input: Write a function to generate an HTML form with a submit button that sends information to a Python program. The form should use the POST method to send data to a file named 'action.py'. If the form is generated correctly, return the HTML string of the form; otherwise, return an empty string.
  assert generate_form()=='<form action=\"action.py\" method=\"post\">\n    <input type=\"submit\" value=\"Submit\">\n</form>'
  assert generate_form()=='<form action="action.py" method="post">\n    <input type="submit" value="Submit">\n</form>'
  assert generate_form()!='<form action="another_action.py" method="post">\n    <input type="submit" value="Submit">\n</form>'
  assert generate_form()!=''
  assert generate_form()!='invalid form'
output: def generate_form():
      return '<form action="action.py" method="post">\n    <input type="submit" value="Submit">\n</form>'