MonkeyType¶
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.
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 3.7+ and the libcst library (for applying type stubs to code files). It generates only Python 3 type annotations (no type comments).
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.
The Details¶
- Configuration
- Tracing function calls
- Generating type annotations
- Storing call traces
- Frequently asked questions
- Why don’t my decorated functions get traces?
- I’m using Django 1.11+ and
monkeytype run manage.py test
generates no traces. - I’m using Django, and I get an AppRegistryNotReady exception when I run monkeytype.
- I run tests against my package installed in site-packages, and I get no traces.
- Why did my test coverage measurement stop working?
- MonkeyType stopped generating TypedDicts.