how to python with statement

how to python with statement

====================1

class Test:
   def __enter__(self):
       print "\n-----in __enter__---self-----", self
       return self

   def __exit__(self, type, value, traceback):
       print "\n-------in __exit__----self, type, value, traceback-----",
self, type, value, traceback

with Test() as thing:
   print "\n-----in with------", thing

OUTPUT
======
-----in __enter__---self----- <__main__.Test instance at 0x7f98f1ebfe60>

-----in with------ <__main__.Test instance at 0x7f98f1ebfe60>

-------in __exit__----self, type, value, traceback----- <
__main__.Test instance at 0x7f98f1ebfe60> None None None

====================2

fp = open("my_file", "r")

print "\n------fp--------", fp

fp.__enter__()

print "\n------fp--------", fp

data = fp.readline()

print "\n------data--------", data

fp.__exit__()

print "\n------fp--------", fp

data = fp.readline()

OUTPUT
======

------fp-------- 

------fp-------- 

------data-------- hello #data red from file

------fp-------- 
Traceback (most recent call last):
File "py_with.py", line 23, in 
data = fp.readline()
ValueError: I/O operation on closed file

====================3

so to open a file, process its contents, and make sure to close it, you can simply do:

with open("my_file") as fp:
   data = fp.read()
   do something with data

====================
More