input: Write a function to count the number of words in a given string. 
  
  assert count_words("Hello world, this is a sentence.")==5
  assert count_words("This is a test.")==4
  assert count_words("SingleWord")==1
  assert count_words("")==0
  assert count_words("  Space  separated  words  ")==3
output: def count_words(s):
      return len(s.split())