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Date: Fri, 4 Sep 2015 02:11:27 +1000
Subject: Re: continue vs. pass in this IO reading and writing
From: Chris Angelico <rosuav@gmail.com>
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Xref: csiph.com comp.lang.python:95953

On Fri, Sep 4, 2015 at 1:57 AM, kbtyo <ahlusar.ahluwalia@gmail.com> wrote:
> I have used CSV and collections. For some reason when I apply this algorithm, all of my files are not added (the output is ridiculously small considering how much goes in - think KB output vs MB input):
>
> from glob import iglob
> import csv
> from collections import OrderedDict
>
> files = sorted(iglob('*.csv'))
> header = OrderedDict()
> data = []
>
> for filename in files:
>     with open(filename, 'r') as fin:
>         csvin = csv.DictReader(fin)
>         header.update(OrderedDict.fromkeys(csvin.fieldnames))
>         data.append(next(csvin))
>
> with open('output_filename_version2.csv', 'w') as fout:
>     csvout = csv.DictWriter(fout, fieldnames=list(header))
>     csvout.writeheader()
>     csvout.writerows(data)

You're collecting up just one row from each file. Since you say your
input is measured in MB (not GB or anything bigger), the simplest
approach is probably fine: instead of "data.append(next(csvin))", just
use "data.extend(csvin)", which should grab them all. That'll store
all your input data in memory, which should be fine if it's only a few
meg, and probably not a problem for anything under a few hundred meg.

ChrisA