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.
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.
Second, you can use the
-mflag (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.
Third, you can use the
--package) flag to specify a package to be (recursively) type checked. This flag is almost identical to the
-mflag 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
htmlpackage (of library stubs). In contrast, if we had used the
-mflag, mypy would have type checked just
__init__.pyfile 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
Fourth, you can also instruct mypy to directly type check small strings as programs by using the
--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
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:
- 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.
- Mypy is able to follow and type check the import, but you did not want mypy to type check that module at all.
- 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.
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-existant 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, you can silence them all at once by using
--ignore-missing-imports flag. We recommend using this flag only
as a last resort: it’s equivalent to adding a
# type: ignore comment
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 the
MYPYPATH environment variable.
If you want to share your work, you can either open a pull request on typeshed or modify the library itself to be PEP 561 compliant.
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
the former has type hints and the latter does not. We run
mypy -m mycode.foo and mypy discovers that
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).
silentbehaves in the same way as
normalbut will additionally suppress any error messages.
skipwill not follow imports and instead will silently replace the module (and anything imported from it) with an object of type
(Note: this option used to be known as
errorbehaves in the same way as
skipbut 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
(the default) or
--follow-imports=error. Either option will help
make sure you are not skipping checking any part of your codebase by
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
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
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
.pyiare 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
- Directories that don’t represent Python packages (i.e. not directly
__init__.py[i]file) are checked as follows:
*.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
added to mypy’s module search path.
On the other hand, if
foo/bar/__init__.py did not exist,
would be added to the module search path instead, and the module name
assumed is just
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
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:
MYPYPATHenvironment variable (a colon-separated list of directories).
- 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.
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
Second, mypy searches for stub files in addition to regular Python files
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
foois found (i.e. a directory
__init__.pyifile) that’s a match.
- If a stub file named
foo.pyiis found, that’s a match.
- If a Python module named
foo.pyis 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.)