On Jan 16, 2017, at 1:27 PM, Terry Reedy <tjre...@udel.edu> wrote:
>
> 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
> FileNotFoundError.
>
>> 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
> AttributeError: __enter__
>
> Is psql_cursor() returning a fake None object with __enter__ and __exit__
> methods?
No, the *context manager*, which I call in the with *does* have __enter__ and
__exit__ methods. It's just that the __enter__ method returns None when it
can't get a connection. So the expression following with *does* evaluate to a
context manager, but the expression following as evaluates to None.
-----------------------------------------------
Israel Brewster
Systems Analyst II
Ravn Alaska
5245 Airport Industrial Rd
Fairbanks, AK 99709
(907) 450-7293
-----------------------------------------------
>
> --
> Terry Jan Reedy
>
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