LibCST matcher for detecting nested f-string expressions in Python AST

Question

I want to create a transformer that converts all quotes of f-strings from a single quote to triple quotes, but leaves nested f-strings intact.

For example, the next expression left intact.

f"""\
Hello {developer_name}
My name is {_get_machine(f"{self.prop_a}, {self.prop_b}")}
"""

But, the transformer result is:

f"""\
Hello {developer_name}
My name is {_get_machine(f"""{self.prop_a}, {self.prop_b}""")}
"""

I tried the following matchers but without success:

class _FormattedStringEscapingTransformer(m.MatcherDecoratableTransformer):

    @m.call_if_not_inside(
        m.FormattedString(
            parts=m.OneOf(
                m.FormattedStringExpression(expression=m.TypeOf(m.FormattedString))
            )
        )
    )
    @m.leave(m.FormattedString())
    def escape_f_string(
        self, original_node: cst.FormattedString, updated_node: cst.FormattedString
    ) -> cst.FormattedString:
        return updated_node.with_changes(start='f"""', end='"""')

class _FormattedStringEscapingTransformer(m.MatcherDecoratableTransformer):

    @m.call_if_not_inside(
        m.FormattedString(
            parts=m.OneOf(
                m.FormattedStringExpression(
                    expression=m.Not(m.FormattedString(parts=m.DoNotCare()))
                )
            )
        )
    )
    @m.leave(m.FormattedString())
    def escape_f_string(
        self, original_node: cst.FormattedString, updated_node: cst.FormattedString
    ) -> cst.FormattedString:
        return updated_node.with_changes(start='f"""', end='"""')

None of them worked.

What is the correct matcher to exclude transformation if inner f-strings expressions?

dROOOze · Accepted Answer · 2024-10-20 10:32:36Z

I don't think call_if_[not_]inside is the correct tool to do this - that will always invoke a method when a node or its parents has the matching condition, not just its parents:

https://libcst.readthedocs.io/en/latest/matchers.html#libcst.matchers.call_if_not_inside

A method that is decorated with this decorator will only be called if it or one of its parents does not match the supplied matcher.

Convert all quotes of f-strings from a single quote to ..., but leave nested f-strings intact translates to always transform an outer f-string, then when leaving child f-strings, don't transform them. A matcher decoratable transformer implementation is only necessary if you have other f-string transforms in the same transformer, but something like this should work:

import libcst as cst
import libcst.matchers as m

class _FormattedStringEscapingTransformer(m.MatcherDecoratableTransformer):
    _escapee_fstring_node: cst.FormattedString | None = None

    @m.visit(m.FormattedString())
    def visit_f_string(self, node: cst.FormattedString) -> None:
        if self._escapee_fstring_node is None:
            self._escapee_fstring_node = node

    @m.leave(m.FormattedString())
    def escape_f_string(
        self, original_node: cst.FormattedString, updated_node: cst.FormattedString
    ) -> cst.FormattedString:
        if original_node is self._escapee_fstring_node:
            self._escapee_fstring_node = None
            return updated_node.with_changes(start='f"""', end='"""')
        return updated_node

>>> mod = cst.parse_module(
... '''
... f"Hello {developer_name}, My name is {_get_machine(f"{self.prop_a}, {self.prop_b}")}"
... '''
... )
...
>>> print(mod.visit(_FormattedStringEscapingTransformer()).code)
f"""Hello {developer_name}, My name is {_get_machine(f"{self.prop_a}, {self.prop_b}")}"""

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