Question

While working through this exercise I ran into a problem.

from sys import argv
from os.path import exists

script, from_file, to_file = argv
print "Copying from %s to %s" % (from_file, to_file)

# we could do these two on one line too, how?
input = open(from_file)
indata = input.read()

print "The input file is %d bytes long" % len(indata)
print "Does the output file exist? %r" % exists(to_file)
print "Ready, hit RETURN to continue, CTRL-C to abort."

raw_input()

output = open(to_file, 'w')
output.write(indata)
print "Alright, all done."
output.close()
input.close()

The line # we could do these two on one line too, how? is what’s confusing me. The only answer I could come up with was:

indata = open(from_file).read()

This performed the way I wanted to, but it requires me to remove:

input.close()

as the input variable no longer exists. How then, can I perform this close operation?

How would you solve this?

Answer 1

The preferred way to work with resources in python is to use context managers:

 with open(infile) as fp:
    indata = fp.read()

The with statement takes care of closing the resource and cleaning up.

You could write that on one line if you want:

 with open(infile) as fp: indata = fp.read()

however, this is considered bad style in python.

You can also open multiple files in a with block:

with open(input, 'r') as infile, open(output, 'w') as outfile:
    # use infile, outfile

Funny enough, I asked exactly the same question back when I started learning python.