f-str.py

from dataclasses import dataclass


class Default(dict[str, str]):
    def __missing__(self, key: str):
        return f"{{{key}}}"


@dataclass
class Pair:
    key: str
    value: str

    def __format__(self, format_spec: str):
        if format_spec == "":
            return self.value
        return format_spec.format_map(Default({self.key: self.value}))


template = "{x} {y}{z}"
x = Pair("x", "hello")
y = Pair("y", "world")
z = Pair("z", "foo")

layer_1_format = "{x:{template}}".format(x=x, template=template)
layer_1_format = "{y:{layer_1_format}}".format(y=y, layer_1_format=layer_1_format)
layer_1_fstr = f"{x:{template}}"
layer_1_fstr = f"{y:{layer_1_fstr}}"


# 3.11 - SyntaxError: f-string: expressions nested too deeply
layer_2_fstr = f"{x:{y:{template}}}"

# ValueError: Max string recursion exceeded
# layer_2_format = "{x:{y:{template}}}".format(x=x, y=y, template=template)

assert layer_1_format == layer_1_fstr == layer_2_fstr == "hello world{z}"

# SyntaxError: f-string: expressions nested too deeply
# layer_3_fstr = f"{x:{y:{z:{template}}}}"