Convert "aaaabbbbbcccccde" to "a4b5c5d1e1"

foo.py

def foo(s):
	if len(s) <= 0:
		return None
	else:
		output, curr_char, curr_count = '', '', 0
		for idx in range(0, len(s)):
			if s[idx] == curr_char:
				curr_count += 1
			else:
				output += curr_char + str(curr_count) if curr_count > 0 else curr_char
				curr_char, curr_count = s[idx], 1
			if idx == len(s)-1:
				output += curr_char + str(curr_count)
		return output

if __name__ == '__main__':
	test_s = 'aaaabbbbbcccccde'
	print test_s
	print
	print foo(test_s)  # outputs "a4b5c5d1e1"