On Apr 30, 2:00 pm, Sneaky Wombat <joe.hr...@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
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