mutable_string.py

from collections import UserString
from typing import Union


class StrIns(UserString):

    def __setitem__(self, key: Union[int, slice], value: str) -> None:
        if isinstance(key, int):
            self._assign_one_element(key, value)
        elif isinstance(key, slice):
            self._assign_slice(key, value)
        else:
            type_name = type(key).__name__
            raise TypeError(f'Setitem does not support type {type_name}')

    def _assign_one_element(self, index: int, value: str) -> None:
        if not isinstance(value, str):
            type_name = type(value).__name__
            raise TypeError('Assignment works with '
                            f'strings only, got {type_name}')

        if len(value) != 1:
            raise ValueError('Index assignment works '
                             f'with single character only, got {value!r}')

        self.data = self.data[:index] + value + self.data[index + 1:]

    def _assign_slice(self, index: slice, value: str) -> None:
        if index.start is None:
            index.start = 0

        if index.stop is None:
            index.stop = len(self.data)

        index = slice(index.start, index.stop, index.step or 1)

        if index.step == 1:
            self._assign_slice_simple(index, value)
        else:
            self._assign_slice_complex(index, value)

    def _assign_slice_simple(self, index: slice, value: str) -> None:
        self.data = (self.data[:index.start]
                     + value + self.data[index.stop + 1:])

    def _assign_slice_complex(self, index: slice, value: str) -> None:
        as_list = list(self.data)
        as_list[index] = list(value)
        self.data = ''.join(as_list)


var = StrIns('some string')
print('Original:', var)  # Original: some string
print('Indexing:', var[0])  # Indexing: s
var[0] = '1'
var[1] = '2'
print('After assignment:', var)  # After assignment: 12me string
print('Find:', var.find('m'))  # Find: 2

var[0:3] = 'some other long'
print('Slice assignment simple:', var)  # Slice assignment simple: some other long string
var[21:4:-2] = 'something'
print('Slice assignment complex:', var)  # Slice assignment complex: some gtnei hotgesmrons