When we work with texts in python,We don't just use it but we surround them using double or single quotes (You can even use triple quotes) like this..
name = "GreenTree" #doublequote
name = 'GreenTree' #singlequote
name = """GreenTree""" #triplequote
name = '''GreenTree'''
for python all of them are same.
when python executes your program and sees quotes "It says ,Oh it's a starting of text and when it hits end quote of string it understands that text has just ended"
That's how python works with texts.
You need to surround your text
black inside pair of quotes.
print len(my_string < -- missed the closing bracket.
If brackets starts with ( , it must be closed with ).
also check this..
read theory again and give it another try,If you get confusion, point them out,I will explain.