How can I extract keywords from a Python format string?

Question

I want to provide automatic string formatting in an API such that:

my_api(\"path/to/{self.category}/{self.name}\", ...)

can be replaced wit

Answer 1

If all placeholders are named, a special dictionary could be used to intercept which keys are tried to be accessed and logged to an array.

def format_keys(str_):
    class HelperDict(dict):
        def __init__(self):
            self._keys = []
        def __getitem__(self, key):
            self._keys.append(key)    
    d = HelperDict()
    str_.format_map(d)
    return d._keys

Note that if there are unnamed placeholders, an IndexError will be raised by .format() (tuple index out of range).