The mypy command line¶
This section documents mypy’s command line interface. You can view
a quick summary of the available flags by running
Command line flags are liable to change between releases.
Specifying what to type check¶
By default, you can specify what code you want mypy to type check by passing in the paths to what you want to have type checked:
$ mypy foo.py bar.py some_directory
Note that directories are checked recursively.
Mypy also lets you specify what code to type check in several other ways. A short summary of the relevant flags is included below: for full details, see Running mypy and managing imports.
Asks mypy to type check the provided module. This flag may be repeated multiple times.
Mypy will not recursively type check any submodules of the provided module.
Asks mypy to type check the provided package. This flag may be repeated multiple times.
Mypy will recursively type check any submodules of the provided package. This flag is identical to
-moduleapart from this behavior.
- Asks mypy to type check the provided string as a program.
This flag makes mypy read configuration settings from the given file.
By default settings are read from
setup.cfgin the current directory, or
.mypy.iniin the user’s home directory. Settings override mypy’s built-in defaults and command line flags can override settings.
--config-file=(with no filename) will ignore all config files.
See The mypy configuration file for the syntax of configuration files.
- This flag makes mypy warn about unused
[mypy-<pattern>]config file sections. (This requires turning off incremental mode using
The following flags customize how exactly mypy discovers and follows imports.
This flag enables import discovery to use namespace packages (see PEP 420). In particular, this allows discovery of imported packages that don’t have an
Namespace packages are found (using the PEP 420 rules, which prefers “classic” packages over namespace packages) along the module search path – this is primarily set from the source files passed on the command line, the
MYPYPATHenvironment variable, and the mypy_path config option.
Note that this only affects import discovery – for modules and packages explicitly passed on the command line, mypy still searches for
__init__.py[i]files in order to determine the fully-qualified module/package name.
This flag makes mypy ignore all missing imports. It is equivalent to adding
# type: ignorecomments to all unresolved imports within your codebase.
Note that this flag does not suppress errors about missing names in successfully resolved modules. For example, if one has the following files:
Then mypy will generate the following errors with
import package.unknown # No error, ignored x = package.unknown.func() # OK. 'func' is assumed to be of type 'Any' from package import unknown # No error, ignored from package.mod import NonExisting # Error: Module has no attribute 'NonExisting'
For more details, see Missing imports.
This flag adjusts how mypy follows imported modules that were not explicitly passed in via the command line.
The default option is
normal: mypy will follow and type check all modules. For more information on what the other options do, see Following imports.
This flag will have mypy collect type information from PEP 561 compliant packages installed for the Python executable
EXECUTABLE. If not provided, mypy will use PEP 561 compliant packages installed for the Python executable running mypy.
See Using installed packages for more on making PEP 561 compliant packages.
This flag will disable searching for PEP 561 compliant packages. This will also disable searching for a usable Python executable.
Use this flag if mypy cannot find a Python executable for the version of Python being checked, and you don’t need to use PEP 561 typed packages. Otherwise, use
- By default, mypy will suppress any error messages generated within PEP 561 compliant packages. Adding this flag will disable this behavior.
By default, mypy will assume that you intend to run your code using the same operating system and Python version you are using to run mypy itself. The following flags let you modify this behavior.
For more information on how to use these flags, see Python version and system platform checks.
This flag will make mypy type check your code as if it were run under Python version X.Y. Without this option, mypy will default to using whatever version of Python is running mypy. Note that the
--py2flags are aliases for
This flag will attempt to find a Python executable of the corresponding version to search for PEP 561 compliant packages. If you’d like to disable this, use the
--no-site-packagesflag (see Import discovery for more details).
- Equivalent to running
This flag will make mypy type check your code as if it were run under the given operating system. Without this option, mypy will default to using whatever operating system you are currently using.
PLATFORMparameter may be any string supported by sys.platform.
- This flag will treat all variables named
NAMEas compile-time constants that are always true. This flag may be repeated.
- This flag will treat all variables named
NAMEas compile-time constants that are always false. This flag may be repeated.
Disallow dynamic typing¶
Any type is used represent a value that has a dynamic type.
--disallow-any family of flags will disallow various uses of the
Any type in
a module – this lets us strategically disallow the use of dynamic typing in a controlled way.
The following options are available:
- This flag disallows usage of types that come from unfollowed imports
(such types become aliases for
Any). Unfollowed imports occur either when the imported module does not exist or when
This flag disallows all expressions in the module that have type
Any. If an expression of type
Anyappears anywhere in the module mypy will output an error unless the expression is immediately used as an argument to
castor assigned to a variable with an explicit type annotation.
In addition, declaring a variable of type
Anyor casting to type
Anyis not allowed. Note that calling functions that take parameters of type
Anyis still allowed.
- This flag disallows functions that have
Anyin their signature after decorator transformation.
- This flag disallows explicit
Anyin type positions such as type annotations and generic type parameters.
- This flag disallows usage of generic types that do not specify explicit
type parameters. Moreover, built-in collections (such as
dict) become disallowed as you should use their aliases from the typing module (such as
This flag reports an error whenever a class subclasses a value of type
Any. This may occur when the base class is imported from a module that doesn’t exist (when using –ignore-missing-imports) or is ignored due to –follow-imports=skip or a
# type: ignorecomment on the
Since the module is silenced, the imported class is given a type of
Any. By default mypy will assume that the subclass correctly inherited the base class even though that may not actually be the case. This flag makes mypy raise an error instead.
Untyped definitions and calls¶
The following flags configure how mypy handles untyped function definitions or calls.
- This flag reports an error whenever a function with type annotations calls a function defined without annotations.
- This flag reports an error whenever it encounters a function definition without type annotations.
- This flag reports an error whenever it encounters a partly annotated function definition.
This flag is less severe than the previous two options – it type checks the body of every function, regardless of whether it has type annotations. (By default the bodies of functions without annotations are not type checked.)
It will assume all arguments have type
Anyand always infer
Anyas the return type.
- This flag reports an error whenever a function with type annotations is decorated with a decorator without annotations.
None and Optional handling¶
The following flags adjust how mypy handles values of type None. For more details, see Disabling strict optional checking.
This flag causes mypy to stop treating arguments with a
Nonedefault value as having an implicit
For example, by default mypy will assume that the
xparameter is of type
Optional[int]in the code snippet below since the default parameter is
def foo(x: int = None) -> None: print(x)
If this flag is set, the above snippet will no longer type check: we must now explicitly indicate that the type is
def foo(x: Optional[int] = None) -> None: print(x)
- This flag disables strict checking of
Nonevalues. With this option, mypy doesn’t generally check the use of
Nonevalues – they are valid everywhere. See Disabling strict optional checking for more about this feature.
Strict optional checking was enabled by default starting in
mypy 0.600, and in previous versions it had to be explicitly enabled
--strict-optional (which is still accepted).
The follow flags enable warnings for code that is sound but is potentially problematic or redundant in some way.
- This flag will make mypy report an error whenever your code uses an unnecessary cast that can safely be removed.
This flag will make mypy report an error whenever your code uses a
# type: ignorecomment on a line that is not actually generating an error message.
This flag, along with the
--warn-redundant-castsflag, are both particularly useful when you are upgrading mypy. Previously, you may have needed to add casts or
# type: ignoreannotations to work around bugs in mypy or missing stubs for 3rd party libraries.
These two flags let you discover cases where either workarounds are no longer necessary.
By default, mypy will generate errors when a function is missing return statements in some execution paths. The only exceptions are when:
- The function has a
- The function has an empty body or a body that is just
...). Empty functions are often used for abstract methods.
--no-warn-no-returnwill disable these error messages in all cases.
- The function has a
- This flag causes mypy to generate a warning when returning a value
Anyfrom a function declared with a non-
Miscellaneous strictness flags¶
This section documents any other flags that do not neatly fall under any of the above sections.
- This flag causes mypy to suppress errors caused by not being able to fully infer the types of global and class variables.
By default, mypy won’t allow a variable to be redefined with an unrelated type. This flag enables redefinion of a variable with an arbitrary type in some contexts: only redefinitions within the same block and nesting depth as the original definition are allowed. Example where this can be useful:
def process(items: List[str]) -> None: # 'items' has type List[str] items = [item.split() for item in items] # 'items' now has type List[List[str]] ...
By default, imported values to a module are treated as exported and mypy allows other modules to import them. This flag changes the behavior to not re-export unless the item is imported using from-as. Note this is always treated as enabled for stub files. For example:
# This won't re-export the value from foo import bar # This will re-export it as bar and allow other modules to import it from foo import bar as bar
By default, mypy allows always-false comparisons like
42 == 'no'. Use this flag to prohibit such comparisons of non-overlapping types, and similar identity and container checks:
from typing import List, Text items: List[int] if 'some string' in items: # Error: non-overlapping container check! ... text: Text if text != b'other bytes': # Error: non-overlapping equality check! ... assert text is not None # OK, check against None is allowed as a special case.
This flag mode enables all optional error checking flags. You can see the list of flags enabled by strict mode in the full
Note: the exact list of flags enabled by running
--strictmay change over time.
Configuring error messages¶
The following flags let you adjust how much detail mypy displays in error messages.
This flag will precede all errors with “note” messages explaining the context of the error. For example, consider the following program:
class Test: def foo(self, x: int) -> int: return x + "bar"
Mypy normally displays an error message that looks like this:
main.py:3: error: Unsupported operand types for + ("int" and "str")
If we enable this flag, the error message now looks like this:
main.py: note: In member "foo" of class "Test": main.py:3: error: Unsupported operand types for + ("int" and "str")
This flag will add column offsets to error messages, for example, the following indicates an error in line 12, column 9 (note that column offsets are 0-based):
main.py:12:9: error: Unsupported operand types for / ("int" and "str")
By default, mypy will store type information into a cache. Mypy will use this information to avoid unnecessary recomputation when it type checks your code again. This can help speed up the type checking process, especially when most parts of your program have not changed since the previous mypy run.
If you want to speed up how long it takes to recheck your code beyond what incremental mode can offer, try running mypy in daemon mode.
This flag disables incremental mode: mypy will no longer reference the cache when re-run.
Note that mypy will still write out to the cache even when incremental mode is disabled: see the
--cache-dirflag below for more details.
By default, mypy stores all cache data inside of a folder named
.mypy_cachein the current directory. This flag lets you change this folder. This flag can also be useful for controlling cache use when using remote caching.
Mypy will also always write to the cache even when incremental mode is disabled so it can “warm up” the cache. To disable writing to the cache, use
- By default, mypy will ignore cache data generated by a different version of mypy. This flag disables that behavior.
The following flags are useful mostly for people who are interested in developing or debugging mypy internals.
- This flag will invoke the Python debugger when mypy encounters a fatal error.
- If set, this flag will display a full traceback when mypy encounters a fatal error.
- This flag lets you use a custom module as a substitute for the
- This flag specifies the directory where mypy looks for typeshed stubs, instead of the typeshed that ships with mypy. This is primarily intended to make it easier to test typeshed changes before submitting them upstream, but also allows you to use a forked version of typeshed.
This flag modifies both the
--disallow-incomplete-defsflags so they also report errors if stubs in typeshed are missing type annotations or has incomplete annotations. If both flags are missing,
--warn-incomplete-stubalso does nothing.
This flag is mainly intended to be used by people who want contribute to typeshed and would like a convenient way to find gaps and omissions.
If you want mypy to report an error when your codebase uses an untyped function, whether that function is defined in typeshed or not, use the
--disallow-untyped-callflag. See Untyped definitions and calls for more details.
--shadow-file SOURCE_FILE SHADOW_FILE
When mypy is asked to type check
SOURCE_FILE, this flag makes mypy read from and type check the contents of
SHADOW_FILEinstead. However, diagnostics will continue to refer to
Specifying this argument multiple times (
--shadow-file X1 Y1 --shadow-file X2 Y2) will allow mypy to perform multiple substitutions.
This allows tooling to create temporary files with helpful modifications without having to change the source file in place. For example, suppose we have a pipeline that adds
reveal_typefor certain variables. This pipeline is run on
mypy --shadow-file original.py temp.py original.pywill then cause mypy to type check the contents of
original.py, but error messages will still reference
If these flags are set, mypy will generate a report in the specified format into the specified directory.
- Causes mypy to generate a text file report documenting how many
expressions of type
Anyare present within your codebase.
- Causes mypy to generate a text file report documenting the functions and lines that are typed and untyped within your codebase.
- Causes mypy to generate a JSON file that maps each source file’s absolute filename to a list of line numbers that belong to typed functions in that file.
Causes mypy to generate a Cobertura XML type checking coverage report.
You must install the lxml library to generate this report.
Causes mypy to generate an HTML type checking coverage report.
You must install the lxml library to generate this report.
Causes mypy to generate a text file type checking coverage report.
You must install the lxml library to generate this report.
- Causes mypy to generate a JUnit XML test result document with type checking results. This can make it easier to integrate mypy with continuous integration (CI) tools.
- This flag will make mypy print out all usages of a class member based on static type information. This feature is experimental.
This flag will give command line arguments that appear to be scripts (i.e. files whose name does not end in
.py) a module name derived from the script name rather than the fixed name
This lets you check more than one script in a single mypy invocation. (The default
__main__is technically more correct, but if you have many scripts that import a large package, the behavior enabled by this flag is often more convenient.)
This flag switches to an improved, experimental implementation of the semantic analyzer (the part of mypy that binds Python names to definitions). The old and the new semantic analyzers mostly behave identically. The new semantic analyzer is better at handling import cycles and forward references to definitions. It also fixes inconsistencies between the daemon and non-daemon modes, and it detects additional error conditions.
Likely, the next mypy release will use the new semantic analyzer by default, and the old semantic analyzer will be removed in the next release after that.