A rough example code for renaming variables using libcst.

libcst_transformer.py

import libcst
import difflib
from termcolor import colored


class RenameTransformer(libcst.CSTTransformer):
    def __init__(self, rename_pairs):
        self.rename_pairs = rename_pairs
        self.restore_keywords = []

    def _rename(self, original_node, renamed_node):
        if original_node.value in self.rename_pairs.keys():
            return renamed_node.with_changes(value=self.rename_pairs[original_node.value])
        else:
            return renamed_node

    def leave_Name(self, original_node, renamed_node):
        """
        LibCST parses variables in f-strings.
        All [Name] nodes are already parsed correctly.
        """
        return self._rename(original_node, renamed_node)

    def visit_Arg(self, node):
        """
        LibCST does not provide APIs to decide whether to visit the child attributes or not.
        Some tricky hack is needed, e.g. restoring the original keyword as follow.
        """
        if node.keyword and node.keyword.value in self.rename_pairs.keys():
            self.restore_keywords.append(node.keyword.value)
        return True

    def leave_Arg(self, original_node, renamed_node):
        try:
            restore = self.restore_keywords.pop()
            return renamed_node.with_changes(keyword=renamed_node.keyword.with_changes(value=restore))
        except IndexError:
            # stack is empty
            return renamed_node


test_code = """
def foo(bar, zar):
    print(f'bar is {bar}, not {zar}')
    print('bar is {bar}, not {zar}'.format(bar=bar, zar=zar))
"""

if __name__ == '__main__':
    rename_candidates = {
        'bar': 'bar_new',
    }
    rename_transformer = RenameTransformer(rename_candidates)
    original_tree = libcst.parse_module(test_code)
    print(original_tree)
    renamed_tree = original_tree.visit(rename_transformer)
    print('#' * 100)
    print('original code:')
    print(test_code)
    print('#' * 100)
    print('renaming bar -> bar_new:')
    # restoring rename tree as code is easy
    diff = "".join(difflib.unified_diff(test_code.splitlines(1), renamed_tree.code.splitlines(1)))
    for l in diff.splitlines():
        if l.startswith('+'):
            print(colored(l, 'green'))
        elif l.startswith('-'):
            print(colored(l, 'red'))
        else:
            print(l)