TDD: String Calculator Kata String Calculator
  
  
  Create a simple String calculator with a method int Add(string numbers).
  
  
  The method can take 0, 1 or 2 numbers, and will return their sum (for an empty string it will return 0).
  For example "" or "1" or "1,2".
  Start with the simplest test case of an empty string and move to 1 and two numbers
  Remember to solve things as simply as possible so that you force yourself to write tests you did not think about.
  Remember to refactor after each passing test.
  
  Allow the Add method to handle an unknown amount of numbers.
  Allow the Add method to handle new lines between numbers (instead of commas).
  
  
  The following input is ok: "1\n2,3" (will equal 6).
  The following input is NOT ok: "1,\n" (not need to prove it - just clarifying).
  
  Support different delimiters
  
  
  to change a delimiter, the beginning of the string will contain a separate line that looks like this: "//[delimiter]\n[numbers…]".
  For example, "//;\n1;2" should return three where the default delimiter is ';'.
  The first line is optional. All existing scenarios should still be supported.
  
  Calling Add with a negative number will throw an exception "negatives not allowed" - and the negative that was passed. If there are multiple negatives, show all of them in the exception message.
  


import re

def string_calculator(string):
    """
    Sums numbers in a string separated by delimiters.

    &gt;&gt;&gt; string_calculator("")
    0
    &gt;&gt;&gt; string_calculator("42")
    42
    &gt;&gt;&gt; string_calculator("473,12")
    485
    &gt;&gt;&gt; string_calculator("1\\n2,3")
    6
    &gt;&gt;&gt; string_calculator("//;\\n1;2")
    3
    &gt;&gt;&gt; string_calculator("-42, 2")
    Traceback (most recent call last):
    ...
    ValueError: negatives not allowed ['-42']
    &gt;&gt;&gt; string_calculator("-42,31,-312")
    Traceback (most recent call last):
    ...
    ValueError: negatives not allowed ['-42', '-312']
    """
    if re.match('-[0-9]+', string):
        raise ValueError("negatives not allowed {}".format(
            re.findall('-[0-9]+', string)))
    separators = ',', '\\n'
    if "//" in string:
        sep_info = re.findall('//.+\n', string)[0]
        separators = sep_info.replace('//','').replace('\\n','')
        string = string.replace(sep_info, '')
    if not string:
        return 0
    if not any(separator in string for separator in separators):
        return int(string)
    return sum(int(number) for number in re.split('|'.join(separators), string))
There's a redundant if branch:

if not any(separator in string for separator in separators):
    return int(string)


removing it still passes all the tests, because

re.split('|'.join(separators), string)


returns a list containing the list itself if there are no separators in the string.