Weird Python behaviour

Jonas Nilsson jmnc at spray.se
Tue Aug 10 18:47:34 CEST 2010


On 10 Ago, 13:58, Jonas Nilsson <j... at spray.se> wrote:
....

> You stumbled in two python common pitfalls at once :-)
> One, the default arguments issue, was already pointed to you.
>
> The other one is that python variables are just names for objects.
> Assigning a variable never mean making a copy, it just means using
> another name for the same object.
> There used to be a very nice (also graphic) explanationor this
> somewhere on the web, but my googling skills failed me this time,
> so instead I'll show you the concept using your own code:
>
>>>> class Family:
> ...     def __init__(self, fName, members = []):
> ...             self.fname = fName
> ...             self.members = members
> ...
>>>> mlist = ["Bill"]
>>>> f1 = Family("Smiths", mlist )
>>>> mlist.append( "John" ) # attempt to not-so-clever reyse of the  
>>>> sme variable
>>>> f2 = Family("Smithers", mlist )
>>>> f1.members
> ['Bill', 'John']
>
> Now my example is a bit contrieved but I'm sure you got the idea : in
> your example is better to copy the list with
>                  self.members = members[:].
>
> Better yet, you could make use of python arguments grouping feature :
>>>> class Family:
> ...     def __init__(self, fName, *members ):
> ...             self.members = list(members) # because members is a
> tuple
> ...             self.fname = fName
> ...
>>>> f1 = Family("Smith")
>>>> f1.members.append("Bill")
>>>> f2 = Family("Smithers")
>>>> f2.members.append("Joe")
>>>> f2.members
> ['Joe']
>>>> f1.members
> ['Bill']
>
> This solves your "no initial member" special case and allows for an
> easier syntax for creating class instances
> (no brackets involved)
>
>>>> f3 = Family("Bochicchio", "Angelo", "Francesco", "Mario")
>>>> f3.members
> ['Angelo', 'Francesco', 'Mario']
>>>>
>
>
> Ciao
> ----
> FB

Thanks everyone.

I was a bit steamed about the problem as it was very unexpected.  
Because I couldn't figure out  what key words to google on, I resorted  
to posting the problem here.

/Jonas



More information about the Python-list mailing list