input: Write a function to replace all occurrences of a given string with an alternate string. The function should take in three parameters: the original input string, the target string to be replaced, and the replacement string. If the target string is found in the input string, all occurrences should be replaced with the replacement string. The function should return the modified string.
  
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Python programming is fun", "Python", "Java")=="Java programming is fun"
  assert replace_string("apple apple orange", "apple", "banana")=="banana banana orange"
  assert replace_string("This is a test", "test", "example")=="This is a example"
  assert replace_string("No occurrences here", "missing", "replace")=="No occurrences here"
output: def replace_string(input_str, target, replacement):
      return input_str.replace(target, replacement)