i wrote a py script to fetch page from web,it just read write permission enough,so my question is when we need execute permission?
Read/write is enough if you want to run it by typing python file.py.  If you want to run it directly as if it were a compiled program, e.g. ./file.py, then you need execute permission (and the appropriate hash-bang line at the top).
when we need chmod +x file.py