error in writing data into file in python

 a='aa'
>>> f=open("key.txt","w")


>>> s=str(a)
>>> f.write(s)

and still the key.txt file remains blank .. why?

Use

f.flush()

to flush the write to disk. Or, if you are done using f, you could use

f.close()

to flush and close the file.

This issue can be avoided completely by making use of the with statement:

with open("key.txt","w") as f:
    s=str(a)
    f.write(s)

The file will be automatically closed when the block completes. Using the with statement you need not worry about this sort of bug creeping into your code.