Python Cookbook

Writing to a File

Credit: Luther Blissett

Problem

You want to write text or data to a file.

Solution

Here is the most convenient way to write one big string to a file:

open('thefile.txt', 'w').write(all_the_text)  # text to a text file
open('abinfile', 'wb').write(all_the_data)    # data to a binary file

However, it is better to bind the file object to a variable so that you can call close on it as soon as you’re done. For example, for a text file:

file_object = open('thefile.txt', 'w')
file_object.write(all_the_text)
file_object.close(  )

More often, the data you want to write is not in one big string but in a list (or other sequence) of strings. In this case, you should use the writelines method (which, despite its name, is not limited to lines and works just as well with binary data as with text files):

file_object.writelines(list_of_text_strings)
open('abinfile', 'wb').writelines(list_of_data_strings)

Calling writelines is much faster than either joining the strings into one big string (e.g., with ''.join) and then calling write, or calling write repeatedly in a loop.

Discussion

To create a file object for writing, you must always pass a second argument to open—either 'w' to write textual data, or 'wb' to write binary data. The same considerations illustrated in Recipe 4.2 also apply here, except that calling close explicitly is even more advisable when you’re writing to a file rather than reading from it. Only by closing the file can you be reasonably sure that the data is actually on ...