What are the advantages of using with for opening files on Python 3?

Question

What are the real performance advantages of using

with open(__file__, 'r') as f:

instead of using:

open(__file__,'r')

in Python 3 for both writing and reading files?

Answer 1

Using with means that the file will be closed as soon as you leave the block. This is beneficial because closing a file is something that can easily be forgotten and ties up resources that you no longer need.

Answer 2

What the with statement basically does is take advantage of two new magic keywords on an object: __enter__ and __exit__ to implement automatic cleanup (c++ destructors, .net IDisposable, etc). So what effectively happens is as follows:

file = open(__file__, 'r')
try:
  # your code here
finally: file.close()

Feel free to read a bit more about the actual implementation in pep-0343

Answer 3

To answer your question of what performance advantage, there is none from a strict CPU/memory standpoint. Your code won't perform any better, but it will be more robust with less typing and it should be more clear and thus easier to maintain. So in a sense, the performance gain will be measured in man-hours in later maintenance, which as we all should know is the true cost of software, so it will have a great "performance advantage". ;)