Trying to understand python with statement and context managers

Question

I am new to this, and am just trying to understand the with statement. I understand that it is supposed to replace the try/except block.

Now suppose I do something like this:

try:
   name='rubicon'/2 # to raise an exception
except Exception as e:
   print "no not possible"
finally:
   print "Ok I caught you"

How do I replace this with a context manager?

Answer 1

with doesn't really replace try/except, but, rather, try/finally. Still, you can make a context manager do something different in exception cases from non-exception ones:

class Mgr(object):
    def __enter__(self): pass
    def __exit__(self, ext, exv, trb):
        if ext is not None: print "no not possible"
        print "OK I caught you"
        return True

with Mgr():
    name='rubicon'/2 #to raise an exception

The return True part is where the context manager decides to suppress the exception (as you do by not re-raising it in your except clause).

Answer 2

The contextlib.contextmanager function decorator provides a handy way of providing a context manager without the need to write a full-fledged ContextManager class of your own (with __enter__ and __exit__ methods, so you don't have to remember the arguments to the __exit__ method, or that the __exit__ method must return True in order to suppress the exception). Instead, you write a function with a single yield at the point you want the with block to run, and you trap any exceptions (that effectively come from the yield) as you normally would.

from contextlib import contextmanager
@contextmanager
def handler():
    # Put here what would ordinarily go in the `__enter__` method
    # In this case, there's nothing to do
    try:
        yield # You can return something if you want, that gets picked up in the 'as'
    except Exception as e:
        print "no not possible"
    finally:
        print "Ok I caught you"

with handler():
    name='rubicon'/2 #to raise an exception

Why go to the extra trouble of writing a context manager? Code re-use. You can use the same context manager in multiple places, without having to duplicate the exception handling. If the exception handling is unique to that situation, then don't bother with a context manager. But if the same pattern crops up again and again (or if it might for your users, e.g., closing a file, unlocking a mutex), it's worth the extra trouble. It's also a neat pattern to use if the exception handling is a bit complicated, as it separates the exception handling from the main line of code flow.

Answer 3

The with in Python is intended for wrapping a set of statements where you should set up and destroy or close resources. It is in a way similar to try...finally in that regard as the finally clause will be executed even after an exception.

A context manager is an object that implements two methods: __enter__ and __exit__. Those are called immediately before and after (respectively) the with block.

For instance, take a look at the classic open() example:

with open('temp.txt', 'w') as f:
    f.write("Hi!")

Open returns a File object that implements __enter__ more or less like return self and __exit__ like self.close().

Answer 4

The Components of Context Manager

1. You should implement an __enter__ method that returns an object
2. Implement a __exit__ method.

Example

I will give a simple example to show you why we need a context manager. During the winter of Xinjiang, China, you should close a door immediately when you open a door. if you forget to close it, you will get cold.

 class Door:
     def __init__(self):
         self.doorstatus='the door was closed when you are not at home'
         print(self.doorstatus)
     def __enter__(self):
         print('I have opened the door')
         return self
     def __exit__(self,*args):
         print('pong!the door has closed')
     def fetchsomethings(self):
         print('I have fetched somethings')

when fetch things at home, you should open a door, fetch somethings and close the door.

 with Door() as dr:
     dr.fetchsomethings()

the output is:

the door was closed when you are not at home
I have opened the door
I have fetched somethings
pong!the door has closed

Explanation

when you initiate a Door class, it will call __init__ method that will print "the door was closed when you are not in home" and __enter__ method that will print "I have opened the door" and return a door instance called dr. when call self.fetchsomethings in with block, the method will print "I have fetched somethings".when the block is finished.the context manager will invoke __exit__ method and it will print "pong!the door has closed" .when you do not use with keyword ,__enter__and __exit__ will not be invoked!!!!

Answer 5

with statements or context managers are there to aid with resources (although may be used for much more).

Let's say you opened a file for writing:

f = open(path, "w")

You now have an open file handle. During the handling of your file, no other program can write to it. In order to let other programs write to it, you must close the file handle:

f.close()

But, before closing your file an error occured:

f = open(path, "w")
data = 3/0  # Tried dividing by zero. Raised ZeroDivisionError
f.write(data)
f.close()

What will happen now is that the function or entire program will exit, while leaving your file with an open handle. (CPython cleans handles on termination and handles are freed together with a program but you shouldn't count on that)

A with statement ensures that as soon as you leave it's indentation, it will close the file handle:

with open(path, "w") as f:
    data = 3/0  # Tried dividing by zero. Raised ZeroDivisionError
    f.write(data)
# In here the file is already closed automatically, no matter what happened.

---

with statements may be used for many more things. For example: threading.Lock()

lock = threading.Lock()
with lock:  # Lock is acquired
   do stuff...
# Lock is automatically released.

Almost everything done with a context manager can be done with try: ... finally: ... but context managers are nicer to use, more comfortable, more readable and by implementing __enter__ and __exit__ provide an easy to use interface.

---

Creating context managers is done by implementing __enter__() and __exit__() in a normal class.

__enter__() tells what to do when a context manager starts and __exit__() when a context manager exists (giving the exception to the __exit__() method if an exception occurred)

A shortcut for creating context managers can be found in contextlib. It wraps a generator as a context manager.