diff.py

import pytest


def diff(first, second):
    len_diff = len(first) - len(second)
    if abs(len_diff) > 1:
        return False
    if len_diff > 0:
        short_str = second
        long_str = first
    else:
        short_str = first
        long_str = second

    offset = 0
    error_counter = 0
    for short_index in range(len(short_str)):
        if short_str[short_index] == long_str[short_index + offset]:
            continue
        if error_counter > 0:
            return False
        error_counter += 1
        if len_diff == 0:
            continue
        offset += 1
        if short_str[short_index] != long_str[short_index + offset]:
            return False
    if len_diff:
        return True
    else: 
        return error_counter == 1
    

def test_equal_str():
    assert not diff("1234", "1234")

def test_replace_sym_in_middle():
    assert diff("1234", "1254")

def test_insert_sym_in_middle():
    assert diff("12345", "125345")

def test_delete_sym_in_middle():
    assert diff("1234", "124")

def test_one_sym_insert_sym_in_the_end():
    assert diff("1", "12")

def test_insert_sym_in_the_end():
    assert diff("1234", "12345")

def test_delete_sym_in_the_end():
    assert diff("12", "1")

def test_insert_2_sym():
    assert not diff("1234", "123474")

def test_too_much_changes():
    assert not diff("12345", "1255555345")

def test_delete_2_sym():
    assert not diff("1234", "12")