Here is your answer:
if "insert_char_or_string_here" in "insert_string_to_search_here":
#DOSTUFF
For checking if it is false:
if not "insert_char_or_string_here" in "insert_string_to_search_here":
#DOSTUFF
OR:
if "insert_char_or_string_here" not in "insert_string_to_search_here":
#DOSTUFF