Generating type annotations

Use the monkeytype command-line script to generate and apply stub files based on recorded call traces.

The script must be able to import your code. It automatically adds the current working directory to the Python path, so ensuring that you run monkeytype from the root of your code is usually sufficient. Alternatively, you can set the PYTHONPATH environment variable.

monkeytype list-modules

The monkeytype list-modules subcommand outputs a list of all modules which have traces present in the trace store. This command respects only the --config option.

monkeytype stub

Run monkeytype stub some.module to generate a stub file for the given module based on call traces queried from the trace store. If the module already has some type annotations, those annotations will be respected and will not be replaced with annotations derived from traced calls.

The generated stub file will be printed to standard output. If you want to save it to a file, redirect the output to a file (e.g. monkeytype stub some.module > some/module.pyi).

You can also run e.g. monkeytype stub some.module:SomeClass or monkeytype stub some.module:somefunc to generate a stub for just one class or function.

MonkeyType must import your code in order to generate annotations for it, so if a module has import side effects, running monkeytype stub on the module will trigger those side effects. For “executable” modules, ensure the execution code is protected with if __name__ == '__main__' to avoid MonkeyType triggering it.

monkeytype apply

If you prefer inline type annotations, monkeytype apply some.module will generate annotations for some.module internally (in exactly the same way as monkeytype stub would), but rather than printing the annotations in stub syntax, it will apply them directly to the code file, modifying it in-place.

Obviously this is best used when the file is tracked in a version-control system, so you can easily see the changes made by MonkeyType and accept or reject them. MonkeyType annotations are rarely suitable exactly as generated; they are a starting point and usually require some adjustment by someone who understands the code.


Both monkeytype stub and monkeytype apply accept the following options:

-c <config-path>, --config <config-path>

The location of the config object defining your call-trace store and other configuration. The config-path should take the form some.module:name, where name is the variable in some.module containing your config instance.

Optionally, the value can also include a () suffix, and MonkeyType will call/instantiate the imported function/class with no arguments to get the actual config instance.

The default value is monkeytype.config:get_default_config(), which tries the config path monkeytype_config:CONFIG and falls back to monkeytype.config:DefaultConfig() if there is no monkeytype_config module. This allows creating a custom config that will be used by default just by creating with a CONFIG instance in it.

-l <limit>, --limit <limit>

The maximum number of call traces to query from your call trace store.

See the query_limit() config method.

Default: 2000


Don’t apply your configured Type rewriters to the output types.


Generate a stub based only on traced calls, ignoring (and overwriting, if applying stubs) any existing type annotations in the code. (By default, existing annotations in the code take precedence over traced types.)

Additionally, monkeytype stub accepts:


Generate a stub that omits any annotations that are already present in the source. (By default, existing annotations in the source are reproduced in the stub.) Because MonkeyType has to replicate existing annotations via runtime introspection, and doesn’t have access to the original string, its replication is often imperfect, which would cause spurious conflicts when applying the stub. Omitting these annotations entirely when generating a stub for application has no cost, since they are already present in the source.

This option is implied by monkeytype apply, since it minimizes the possibility of a conflict when attempting to apply annotations.

This option is mutually exclusive with --ignore-existing-annotations.


Generate a textual diff between stubs generated by preserving existing annotations and ignoring them. Use this to see how accurately your annotations represent what is seen in production.

Additionally, monkeytype apply accepts:


Adds from __future__ import annotations to the applied file, and confines all the newly added imports from the stub in the if TYPE_CHECKING block to prevent potential circular dependencies.

Type rewriters

MonkeyType’s built-in type generation is quite simple: it just makes a Union of all the types seen in traces for a given argument or return value, and shrinks that Union to remove redundancy. All additional type transformations are performed through configured type rewriters.

class monkeytype.typing.TypeRewriter

The TypeRewriter class provides a type-visitor that can be subclassed to easily implement custom type transformations.

Subclasses can implement arbitrary rewrite_Foo methods for rewriting a type named Foo. TypeRewriter itself implements only rewrite_Dict, rewrite_List, rewrite_Set, rewrite_Tuple, rewrite_Union (in addition to the methods listed below). These methods just recursively rewrite all type arguments of the container types.

For example type rewriter implementations, see the source code of the subclasses listed below.

rewrite(typ: type) → type

Public entry point to rewrite given type; return rewritten type.

generic_rewrite(typ: type) → type

Fallback method when no specific rewrite_Foo method is available for a visited type.

class monkeytype.typing.RemoveEmptyContainers

Rewrites e.g. Union[List[Any], List[int]] to List[int]. The former type frequently occurs when a method that takes List[int] also sometimes receives the empty list, which will be typed as List[Any].

class monkeytype.typing.RewriteConfigDict

Takes a generated type like Union[Dict[K, V1], Dict[K, V2]] and rewrites it to Dict[K, Union[V1, V2]].

class monkeytype.typing.RewriteLargeUnion(max_union_len: int = 5)

Rewrites large unions (by default, more than 5 elements) to simply Any, for better readability of functions that aren’t well suited to static typing.

class monkeytype.typing.ChainedRewriter(rewriters: Iterable[TypeRewriter])

Accepts a list of rewriter instances and applies each in order. Useful for composing rewriters, since the type_rewriter config method only allows returning a single rewriter.

class monkeytype.typing.NoOpRewriter

Does nothing. The default type rewriter in the base Config.