input: Write a python function to calculate word frequency in a given string.
  assert word_frequency("hello world, this is a test string") == {"hello": 1, "world,": 1, "this": 1, "is": 1, "a": 1, "test": 1, "string": 1}
  assert word_frequency("repeat repeat repeat") == {"repeat": 3}
  assert word_frequency("word frequency test, word frequency") == {"word": 2, "frequency": 2, "test,": 1}
output: def word_frequency(s):
      words = s.split()
      freq = {}
      for word in words:
          if word in freq:
              freq[word] += 1
          else:
              freq[word] = 1
      return freq