Print "diagram not found" only after searching the entire file

file.txt

something in 1 line
2nd line
3rd line
my_diagram with other data
some other lines
one more line

with open('file.txt', 'r') as fp:
    ad = fp.readlines()
    for lines in ad:
        if lines.__contains__('my_diagram'):
            print('Got it')
        else:
            print('diagram not found')

Every time it reads a line that doesn't have the string it prints "diagram not found". I only want to print if it's not found anywhere in the file. How can I do that?