Writing lists to a file

After creating my list from the previous post I needed to save this to a text file to send back to the person who had requested the information. I just wrote a standard loop over the list but this got me thinking about a better way to do this as it’s a common task. As it turns out not really but it did bring up a few titbits worth mentioning.

If you are from a PERL background you might be tempted to use a single write and a join to turn the list into a string as follows


Don’t. This creates a new string in memory from the list and then writes that. Ok if it is a small list but a huge waste of memory for larger ones.

For the more experienced Python user, using a generator produces arguably a cleaner syntax and saves a line of typing. As with all generators, you can make it conditonal by adding an if clause to the end.

with open(r'x:\path\to\file.txt','w') as txtfile:
  txtfile.writelines( "%s\n" % item for item in mylist )

That covers off the text file, but there may be a better option depending upon what is going to be done with the resulting file. I’ll finish the following couple of examples.

# if the file is going to Excel, maybe a CSV file
import csv
with open(r'x:\path\to\file.csv','w') as csvfile:
  csv = csv.writer(csvfile)
  for item in mylist:

# or how about json if it is going to another program
import json # Python 2.5 use simplejson
with open(r'x:\path\to\file.json','w') as jsonfile:

See the Python docs for the csv module and json module.


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