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
-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.
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 justhtml
’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
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).
You can also use the files
option in your mypy.ini
file to specify which
files to check, in which case you can simply run mypy
with no arguments.
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.
Mapping file paths to modules¶
One of the main ways you can tell mypy what to type check is by providing mypy a list of paths. 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.
Mypy will check all paths provided that correspond to files.
Mypy will recursively discover and check all files ending in
.py
or.pyi
in directory paths provided, after accounting for--exclude
.For each file to be checked, mypy will attempt to associate the file (e.g.
project/foo/bar/baz.py
) with a fully qualified module name (e.g.foo.bar.baz
). The directory the package is in (project
) is then added to mypy’s module search paths.
How mypy determines fully qualified module names depends on if the options
--no-namespace-packages
and
--explicit-package-bases
are set.
If
--no-namespace-packages
is set, mypy will rely solely upon the presence of__init__.py[i]
files to determine the fully qualified module name. That is, mypy will crawl up the directory tree for as long as it continues to find__init__.py
(or__init__.pyi
) files.For example, if your directory tree consists of
pkg/subpkg/mod.py
, mypy would requirepkg/__init__.py
andpkg/subpkg/__init__.py
to exist in order correctly associatemod.py
withpkg.subpkg.mod
The default case. If
--namespace-packages
is on, but--explicit-package-bases
is off, mypy will allow for the possibility that directories without__init__.py[i]
are packages. Specifically, mypy will look at all parent directories of the file and use the location of the highest__init__.py[i]
in the directory tree to determine the top-level package.For example, say your directory tree consists solely of
pkg/__init__.py
andpkg/a/b/c/d/mod.py
. When determiningmod.py
’s fully qualified module name, mypy will look atpkg/__init__.py
and conclude that the associated module name ispkg.a.b.c.d.mod
.You’ll notice that the above case still relies on
__init__.py
. If you can’t put an__init__.py
in your top-level package, but still wish to pass paths (as opposed to packages or modules using the-p
or-m
flags),--explicit-package-bases
provides a solution.With
--explicit-package-bases
, mypy will locate the nearest parent directory that is a member of theMYPYPATH
environment variable, themypy_path
config or is the current working directory. Mypy will then use the relative path to determine the fully qualified module name.For example, say your directory tree consists solely of
src/namespace_pkg/mod.py
. If you run the following command, mypy will correctly associatemod.py
withnamespace_pkg.mod
:$ MYPYPATH=src mypy --namespace-packages --explicit-package-bases .
If you pass a file not ending in .py[i]
, the module name assumed is
__main__
(matching the behavior of the Python interpreter), unless
--scripts-are-modules
is passed.
Passing -v
will show you the files and associated module
names that mypy will check.
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.
Missing imports¶
When you import a module, mypy may report that it is unable to follow the import. This can cause errors that look like the following:
main.py:1: error: Skipping analyzing 'django': module is installed, but missing library stubs or py.typed marker
main.py:2: error: Library stubs not installed for "requests"
main.py:3: error: Cannot find implementation or library stub for module named "this_module_does_not_exist"
If you get any of these errors on an import, mypy will assume the type of that
module is Any
, the dynamic type. This means attempting to access any
attribute of the module will automatically succeed:
# Error: Cannot find implementation or library stub for module named 'does_not_exist'
import does_not_exist
# But this type checks, and x will have type 'Any'
x = does_not_exist.foobar()
This can result in mypy failing to warn you about errors in your code. Since
operations on Any
result in Any
, these dynamic types can propagate
through your code, making type checking less effective. See
Dynamically typed code for more information.
The next sections describe what each of these errors means and recommended next steps; scroll to the section that matches your error.
Missing library stubs or py.typed marker¶
If you are getting a Skipping analyzing X: module is installed, but missing library stubs or py.typed marker
,
error, this means mypy was able to find the module you were importing, but no
corresponding type hints.
Mypy will not try inferring the types of any 3rd party libraries you have installed
unless they either have declared themselves to be
PEP 561 compliant stub package (e.g. with a py.typed
file) or have registered
themselves on typeshed, the repository
of types for the standard library and some 3rd party libraries.
If you are getting this error, try to obtain type hints for the library you’re using:
Upgrading the version of the library you’re using, in case a newer version has started to include type hints.
Searching to see if there is a PEP 561 compliant stub package corresponding to your third party library. Stub packages let you install type hints independently from the library itself.
For example, if you want type hints for the
django
library, you can install the django-stubs package.Writing your own stub files containing type hints for the library. You can point mypy at your type hints either by passing them in via the command line, by using the
files
ormypy_path
config file options, or by adding the location to theMYPYPATH
environment variable.These stub files do not need to be complete! A good strategy is to use stubgen, a program that comes bundled with mypy, to generate a first rough draft of the stubs. You can then iterate on just the parts of the library you need.
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.
If you are unable to find any existing type hints nor have time to write your own, you can instead suppress the errors.
All this will do is make mypy stop reporting an error on the line containing the
import: the imported module will continue to be of type Any
, and mypy may
not catch errors in its use.
To suppress a single missing import error, add a
# type: ignore
at the end of the line containing the import.To suppress all missing import errors from a single library, add a per-module section to your mypy config file setting
ignore_missing_imports
to True for that library. For example, suppose your codebase makes heavy use of an (untyped) library namedfoobar
. 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 offoobar
in your codebase. For more information, see the documentation about configuring import discovery in config files. The.*
afterfoobar
will ignore imports offoobar
modules and subpackages in addition to thefoobar
top-level package namespace.To suppress all missing import errors for all untyped libraries in your codebase, use
--disable-error-code=import-untyped
. See Check that import target can be found [import-untyped] for more details on this error code.You can also set
disable_error_code
, like so:[mypy] disable_error_code = import-untyped
You can also set the
--ignore-missing-imports
command line flag or set theignore_missing_imports
config file option to True in the global section of your mypy config file. We recommend avoiding--ignore-missing-imports
if possible: it’s equivalent to adding a# type: ignore
to all unresolved imports in your codebase.
Library stubs not installed¶
If mypy can’t find stubs for a third-party library, and it knows that stubs exist for the library, you will get a message like this:
main.py:1: error: Library stubs not installed for "yaml"
main.py:1: note: Hint: "python3 -m pip install types-PyYAML"
main.py:1: note: (or run "mypy --install-types" to install all missing stub packages)
You can resolve the issue by running the suggested pip commands.
If you’re running mypy in CI, you can ensure the presence of any stub packages
you need the same as you would any other test dependency, e.g. by adding them to
the appropriate requirements.txt
file.
Alternatively, add the --install-types
to your mypy command to install all known missing stubs:
mypy --install-types
This is slower than explicitly installing stubs, since it effectively runs mypy twice – the first time to find the missing stubs, and the second time to type check your code properly after mypy has installed the stubs. It also can make controlling stub versions harder, resulting in less reproducible type checking.
By default, --install-types
shows a confirmation prompt.
Use --non-interactive
to install all suggested
stub packages without asking for confirmation and type check your code:
If you’ve already installed the relevant third-party libraries in an environment
other than the one mypy is running in, you can use --python-executable
flag to point to the Python executable for that
environment, and mypy will find packages installed for that Python executable.
If you’ve installed the relevant stub packages and are still getting this error, see the section below.
Cannot find implementation or library stub¶
If you are getting a Cannot find implementation or library stub for module
error, this means mypy was not able to find the module you are trying to
import, whether it comes bundled with type hints or not. If you are getting
this error, try:
Making sure your import does not contain a typo.
If the module is a third party library, making sure that mypy is able to find the interpreter containing the installed library.
For example, if you are running your code in a virtualenv, make sure to install and use mypy within the virtualenv. Alternatively, if you want to use a globally installed mypy, set the
--python-executable
command line flag to point the Python interpreter containing your installed third party packages.You can confirm that you are running mypy from the environment you expect by running it like
python -m mypy ...
. You can confirm that you are installing into the environment you expect by running pip likepython -m pip ...
.Reading the How imports are found section below to make sure you understand how exactly mypy searches for and finds modules and modify how you’re invoking mypy accordingly.
Directly specifying the directory containing the module you want to type check from the command line, by using the
mypy_path
orfiles
config file options, or by using theMYPYPATH
environment variable.Note: if the module you are trying to import is actually a submodule of some package, you should specify the directory containing the entire package. For example, suppose you are trying to add the module
foo.bar.baz
which is located at~/foo-project/src/foo/bar/baz.py
. In this case, you must runmypy ~/foo-project/src
(or set theMYPYPATH
to~/foo-project/src
).
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 list of directories, colon-separated on UNIX systems, semicolon-separated on Windows).The
mypy_path
config file option.The directories containing the sources given on the command line (see Mapping file paths to modules).
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 stub-only package (PEP 561) via the MYPYPATH
, it must be
installed (see PEP 561 support)
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 directoryfoo
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.)
Setting mypy_path
/MYPYPATH
is mostly useful in the case
where you want to try running mypy against multiple distinct
sets of files that happen to share some common dependencies.
For example, if you have multiple projects that happen to be
using the same set of work-in-progress stubs, it could be
convenient to just have your MYPYPATH
point to a single
directory containing the stubs.
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
? Mypy’s behaviour here is
configurable – although we strongly recommend using the default –
by using the --follow-imports
flag. This flag
accepts one of four string values:
normal
(the default, recommended) 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 asnormal
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 typeAny
.error
behaves in the same way asskip
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.
Only 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.
Adjusting import following behaviour is often most useful when restricted to
specific modules. This can be accomplished by setting a per-module
follow_imports
config option.