Submitting a PR

CPython bugs are listed, discussed, and tracked at https://bugs.python.org/. After completing your CLA registration, you can find an issue to work on there and make a contribution.

Example PR

Let’s walk through how to submit a PR using a real example.

I’ll pick https://bugs.python.org/issue29418 as the issue. The person who opened it reported that isroutine from the inspect module returns False for some built-in functions. In the example they provided:

>>> inspect.isroutine(object().__str__)
False

>>> object().__str__
<method-wrapper '__str__' of object object at 0x7fb92f30a0d0>

this object is a method-wrapper. So what we need to fix is to make inspect.isroutine return True for method-wrapper objects as well. Let’s look at the function’s definition:

Return True if the object is a user-defined or built-in function or method.

According to the definition, the function should return True if the object is a user-defined or built-in function or method. Since a method-wrapper is a built-in method, expecting it to return True is perfectly reasonable.

Forking CPython

Now that we know what to do, the first step is to fork CPython. Go to the CPython GitHub repository and click the Fork button in the top right to fork it to your own account.

Now open a git console on your machine and run the following commands in order:

git clone https://github.com/{username}/cpython
cd cpython
./configure
make

We’ve now cloned CPython and compiled it on our machine. To test the compiled version:

./python.exe
Python 3.9.0a5+ (heads/master-dirty:0bae8d6f45, Apr 17 2020, 17:54:17)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

The latest version at the time of writing this post was 3.9.0a5+.

Finding the Code

The inspect.isroutine function lives in Lib/inspect.py. Let’s open the file and find the relevant function:

def isroutine(object):
    """Return true if the object is any kind of function or method."""
    return (isbuiltin(object) or isfunction(object) or
            ismethod(object) or ismethoddescriptor(object))

The function performs four different checks but doesn’t include any check for method-wrapper objects. Let’s see what type method-wrapper objects belong to:

>>> type(object().__str__)
<class 'method-wrapper'>
>>> import types
>>> isinstance(object().__str__, types.MethodWrapperType)
True

We can detect method-wrapper objects using types.MethodWrapperType.

Making the Fix

Let’s add a new ismethodwrapper function to Lib/inspect.py and update the isroutine function:

def ismethodwrapper(object):
    """Return true if the object is a method-wrapper."""
    return isinstance(object, types.MethodWrapperType)


def isroutine(object):
    """Return true if the object is any kind of function or method."""
    return (isbuiltin(object) or isfunction(object) or
            ismethod(object) or ismethoddescriptor(object) or
            ismethodwrapper(object))

Don’t forget to add the new function to the module’s __all__ list as well.

Running the Tests

After making changes to CPython, you are expected to add a test to Lib/test/test_inspect.py. Look at the existing tests and add an appropriate one:

def test_ismethodwrapper(self):
    self.assertTrue(inspect.ismethodwrapper(object().__str__))
    self.assertTrue(inspect.ismethodwrapper(object().__repr__))
    self.assertFalse(inspect.ismethodwrapper(inspect.ismethodwrapper))
    # isroutine should now return True for method-wrappers
    self.assertTrue(inspect.isroutine(object().__str__))

To run the tests:

./python.exe -m unittest Lib.test.test_inspect -v

Once the tests pass, we can move on to creating a branch.

Creating a Branch and Committing

For CPython contributions, always create a topic-specific branch — never work directly on main:

git checkout -b fix-isroutine-method-wrapper

Stage your changes and commit:

git add Lib/inspect.py Lib/test/test_inspect.py
git commit -m "bpo-29418: Add ismethodwrapper() to the inspect module"

Opening the PR

Push the branch to your own fork:

git push origin fix-isroutine-method-wrapper

Then go to your fork on GitHub. GitHub will automatically offer you a “Compare & pull request” option. Click it and create your PR.

When writing the PR description, pay attention to:

• Title: bpo-29418: Add ismethodwrapper() to the inspect module
• Description: Briefly explain what you fixed, why you fixed it, and how you tested it
• Issue link: Add the line Fixes https://bugs.python.org/issue29418

Conclusion

You’ve submitted your first CPython PR! Wait for the core developers to review it; you may be asked to make changes based on their feedback. Be patient — contributing to a large open source project is both exciting and educational.

Note: This post is based on a contribution made in 2020. Since then, CPython’s bug tracking system has been fully migrated to GitHub Issues. bugs.python.org is now only accessible for historical records.