is this foolish?

[Jean-Michel Pichavant]

Cameron Simpson wrote:
> I've found myself using a Python gotcha as a feature.
>
> I've got a budding mail filter program which keeps rule state in a
> little class instance. Slightly paraphrased:
>
>     class RuleState(object):
>         def __init__(self, M, maildb_path, maildirs={}):
>             [...]
>             self.maildirs = maildirs
>
> The maildirs property is a cache of Maildir objects mapped by their
> pathname to avoid opening Maildirs every time they're mentioned. I
> create a new RuleState every time I file a message, and of course I want
> to share the cache between instances.
>
> Normally we look on the Python default parameter value as a gotcha which
> causes the unwary to reuse a single object across the board, causing
> unwanted persistence of state.
>
> But here I actually think this is a sane way to make an anonymous single
> shared state object for the maildirs cache, absent the caller's intent
> to use their own.
>
> I can think of a few potential downsides, but on the whole this is going
> to do exactly what I want in this case.
>
> Would experienced users please mock me?
>   
Instance attributes are not meant to be shared among all instances. 
Assigning a persistant object over instances to an instance attribute is 
perverting your design.
Remember that experienced programmers will automaticcaly trigger a 
warning in their mind when reading your mutable maildirs. They 'll lose 
time to realize that it was intended. Since there is much more standard 
ways to do this (class attribute for instance), you should not use that 
form.

Cleverness is the step mother of obfuscation.

JM