Re: Basic list/dictionary question

[Ralax]

On Nov 11, 8:58 pm, Chris Rebert  wrote:
> On Wed, Nov 11, 2009 at 4:16 AM, Daniel Jowett  
> wrote:
> > Greetings,
>
> > I'm trying to categorize items in a list, by copying them into a
> > dictionary...
> > A simple example with strings doesn't seem to work how I'd expect:
>
>  basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
>  d = {}
>  d = d.fromkeys(basket, [])
>  d
> > {'orange': [], 'pear': [], 'apple': [], 'banana': []}
>  for fruit in basket:
> > ... d[fruit].append(fruit)
> > ...
>
> > No if I print d I'd EXPECT
>  d
> > {'orange': ['orange', 'orange'], 'pear': ['pear'], 'apple': ['apple',
> > 'apple'], 'banana': ['banana']}
>
> > But what I GET is
>  d
> > {'orange': ['apple', 'orange', 'apple', 'pear', 'orange', 'banana'], 'pear':
> > ['apple', 'orange', 'apple', 'pear', 'orange', 'banana'], 'apple': ['apple',
> > 'orange', 'apple', 'pear', 'orange', 'banana'], 'banana': ['apple',
> > 'orange', 'apple', 'pear', 'orange', 'banana']}
>
> > From what I can work out, there is only ONE list that is referenced from the
> > dictionary 4 times. Which would be because the same empty list is assigned
> > to every key in the dictionary by the "fromkeys" line. But that seems
> > seriously counter-intuitive to me...
>
> Python doesn't do any extra copying in most places unless you
> /explicitly/ do so yourself or ask it to; so yes, in this case, Python
> just copies references to the same object and does not copy the object
> itself.
>
> You'd probably be better off using a defaultdict in your particular 
> usecase:http://docs.python.org/library/collections.html#collections.defaultdict
>
> Or and so you avoid running into it, default argument values aren't
> copied either:
> In [2]: def foo(z, a=[]):
>    ...:         a.append(z)
>    ...:     return a
>    ...:
>
> In [3]: foo(1)
> Out[3]: [1]
>
> In [4]: foo(2)
> Out[4]: [1, 2]
>
> In [5]: foo(2)
> Out[5]: [1, 2, 2]
>
> In [6]: foo(3)
> Out[6]: [1, 2, 2, 3]
>
> In [7]: foo(4,[])
> Out[7]: [4]
>
> In [8]: foo(5)
> Out[8]: [1, 2, 2, 3, 5]
>
> Cheers,
> Chris
> --http://blog.rebertia.com

Usefull!
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Re: Unexpected python exception

[Ralax]

On Nov 11, 6:59 pm, Richard Purdie  wrote:
> I've been having problems with an unexpected exception from python which
> I can summarise with the following testcase:
>
> def A():
>     import __builtin__
>     import os
>
>     __builtin__.os = os
>
> def B():
>     os.stat("/")
>     import os
>
> A()
> B()
>
> which results in:
>
> Traceback (most recent call last):
>   File "./test.py", line 12, in 
>     B()
>   File "./test.py", line 8, in B
>     os.stat("/")
> UnboundLocalError: local variable 'os' referenced before assignment
>
> If I remove the "import os" from B(), it works as expected.
>
> >From what I've seen, its very unusual to have something operate
>
> "backwards" in scope in python. Can anyone explain why this happens?
>
> Cheers,
>
> Richard

One word, Python treat objects in a function or method-scope as
locals.
So os is not defined in B(), is it right?
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