input: Write a function to count the occurrence of each character in a given string. The function should return a dictionary where the keys are the characters and the values are the counts of those characters in the string. If the string is empty, the function should return an empty dictionary.
  def char_occurrences(a_string):
   char_count = {}
   for char in a_string:
    if char in char_count:
     char_count[char] += 1
    else:
     char_count[char] = 1
   return char_count
  
  assert char_occurrences("Hello, World!")=={'H': 1, 'e': 1, 'l': 3, 'o': 2, ',': 1, ' ': 1, 'W': 1, 'r': 1, 'd': 1, '!': 1}
  assert char_occurrences("Python")=={'P': 1, 'y': 1, 't': 1, 'h': 1, 'o': 1, 'n': 1}
  assert char_occurrences("")=={}
  assert char_occurrences("aA")=={'a': 1, 'A': 1}
  assert char_occurrences("123")=={'1': 1, '2': 1, '3': 1}
output: def char_occurrences(a_string):
   char_count = {}
   for char in a_string:
    if char in char_count:
     char_count[char] += 1
    else:
     char_count[char] = 1
   return char_count