02_add_python_function.md

Exercise 02: Add a Python function to CPython

You probably already know that the Lib/ directory within CPython contains all the Python-based modules that Python includes. You are probably familiar with many of them, so the goal will be to pick one of them, so you can add a new one.

This might sound too complicated, but the recommendation is to pick a module that has a function, and maybe re-write a variation for that function, by removing one or more keyword arguments, and provide a simplified version.

You can do this with any function you want, but in case you don't know, you can follow the example below.

Example

Let's consider the module glob that contains a function with the same name, that has the following arguments:

def glob(pathname, *, root_dir=None, dir_fd=None, recursive=False,
        include_hidden=False):

If you are not familiar with this function, check the following example:

from glob import glob

>>> glob("tutorial/**")
['tutorial/exercises', 'tutorial/code', 'tutorial/README.md']

The pattern ** means that everything found inside the tutorial directory will be listed, but as you probably notice, the tutorial/exercises directory content was not listed. For that, glob provides a way to do it recursively, so you can call the function like this:

>>> glob("tutorial/**", recursive=True)
['tutorial/', 'tutorial/exercises', 'tutorial/exercises/10_rewrite_python_functionality.md', 'tutorial/exercises/09_module_with_functions.md', 'tutorial/exercises/08_helloworld_rust.md', 'tutorial/exercises/07_helloworld_zig.md', 'tutorial/exercises/06_helloworld_c.md', 'tutorial/exercises/05_add_grammar_alias.md', 'tutorial/exercises/04_add_container_method.md', 'tutorial/exercises/03_add_c_function.md', 'tutorial/exercises/02_add_python_function.md', 'tutorial/exercises/01_compile_cpython.md', 'tutorial/exercises/00_compile_cprogram.md', 'tutorial/code', 'tutorial/code/hello_world_zig', 'tutorial/code/hello_world_zig/src', 'tutorial/code/hello_world_zig/src/hello.zig', 'tutorial/code/hello_world_zig/pyproject.toml', 'tutorial/code/hello_world_zig/build.py', 'tutorial/code/hello_world_rust', 'tutorial/code/hello_world_rust/src', 'tutorial/code/hello_world_rust/src/lib.rs', 'tutorial/code/hello_world_rust/pyproject.toml', 'tutorial/code/hello_world_rust/Cargo.toml', 'tutorial/code/hello_world_c', 'tutorial/code/hello_world_c/pyproject.toml', 'tutorial/code/hello_world_c/hello.c', 'tutorial/README.md']

To have this behavior by default, we could create rglob to be a variation of glob that has the recursive parameter set to True by default and cannot be changed:

def rglob(pathname, *, root_dir=None, dir_fd=None, include_hidden=False):
    return glob(pathname, root_dir=root_dir, dir_fd=dir_fd, recursive=True,
    include_hidden=include_hidden)

For this particular case, we need to also modify the __all__ variable from:

__all__ = ["glob", "iglob", "escape", "translate"]

to

__all__ = ["glob", "rglob", "iglob", "escape", "translate"]

Repeating the values from the parameters when calling glob allows us to have control over the behavior but only one is set to True which is recursive.

After recompiling CPython, I will be able to do:

>>> from glob import rglob
>>> rglob("tutorial/**")
['tutorial/', 'tutorial/exercises', 'tutorial/exercises/10_rewrite_python_functionality.md', 'tutorial/exercises/09_module_with_functions.md', 'tutorial/exercises/08_helloworld_rust.md', 'tutorial/exercises/07_helloworld_zig.md', 'tutorial/exercises/06_helloworld_c.md', 'tutorial/exercises/05_add_grammar_alias.md', 'tutorial/exercises/04_add_container_method.md', 'tutorial/exercises/03_add_c_function.md', 'tutorial/exercises/02_add_python_function.md', 'tutorial/exercises/01_compile_cpython.md', 'tutorial/exercises/00_compile_cprogram.md', 'tutorial/code', 'tutorial/code/hello_world_zig', 'tutorial/code/hello_world_zig/src', 'tutorial/code/hello_world_zig/src/hello.zig', 'tutorial/code/hello_world_zig/pyproject.toml', 'tutorial/code/hello_world_zig/build.py', 'tutorial/code/hello_world_rust', 'tutorial/code/hello_world_rust/src', 'tutorial/code/hello_world_rust/src/lib.rs', 'tutorial/code/hello_world_rust/pyproject.toml', 'tutorial/code/hello_world_rust/Cargo.toml', 'tutorial/code/hello_world_c', 'tutorial/code/hello_world_c/pyproject.toml', 'tutorial/code/hello_world_c/hello.c', 'tutorial/README.md']

Optional challenge

Add a new Python-based module (separate file) to Lib/ so you can include "more batteries" into your local Python.