using zip() and dictionaries

[Simon Forman]

On Apr 30, 2:00 pm, Sneaky Wombat <joe.hr... at gmail.com> wrote:
> quick update,
>
> #change this line:
> for (k,v) in zip(header,[[]]*len(header)):
> #to this line:
> for (k,v) in zip(header,[[],[],[],[]]):
>
> and it works as expected.  Something about the [[]]*len(header) is
> causing the weird behavior.  I'm probably using it wrong, but if
> anyone can explain why that would happen, i'd appreciate it.  My guess
> is that it's iterating through the the whole dictionary because of the
> value on the right in zip().
>
> On Apr 30, 12:45 pm, Sneaky Wombat <> wrote:
>
> > I'm really confused by what is happening here.  If I use zip(), I
> > can't update individual dictionary elements like I usually do.  It
> > updates all of the dictionary elements.  It's hard to explain, so here
> > is some output from an interactive session:
>
> > In [52]: header=['a','b','c','d']
> > In [53]: columnMap={}
> > In [54]: for (k,v) in zip(header,[[]]*len(header)):
> >    ....:     #print "%s,%s"%(k,v)
> >    ....:     columnMap[k] = v
> >    ....:
> > In [55]: columnMap
> > Out[55]: {'a': [], 'b': [], 'c': [], 'd': []}
> > In [56]: columnMap['a'].append('something')
> > In [57]: columnMap
> > Out[57]:
> > {'a': ['something'],
> >  'b': ['something'],
> >  'c': ['something'],
> >  'd': ['something']}
>
> > Why does ['something'] get attached to all columnMap elements instead
> > of just element 'a'?
>
> > In [58]: columnMap={'a': [], 'b': [], 'c': [], 'd': []}
> > In [59]: columnMap['a'].append('something')
> > In [60]: columnMap
> > Out[60]: {'a': ['something'], 'b': [], 'c': [], 'd': []}
>
> > creating the dictionary without using zip, it works as normal.
>
>

You figured out your original question, I just wanted to point out
that your code as posted is less efficient than it could be.
Rather than creating a list of empty lists to feed to zip() you could
just assign a new empty list to columnMap in your loop body, like so:

In [54]: for k in header:
   ....:     columnMap[k] = []

That avoids creating a list of lists just to pass to zip() the call to
zip() too.
~S