Error handling in context managers
tjreedy at udel.edu
Mon Jan 16 17:27:56 EST 2017
On 1/16/2017 1:06 PM, Israel Brewster wrote:
> I generally use context managers for my SQL database connections, so I can just write code like:
> with psql_cursor() as cursor:
> <do whatever>
> And the context manager takes care of making a connection (or getting a connection from a pool, more likely), and cleaning up after the fact (such as putting the connection back in the pool), even if something goes wrong. Simple, elegant, and works well.
> The problem is that, from time to time, I can't get a connection, the result being that cursor is None,
This would be like open('bad file') returning None instead of raising
> and attempting to use it results in an AttributeError.
Just as None.read would.
Actually, I have to wonder about your claim. The with statement would
look for cursor.__enter__ and then cursor.__exit__, and None does not
have those methods. In other words, the expression following 'with'
must evaluate to a context manager and None is not a context manager.
>>> with None: pass
Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
with None: pass
Is psql_cursor() returning a fake None object with __enter__ and
Terry Jan Reedy
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