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MonkeyType collects runtime types of function arguments and return values, and can automatically generate stub files or even add draft type annotations directly to your Python code based on the types collected at runtime.


Say some/ originally contains:

def add(a, b):
    return a + b

And contains:

from some.module import add

add(1, 2)

Now we want to infer the type annotation of add in some/ by running with MonkeyType. One way is to run:

$ monkeytype run

By default, this will dump call traces into a SQLite database in the file monkeytype.sqlite3 in the current working directory. You can then use the monkeytype command to generate a stub file for a module, or apply the type annotations directly to your code.

Running monkeytype stub some.module will output a stub:

def add(a: int, b: int) -> int: ...

Running monkeytype apply some.module will modify some/ to:

def add(a: int, b: int) -> int:
    return a + b

This example demonstrates both the value and the limitations of MonkeyType. With MonkeyType, it’s very easy to add annotations that reflect the concrete types you use at runtime, but those annotations may not always match the full intended capability of the functions. For instance, add is capable of handling many more types than just integers. Similarly, MonkeyType may generate a concrete List annotation where an abstract Sequence or Iterable would be more appropriate. MonkeyType’s annotations are an informative first draft, to be checked and corrected by a developer.


Readability and static analysis are the primary motivations for adding type annotations to code. It’s already common in many Python style guides to document the argument and return types for a function in its docstring; annotations are a standardized way to provide this documentation, which also permits static analysis by a typechecker such as mypy.

For more on the motivation and design of Python type annotations, see PEP 483 and PEP 484.


MonkeyType requires Python 3.7+ and the libcst library (for applying type stubs to code files). It generates only Python 3 type annotations (no type comments).


Install MonkeyType with pip:

pip install MonkeyType

How MonkeyType works

MonkeyType uses the sys.setprofile hook provided by Python to interpose on function calls, function returns, and generator yields, and record the types of arguments / return values / yield values.

It generates stub files based on that data, and can use libcst to apply those stub files directly to your code.

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