Specify types with docstrings

Introduction

You debug your code permanently, and now in the course of debugging you can also collect type information and specify these types in docstrings.

PyCharm provides an intention action that makes it possible to collect type information at runtime, and define type specifications.

However, it is quite possible to specify the types of parameters manually, without the debugger.

Both cases are explored in the section Examples.

Parameter type specification

To specify the parameter types, follow these general steps

Press Ctrl+Alt+S and go to Editor | General |Smart Keys.
Select the Insert type placeholders checkbox in the Smart Keys page of the editor settings.
Place the caret at the function name, and press Alt+Enter.
In the list of intention actions that opens, choose Insert documentation string stub. PyCharm creates a documentation stub, according to the selected docstring format, with the type specification, collected during the debugger session.

Note that reStructuredText is used for all the subsequent examples, but it is possible to use any of the supported formats of the documentation strings, whether it is plain text, Epytext, Google or NumPy. Refer to the description of the page Python Integrated Tools for details.

Example

Consider the following code:

            import math


            class SimpleEquation:
                def demo(self, a, b, c):
                    d = math.sqrt(abs(b ** 2 - 4 * a * c))
                    root1 = (-b + d) / (2 * a)
                    root2 = (-b - d) / (2 * a)
                    print(root1, root2)


            SimpleEquation().demo(3, 2, 1)
        

Let us suppose that reStructuredText has been selected as the docstring format on the page Python Integrated Tools.

Specify types manually

Place the caret at the name of the function (here it is demo). The suggested intention action is Insert documentation string stub (refer to the section Create documentation comments for details).
Click this intention to produce the documentation comment stub in the reStructuredText format:
Manually specify the desired types of the parameters:
By the way, you can use quick documentation for the function. If you position the caret at the function name and press Ctrl+Q, you will see:

Note that for reStructuredText it's possible to specify types in two formats:

:param param_type param_name: parameter description (type description is on the same line as the parameter description).
:type param_name: param_type (type description is on a separate line)

Both variants are shown below:

Specify types with the aid of the debugger

Press Ctrl+Alt+S and go to Build, Execution, Deployment | Python Debugger. In the Python Debugger page, select the Collect runtime information for code insight checkbox.
Debug the function call, and use the intention action Insert documentation string stub again. Information about the arguments and return values obtained during the debugging session will be used to pre-populate type annotations in a docstring.
Inspect the result:

Last modified: 08 March 2021