Create a compress function in Python?

Question

I need to create a function called compress that compresses a string by replacing any repeated letters with a letter and number. My function should return the shortened vers

Answer 1

For a coding interview, where it was about the algorithm, and not about my knowledge of Python, its internal representation of data structures, or the time complexity of operations such as string concatenation:

def compress(message: str) -> str:
    output = ""
    length = 0
    previous: str = None
    for char in message:
        if previous is None or char == previous:
            length += 1
        else:
            output += previous
            if length > 1:
                output += str(length)
            length = 1
        previous = char
    if previous is not None:
        output += previous
        if length > 1:
            output += str(length)
    return output

For code I'd actually use in production, not reinventing any wheels, being more testable, using iterators until the last step for space efficiency, and using join() instead of string concatenation for time efficiency:

from itertools import groupby
from typing import Iterator


def compressed_groups(message: str) -> Iterator[str]:
    for char, group in groupby(message):
        length = sum(1 for _ in group)
        yield char + (str(length) if length > 1 else "")


def compress(message: str) -> str:
    return "".join(compressed_groups(message))

Taking things a step further, for even more testability:

from itertools import groupby
from typing import Iterator
from collections import namedtuple


class Segment(namedtuple('Segment', ['char', 'length'])):

    def __str__(self) -> str:
        return self.char + (str(self.length) if self.length > 1 else "")


def segments(message: str) -> Iterator[Segment]:
    for char, group in groupby(message):
        yield Segment(char, sum(1 for _ in group))


def compress(message: str) -> str:
    return "".join(str(s) for s in segments(message))

Going all-out and providing a Value Object CompressedString:

from itertools import groupby
from typing import Iterator
from collections import namedtuple


class Segment(namedtuple('Segment', ['char', 'length'])):

    def __str__(self) -> str:
        return self.char + (str(self.length) if self.length > 1 else "")


class CompressedString(str):

    @classmethod
    def compress(cls, message: str) -> "CompressedString":
        return cls("".join(str(s) for s in cls._segments(message)))

    @staticmethod
    def _segments(message: str) -> Iterator[Segment]:
        for char, group in groupby(message):
            yield Segment(char, sum(1 for _ in group))


def compress(message: str) -> str:
    return CompressedString.compress(message)