.

Following imports

Mypy is designed to doggedly follow all imports, even if the imported module is not a file you explicitly wanted mypy to check.

For example, suppose we have two modules mycode.foo and mycode.bar: the former has type hints and the latter does not. We run mypy -m mycode.foo and mypy discovers that mycode.foo imports mycode.bar.

How do we want mypy to type check mycode.bar? We can configure the desired behavior by using the --follow-imports flag. This flag accepts one of four string values:

  • normal (the default) follows all imports normally and type checks all top level code (as well as the bodies of all functions and methods with at least one type annotation in the signature).

  • silent behaves in the same way as normal but will additionally suppress any error messages.

  • skip will not follow imports and instead will silently replace the module (and anything imported from it) with an object of type Any.

  • error behaves in the same way as skip but is not quite as silent – it will flag the import as an error, like this:

    main.py:1: note: Import of 'mycode.bar' ignored
    main.py:1: note: (Using --follow-imports=error, module not passed on command line)
    

If you are starting a new codebase and plan on using type hints from the start, we recommend you use either --follow-imports=normal (the default) or --follow-imports=error. Either option will help make sure you are not skipping checking any part of your codebase by accident.

If you are planning on adding type hints to a large, existing code base, we recommend you start by trying to make your entire codebase (including files that do not use type hints) pass under --follow-imports=normal. This is usually not too difficult to do: mypy is designed to report as few error messages as possible when it is looking at unannotated code.

If doing this is intractable, we recommend passing mypy just the files you want to type check and use --follow-imports=silent. Even if mypy is unable to perfectly type check a file, it can still glean some useful information by parsing it (for example, understanding what methods a given object has). See Using mypy with an existing codebase for more recommendations.

We do not recommend using skip unless you know what you are doing: while this option can be quite powerful, it can also cause many hard-to-debug errors.

Mapping file paths to modules

One of the main ways you can tell mypy what files to type check is by providing mypy the paths to those files. For example:

$ mypy file_1.py foo/file_2.py file_3.pyi some/directory

This section describes how exactly mypy maps the provided paths to modules to type check.

  • Files ending in .py (and stub files ending in .pyi) are checked as Python modules.
  • Files not ending in .py or .pyi are assumed to be Python scripts and checked as such.
  • Directories representing Python packages (i.e. containing a __init__.py[i] file) are checked as Python packages; all submodules and subpackages will be checked (subpackages must themselves have a __init__.py[i] file).
  • Directories that don’t represent Python packages (i.e. not directly containing an __init__.py[i] file) are checked as follows:
    • All *.py[i] files contained directly therein are checked as toplevel Python modules;
    • All packages contained directly therein (i.e. immediate subdirectories with an __init__.py[i] file) are checked as toplevel Python packages.

One more thing about checking modules and packages: if the directory containing a module or package specified on the command line has an __init__.py[i] file, mypy assigns these an absolute module name by crawling up the path until no __init__.py[i] file is found.

For example, suppose we run the command mypy foo/bar/baz.py where foo/bar/__init__.py exists but foo/__init__.py does not. Then the module name assumed is bar.baz and the directory foo is added to mypy’s module search path.

On the other hand, if foo/bar/__init__.py did not exist, foo/bar would be added to the module search path instead, and the module name assumed is just baz.

If a script (a file not ending in .py[i]) is processed, the module name assumed is __main__ (matching the behavior of the Python interpreter), unless --scripts-are-modules is passed.

How imports are found

When mypy encounters an import statement or receives module names from the command line via the --module or --package flags, mypy tries to find the module on the file system similar to the way Python finds it. However, there are some differences.

First, mypy has its own search path. This is computed from the following items:

  • The MYPYPATH environment variable (a colon-separated list of directories).
  • The mypy_path config file option.
  • The directories containing the sources given on the command line (see below).
  • The installed packages marked as safe for type checking (see PEP 561 support)
  • The relevant directories of the typeshed repo.

Note

You cannot point to a PEP 561 package via the MYPYPATH, it must be installed (see PEP 561 support)

For sources given on the command line, the path is adjusted by crawling up from the given file or package to the nearest directory that does not contain an __init__.py or __init__.pyi file.

Second, mypy searches for stub files in addition to regular Python files and packages. The rules for searching for a module foo are as follows:

  • The search looks in each of the directories in the search path (see above) until a match is found.
  • If a package named foo is found (i.e. a directory foo containing an __init__.py or __init__.pyi file) that’s a match.
  • If a stub file named foo.pyi is found, that’s a match.
  • If a Python module named foo.py is found, that’s a match.

These matches are tried in order, so that if multiple matches are found in the same directory on the search path (e.g. a package and a Python file, or a stub file and a Python file) the first one in the above list wins.

In particular, if a Python file and a stub file are both present in the same directory on the search path, only the stub file is used. (However, if the files are in different directories, the one found in the earlier directory is used.)