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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.

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.

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 .

For more on the motivation and design of Python type annotations, see PEP and (PEP) .

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).

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.