cache backends should implement __contains__ so that the in operator may be used

[Gary Wilson <gary.wilson@…>]

So instead of

cache.has_key(mykey)

one could do

mykey in cache

[Gary Wilson <gary.wilson@…>]

added __contains__ to BaseCache class

[Gary Wilson <gary.wilson@…>]

So in my patch, the __contains__ method in BaseCache simply calls has_key. This is nice since all the cache subclasses automatically pick this up for free, but also means that a

mykey in cache

statement becomes slower than

cache.has_key(mykey)

because of the extra function call. If we care about performance here, we might want to implement __contains__ for each cache class without calling that class's has_key method. Looking at the python standard library, it seems that classes with both __contains__ and has_key methods just put the same code in each instead of having one call the other.

[Malcolm Tredinnick]

Not yet ready to checkin, due to Gary's last comment. Checking in an optimisation that slows the code down seems like something we should avoid.

Note sure why this was marked as needing documentation, either, but if you think it needs documentation, Gary, by all means write some.

[Chris Beaven]

optimized version

[Chris Beaven]

Unless I'm missing something, can't we just do this? (see patch)

[Gary Wilson <gary.wilson@…>]

Replying to mtredinnick:

Note sure why this was marked as needing documentation, either, but if you think it needs documentation, Gary, by all means write some.

I was just thinking it might be good to be mentioned in the cache documentation (there is no mention of has_key() method currently).

[Malcolm Tredinnick]

Replying to SmileyChris:

Unless I'm missing something, can't we just do this? (see patch)

Yes, it is possible to make __contains__ and has_key be aliases for each other like this. In the early days, when __contains__ was first introduced into the language, it wasn't clear if this was going to be a good identification, but nowadays, it's accepted as correct; to the point that even Python core checkins are using this idiom in places.

What that patch doesn't need is the comment: it's the equivalent of saying "add one to i" in a comment. People know what __contains__ does already.

[Malcolm Tredinnick]

(In [5171]) Fixed #4041 -- Added a contains method to cache backends. Thanks, Gary
Wilson and SmileyChris.