`str.translate` caching values causes incorrect result

Bug report

Bug description:

It seems that str.translate is caching values internally and this is causing unintended results.

When the translation table returns an ordinal or a string ordinal string, it caches the values returned by __getitem__ and will not call the function again. However if you are returning a 2-length string this doesn't occur (and in fact calls __getitem__ an additional time for some reason?).

I expect that the __getitem__ be executed for every character since the function documentation does not mention this caching value and more-so the function behaves inconsistently.

This is likely caused by #65317.

class counting_translate_ord(dict[int, int]):
    def __init__(self) -> None:
        super().__init__()
        self.missing_calls = 0

    def __getitem__(self, key: int) -> int:
        self.missing_calls += 1
        return ord(str(self.missing_calls % 10))


table = counting_translate_ord()

print(("----").translate(table))  # 1111
print(table.missing_calls) # 1
print(("!@#$").translate(table))  # 2345
print(table.missing_calls) # 5


class counting_translate_str(dict[int, str]):
    def __init__(self) -> None:
        super().__init__()
        self.missing_calls = 0

    def __getitem__(self, key: int) -> str:
        self.missing_calls += 1
        return str(self.missing_calls % 10)


table = counting_translate_str()

print(("----").translate(table))  # 1111
print(table.missing_calls) # 1
print(("!@#$").translate(table))  # 2345
print(table.missing_calls) # 5


class counting_translate_2str(dict[int, str]):
    def __init__(self) -> None:
        super().__init__()
        self.missing_calls = 0

    def __getitem__(self, key: int) -> str:
        self.missing_calls += 1
        return format(self.missing_calls, "02d")[:2]


table = counting_translate_2str()

print(("----").translate(table))  # 02030405
print(table.missing_calls) # 5
print(("!@#$").translate(table))  # 07080910
print(table.missing_calls) # 10

CPython versions tested on:

3.12

Operating systems tested on:

Linux

首先，使用提供的 Python 复现程序运行三个计数翻译表案例，并比较它们的 getitem 调用次数和结果。然后定位 CPython 中 str.translate 的入口点和测试；当行为与文档化的契约一致，并且有回归测试覆盖时，即视为完成。