How to write header row with csv DictWriter

Question

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

User · Answer

Another way to do this would be to add before adding lines in your output, the following line :

output.writerow(dict(zip(dr.fieldnames, dr.fieldnames)))

The zip would return a list of doublet containing the same value. This list could be used to initiate a dictionary.

User · Answer

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

User · Answer

A few options    1  Laboriously make an identity-mapping  i e  do-nothing  dict out of your fieldnames so that csv DictWriter can convert it back to a list and pass it to a csv writer instance     2  The documentation mentions  the underlying writer instance      so just use it  example at the end    dw writer writerow dw fieldnames     3  Avoid the csv Dictwriter overhead and do it yourself with csv writer  Writing data   w writerow  d k  for k in fieldnames     or  w writerow  d get k  restval  for k in fieldnames     Instead of the extrasaction  functionality   I d prefer to code it myself  that way you can report ALL  extras  with the keys and values  not just the first extra key  What is a real nuisance with DictWriter is that if you ve verified the keys yourself as each dict was being built  you need to remember to use extrasaction  ignore  otherwise it s going to SLOWLY  fieldnames is a list  repeat the check   wrong fields    k for k in rowdict if k not in self fieldnames                   gt  gt  gt  f   open  csvtest csv    wb    gt  gt  gt  import csv  gt  gt  gt  fns    foo bar zot  split    gt  gt  gt  dw   csv DictWriter f  fns  restval  Huh      dw writefieldnames fns  -- no such animal  gt  gt  gt  dw writerow fns    no such luck  it can t imagine what to do with a list Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File  C  python26 lib csv py   line 144  in writerow     return self writer writerow self  dict to list rowdict     File  C  python26 lib csv py   line 141  in  dict to list     return  rowdict get key  self restval  for key in self fieldnames  AttributeError   list  object has no attribute  get   gt  gt  gt  dir dw      doc        init        module       dict to list    extrasaction    fieldnam es    restval    writer    writerow    writerows     eureka  gt  gt  gt  dw writer writerow dw fieldnames   gt  gt  gt  dw writerow   foo   oof     gt  gt  gt  f close    gt  gt  gt  open  csvtest csv    rb   read    foo bar zot r noof Huh  Huh  r n   gt  gt  gt

[python] How to write header row with csv.DictWriter?

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