MonkeyType 18.1.11

Example

Say some/module.py originally contains:

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

And myscript.py contains:

from some.module import add

add(1, 2)

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

$ monkeytype run myscript.py

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/module.py 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.

Motivation

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.

Requirements

MonkeyType requires Python 3.6+ and the retype library (for applying type stubs to code files). It generates only Python 3 type annotations (no type comments).

Installing

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 retype to apply those stub files directly to your code.

See the full documentation for details.

Troubleshooting

Check if your issue is mentioned in the frequently asked questions list.

LICENSE

MonkeyType is BSD licensed.

18.1.11

• Fix crash in RewriteEmptyContainers rewriter if a parameter has only empty container types in traces (and more than one). Fixes #53.

18.1.10

• Display retype errors when stub application fails. Merge of #52, fixes #49.

• Add --sample-count option to show the number of traces a given stub is based on. Merge of #50, fixes #7. Thanks Tai-Lin.

• Add monkeytype run -m for running a module as a script. Merge of #41. Thanks Simon Gomizelj.

• Add support for Django’s cached_property decorator. Merge of #46, fixes #9. Thanks Christopher J Wang.

• Catch and log serialization exceptions instead of crashing. Fixes #38, merge of #39.

• Fix bug in default code filter when Python lib paths are symlinked. Merge of #40. Thanks Simon Gomizelj.

17.12.3

• Rewrite imports from _io module to io. (#1, merge of #32). Thanks Radhans Jadhao.

• Add Config.cli_context() as a hook for custom CLI initialization and cleanup logic (#28; merge of #29). Thanks Rodney Folz.

17.12.2

• Exclude “frozen importlib” functions in default code filter.

• Fix passing args to script run with monkeytype run (#18; merge of #21). Thanks Rodney Folz.

• Fix generated annotations for NewType types (#22; merge of #23). Thanks Rodney Folz.

17.12.1

• Fix using MonkeyType outside a virtualenv (#16). Thanks Guido van Rossum for the report.

17.12.0

• Initial public version.