String compression interview question.

string_transformation.feature

Feature: String Transformation Puzzles
  In order to get a job,
  As an interviewee,
  I need to be able to implement stupid, meaningless string transformations.

  Scenario: Compress consecutive characters to a number and the character itself.

    Write a function that takes in a char *string and converts series of
    consecutive identical characters to the number of consecutive characters and
    the character itself.

    E.g. (aaabbbbcccccdde => 3a4b5c2de)

    Given the string "aaabbbbcccccdde"
    When I compress the string
    Then I get the string "3a4b5c2de"

    Given the string "aaabbbbcccccddeaaa"
    When I compress the string
    Then I get the string "3a4b5c2de3a"

transform.py

def compress(subject):
    """Compress consecutive characters to a number and the character itself.
    """
    def gen():
        count = 0
        last_char = None
        out = lambda: str(count) + last_char if count > 1 else last_char
        for char in subject:
            if char != last_char and last_char is not None:
                yield out()
                count = 0
            last_char = char
            count += 1

        yield out()

    return ''.join(gen())