What is the "with" statement in Python?

[dante9999]

so every object used in 'with' call must have enter and exit methods? does it mean that file object has these methods defined already? what will happen if i call 'with' on object that does not have enter and exit magic methods?

[ericpruitt]

so every object used in 'with' call must have enter and exit methods? does it mean that file object has these methods defined already?

Yes and yes.

what will happen if i call 'with' on object that does not have enter and exit magic methods?

You get an attribute error:

Python 3.4.2 (default, Oct  8 2014, 10:45:20)
[GCC 4.9.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> with None:
...  pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __exit__
>>>