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Appending to dictionary of lists

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  Appending to dictionary of lists "Alex van der Spek" <zdoor@xs4all.nl> - 2011-05-03 21:50 +0200
    Re: Appending to dictionary of lists Paul Rubin <no.email@nospam.invalid> - 2011-05-03 12:56 -0700
      Re: Appending to dictionary of lists "Alex van der Spek" <zdoor@xs4all.nl> - 2011-05-03 22:20 +0200

#4569 — Appending to dictionary of lists

I open a csv file and create a DictReader object. Subsequently, reading 
lines from this file I try to update a dictionary of lists:

csvf=open(os.path.join(root,fcsv),'rb')
csvr=csv.DictReader(csvf)
refd=dict.fromkeys(csvr.fieldnames,[])
for row in csvr:
     for (k,v) in row.items():
           refd[k].append(v)

I do not understand why this appends v to every key k each time.

Thanks in advance for any tips you can pass on.

Alex van der Spek 

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#4571

"Alex van der Spek" <zdoor@xs4all.nl> writes:
> refd=dict.fromkeys(csvr.fieldnames,[])  ...
> I do not understand why this appends v to every key k each time.

You have initialized every element of refd to the same list.  Try

    refd = dict((k,[]) for k in csvr.fieldnames)

instead.

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#4572

Thank you! Would never have found that by myself.


"Paul Rubin" <no.email@nospam.invalid> wrote in message 
news:7x7ha75zib.fsf@ruckus.brouhaha.com...
> "Alex van der Spek" <zdoor@xs4all.nl> writes:
>> refd=dict.fromkeys(csvr.fieldnames,[])  ...
>> I do not understand why this appends v to every key k each time.
>
> You have initialized every element of refd to the same list.  Try
>
>    refd = dict((k,[]) for k in csvr.fieldnames)
>
> instead. 

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