When to use "with" in python

Question

I saw this question, and I understand when you would want to use with foo() as bar:, but I don't understand when you would just want to do:

bar = foo()
with bar:
   ....

Doesn't that just remove the tear-down benefits of using with ... as, or am I misunderstanding what is happening? Why would someone want to use just with?

Answer 1

To expand a bit on @freakish's answer, with guarantees entry into and then exit from a "context". What the heck is a context? Well, it's "whatever the thing you're with-ing makes it". Some obvious ones are:

• locks: you take a lock, manipulate some data, and release the lock.
• external files/streams: you open a file, read or write it, and close it.
• database records: you find a record (which generally also locks it), use some fields and maybe change them, and release the record (which also unlocks it).

Less obvious ones might even include certain kinds of exception trapping: you might catch divide by zero, do some arithmetic, and then stop catching it. Of course that's built into the Python syntax: try ... except as a block! And, in fact, with is simply a special case of Python's try/except/finally mechanisms (technically, try/finally wrapped around another try block; see comments).

The as part of a with block is useful when the context-entry provides some value(s) that you want to use inside the block. In the case of the file or database record, it's obvious that you need the newly-opened stream, or the just-obtained record. In the case of catching an exception, or holding a lock on a data structure, there may be no need to obtain a value from the context-entry.

Answer 2

For example when you want to use Lock():

from threading import Lock
myLock = Lock()
with myLock:
   ...

You don't really need the Lock() object. You just need to know that it is on.