squiddy / libcst_easy_matcher

An experiment to create matchers from source code when working with LibCST

libcst_easy_matcher

An experiment to create matchers from source code when working with LibCST. A matcher enables you to check whether a certain node in the tree generated by LibCST matches a certain pattern.

Status

a = 3
a = __
__ = 3
b = bar(x=3)
b = bar(x=__)
b = __(x=3)

See tests/test_libcst_easy_matcher.py for all examples.

Motivation

If I want to check that a statement matches

client["cookies"] = get_cookie(request)

I'd write something like this:

m.Assign(
    targets=[
        m.AssignTarget(
            target=m.Subscript(
                value=m.Attribute(
                    value=m.Name("client"),
                    attr=m.Name("cookies"),
                )
            )
        )
    ],
    value=m.Call(
        func=m.Name(
            value='get_cookie'
        ),
        args=[
            m.Arg(
                value=m.Name(
                    value='request'
                )
            )
        ]
    )
)

I'm experimenting here to instead do it like this:

create_matcher("client['cookies'] = get_cookie(request)")

Placeholders

Just matching against concrete values only get's you so far, so I'm evaluating the use of placeholders like this:

__ = get_cookie(request)
client[__] = get_cookie(request)
client['cookies'] = __(request)
client['cookies'] = get_cookie(__)

All of which should match the example code. __ might not be the best choice in the long run, but I needed a valid identifier to be able to still parse the code with LibCST.

License

MIT