Trying to understand python with statement and context managers

python contextmanager
user444997 picture user444997 · Sep 12, 2010 · Viewed 12k times · Source

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

Alex Martelli picture Alex Martelli · Sep 12, 2010

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).

Related questions

Create a "with" block on several context managers?

Suppose you have three objects you acquire via context manager, for instance A lock, a db connection and an ip socket. You can acquire them by: with lock: with db_con: with socket: #do stuff But is there a way …

Read More
File open and close in python

I have read that when file is opened using the below format with open(filename) as f: #My Code f.close() explicit closing of file is not required . Can someone explain why is it so ? Also if someone does explicitly …

Read More
StringIO and compatibility with 'with' statement (context manager)

I have some legacy code with a legacy function that takes a filename as an argument and processes the file contents. A working facsimile of the code is below. What I want to do is not have to write to …

Read More