Replace Line in Textfile

[Deadline]

I have a really simple query, but I can't find an answer to it. I'm new to Python and am learning for a project I'm setting up, but I seem to have fallen at the first hurdle.

All I want to do is replace a line in a text file. That's it. So, I've written the following:

f = open("test.txt", "rt")
data = f.read()
data = data.replace('python', 'Hello')
f.close()
f = open("test.txt", "wt")
f.write(data)
f.close()

which replaces the word 'python' with the word 'hello'. Easy.

But, how do I combine that with an If statement? Let's say the file only contains a single word: python. How do I get the system to check the single line in the file and, if it says 'python' replace it with the word 'hello', if it says anything else, print whatever the file contains?

It feels like it should be really simple, I just can't get there.