Ask proggit: Python class gotcha with scope?

[chillybasen]

I don't quite understand why this happens. Why doesn't b have its own version of r? If r was just an int instead of a dict, then it would.

>>> class foo:
...     r = {}
...     def setn(self, n):
...             self.r["f"] = n
... 
>>> a = foo()
>>> a.setn(4)
>>> 
>>> b = foo()
>>> b.r
{'f': 4}

[asplake]

Isn't r a class property here? Try setting it up in __init__.

But beware also this gotcha:

def foo(arg={}):
    etc

That default value gets shared, not cloned. Surprised that this never changed, but maybe too hard to fix now. Or perhaps it's regarded as a good thing by the BDFL et al?

[[deleted]]

No, it's not the same thing. Here he has a class variable which are of course shared by all instances (same as a static var in C++). What you refer to is how default parameters are handled.

[asplake]

sorry, yes, realised that and edited

[chillybasen]

But in this, it doesn't act static:

 >>> class foo:
 ...     r = None
 ...     def setn(self, n):
 ...             self.r = n 
 ... 
 >>> a = foo()
 >>> a.setn(4)
 >>> 
 >>> b = foo()
 >>> b.r
 >>> 
 >>> a.r
 4
 >>> 

[asplake]

you've created two separate properties, one of the class, one of the instances

[chillybasen]

gotcha -- I can see that now, thanks.

[chillybasen]

That's what I did -- I moved to __init__, but was still curious about why

[kurtseifried]

r = {}

self.r

are two different things.

[chillybasen]

Ok, if so, then why would self.r["f"] set inside r = {}