Appending to dictionary of lists

[Alex van der Spek]

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

 

[Paul Rubin]

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.

 

[Alex van der Spek]

Thank you! Would never have found that by myself.

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.