writing to file very slow

Jack Diederich jack at performancedrivers.com
Wed Mar 26 16:55:13 CET 2003


On Wed, Mar 26, 2003 at 03:54:20PM +0100, Moritz Lennert wrote:
> Hello,
> 
> I have written a cgi script that uses PyGreSQL to query a PostgreSQL
> database on the basis of the input of an html form. The results of the
> query are stored in a file for the user to download.
> 
> Everything seems to work fine, except for the fact that writing the file
> is very slow (example: 4 minutes for 4 thousand lines). I've even had the
> problem that the program stopped writing the file before coming to the end
> of the results, then launches the same query again and starts writing a
> new file, which "crashes" again, triggering a new query, etc.
> 
> I use the following modules:
> 
> import cgi
> import os
> import _pg
> import string
> import tempfile
> 
> And here is the relevant function for writing the file:
> 
> def fichier_resultats(results):
>   temdir="/tmp"
>   tfilename = tempfile.mktemp('rec.txt')
>   f=open(tfilename,'w')
> 
>   varnames=""
>   for z in range(len(results.listfields())-1):
>     varnames += str(results.fieldname(z))+'|'
>   varnames += str(results.fieldname(len(results.listfields())-1))
>   f.write(varnames)
>   f.write("\n")
>   for x in range(results.ntuples()):
>    var = ""
>    for y in range(len(results.listfields())-1):
>      var+=str(results.getresult()[x][y])+'|'
>    var+=str(results.getresult()[x][len(results.listfields())-1])
>    f.write(var)
>    f.write('\n')
>   f.close()
>   return(tfilename)

What everyone else said plus,
  you call getresult() alot, does it's return value ever change?
  is it doing alot of work?  you could call it just once
 
  if the list of fields never changes, just calculate the string once
  and output it many times

  listfields() is used only for its length

  see my comment below, do you really only want to output the last
  value in each row of getresult() ?

modified function below (!warning, untested!)
I use lambdas unapoligetically, feel free to translate into list comps

def fichier_resultats(results):
  temdir="/tmp"
  tfilename = tempfile.mktemp('rec.txt')
  f=open(tfilename,'w')

  # you could also 
  varnames = '|'.join(map(lambda i: str(results.fieldname(i)), range(len(results.listfields()))))
  f.write(varnames)
  f.write("\n")
  gotten_result = results.getresult() # we only call this once
  field_str = '|'.join(map(str, gotten_result[0]))
  for row in gotten_result:
    f.write(field_str)
    f.write(row[-1]) # is this really what you want? translation of below line
    # var+=str(results.getresult()[x][len(results.listfields())-1])
    f.write('\n')
  f.close()
  return(tfilename)


-jackdied





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