Writing Files

Python is equally good at writing to files as it is at reading them. Open a new Python script nano write_times_table.py and type;

from __future__ import print_function
import sys

filename = sys.argv[1]
n = int( sys.argv[2] )

FILE = open( filename, "w" )

for i in range(1, 11):
    print("%d times %d equals %d" % ( i, n, i*n ), file=FILE)


Run this script by typing;

python write_times_table.py five.txt 5

This should result in the five times table being written to the file five.txt in the current directory.

The part of the line FILE = open( filename, "w" ) opens the file whose path is the variable filename and connects it to the filehandle FILE. This time however, the file is opened using mode "w", so the file is opened for writing, not reading. If the file does not exist, then the file is created, and it it does exist, then the file is overwritten (so be careful not to overwrite any of your important files!).

There are three different modes for opening files;

To write to the file, supply the filehandle to the print command, e.g. as in the script type print("%d times %d equals %d" % ( i, n, i*n ) file=FILE). The filehandle is supplied using the file=FILE at the end of the print function.

Finally, when you have finished writing to a file you should close it using the close command. This ensures that what you have written is properly copied to disc (as it may up to this point be buffered in memory).

Filehandles allow you to refer to more than one file at a time. For example, we could modify the script that numbered each line of the file so that it wrote the numbered lines to another file. For example;

from __future__ import print_function
import sys

filename = sys.argv[1]

numbered_filename = "%s_numbered" % filename

RFILE = open( filename, "r" )
WFILE = open( numbered_filename, "w" )

lines = RFILE.readlines()

for i in range( 0, len(lines) ):
    print("%4d: %s" % ( i, lines[i] ), file=WFILE, end="")


(note that numbered_filename = "%s_numbered" % filename uses the same syntax as print, except now the output is returned to a new string variable, rather than printed to the screen. So if filename contained the string file.txt, then numbered_filename would be set equal to file.txt_numbered)

Compare with Perl

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