Issue
Assume I have a csv.DictReader
object and I want to write it out as a CSV file. How can I do this?
I know that I can write the rows of data like this:
dr = csv.DictReader(open(f), delimiter='\t')
# process my dr object
# ...
# write out object
output = csv.DictWriter(open(f2, 'w'), delimiter='\t')
for item in dr:
output.writerow(item)
But how can I include the fieldnames?
Solution
Edit:
In 2.7 / 3.2 there is a new writeheader()
method. Also, John Machin's answer provides a simpler method of writing the header row.
Simple example of using the writeheader()
method now available in 2.7 / 3.2:
from collections import OrderedDict
ordered_fieldnames = OrderedDict([('field1',None),('field2',None)])
with open(outfile,'wb') as fou:
dw = csv.DictWriter(fou, delimiter='\t', fieldnames=ordered_fieldnames)
dw.writeheader()
# continue on to write data
Instantiating DictWriter requires a fieldnames argument.
From the documentation:
The fieldnames parameter identifies the order in which values in the dictionary passed to the writerow() method are written to the csvfile.
Put another way: The Fieldnames argument is required because Python dicts are inherently unordered.
Below is an example of how you'd write the header and data to a file.
Note: with
statement was added in 2.6. If using 2.5: from __future__ import with_statement
with open(infile,'rb') as fin:
dr = csv.DictReader(fin, delimiter='\t')
# dr.fieldnames contains values from first row of `f`.
with open(outfile,'wb') as fou:
dw = csv.DictWriter(fou, delimiter='\t', fieldnames=dr.fieldnames)
headers = {}
for n in dw.fieldnames:
headers[n] = n
dw.writerow(headers)
for row in dr:
dw.writerow(row)
As @FM mentions in a comment, you can condense header-writing to a one-liner, e.g.:
with open(outfile,'wb') as fou:
dw = csv.DictWriter(fou, delimiter='\t', fieldnames=dr.fieldnames)
dw.writerow(dict((fn,fn) for fn in dr.fieldnames))
for row in dr:
dw.writerow(row)
Answered By - mechanical_meat Answer Checked By - Timothy Miller (PHPFixing Admin)
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