[Tutor] Problem iterating over csv.DictReader

Serdar Tumgoren zstumgoren at gmail.com
Mon Apr 26 17:57:09 CEST 2010

> I have noticed this odd behaviour in the CSV DictReader Class, and at a
> loss to understand/ get around it.
> The aim is to read in a CSV file, and then iterate over the lines. The
> problem (seems) to be that once you have iterated over it once, you can't do
> it again.
> This is expected behavior. See below from the Python docs:


If you'd like to make multiple passes over the lines from your CSV, store
them in a variable when you first read them in and then loop over that
variable instead.

One approach (though it may not be the best if you're dealing with huge
quantities of data):

reader = csv.DictReader(open(fname, 'r'), delimiter = ',', quotechar = '"')
data = [row for row in reader]
# now you can make multiple passes over the dictionaries stored in "data"

> I don't know if this is me (and it may well be) but it seems to be a
> recurrent issue, and means that a csv.DictReader doesn't behave in the same
> way as a normal dict object.

Correct. It's not supposed to behave like a normal dict. It behaves like a
reader object. See the python docs:

"Create an object which operates like a regular reader but maps the
information read into a dict whose keys are given by the optional *
fieldnames* parameter."

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