How Do You Write a Python Function to Determine if There Are Three or More Unique Characters in a String?

Problem scenario
You want a function to tell you if there are three unique characters in a string. For example "ababababa" would have fewer than three unique characters. But "abc" would have three or more unique characters. How do you write a function that does this that solves in linear time?

Solution

variable_to_test = "aaaaaaaaaaaabbbbbb"

def two_unique(s):
   flag = "There are only two unique characters in the string!"
   dict_a = {}
   for string_portion in s:
     if string_portion in dict_a:
         pass
     else:
       dict_a[string_portion] = 1
   key_counter = 0
   for key in dict_a.keys():
     key_counter = key_counter + 1;
     if key_counter > 2:
       flag = True
       return flag
   return False


answer = two_unique(variable_to_test)
if(answer):
  print("There are more than two unique characters!")
else:
  print("There are two or fewer unique characters in the string!")

variable_to_test = "aaaaaaaaaaaabbbbbbc"

def two_unique(s):
   flag = "There are only two unique characters in the string!"
   dict_a = {}
   for string_portion in s:
     if string_portion in dict_a:
       dict_a[string_portion] = dict_a[string_portion] + 1
     else:
       dict_a[string_portion] = 1
   key_counter = 0
   for key in dict_a.keys():
     key_counter = key_counter + 1;
     if key_counter > 2:
       flag = True
       return flag
   return False


answer = two_unique(variable_to_test)
if(answer):
  print("There are more than two unique characters!")
else:
  print("There are two or fewer unique characters in the string!")