Writing to text files

The mechanics of writing to text files are very straightforward. Suppose we need to store a list of coordinates to a text file, e.g.

coords = [[12, 31], [75, 19], [28, 51]]

Writing them to a file could be done by code like this,

# write_coords.py
coords = [[12, 31], [75, 19], [28, 51]]
fname = input('Name of file to create? ')
f = open(fname, 'w')
for coord in coords:
    f.write(str(coord[0]) + ' ' + str(coord[1]) + '\n')
f.close()

The pattern is similar to reading from a file:

• We open the file, but with a mode of w for write. Note the wording of the prompt 'Name of file to create? ' Opening a file for writing deletes an existing file of the same name if one exists! In production code it is critical to either know you can clobber existing files, or to check whether a file name has already been used, and then prompt the user for another name if existing files shouldn't be clobbered.
• We direct output to the file using the file object's write method. This method wants a string argument so we need to format our coordinates accordingly (f.write(coord[0])) would throw an error). Note that we also have to add the newline character ourselves.
• When we are done we close the file.

The resulting file looks like this if you open it in a text editor, e.g. IDLE,

12 31
75 19
28 51