Exercise 17: More Files

Now let's do a few more things with files. We're going to actually write a Python script to copy one file to another. It'll be very short but will give you some ideas about other things you can do with files.

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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()

You should immediately notice that we import another handy command named exists. This returns True if a file exists, based on it's name in a string as an argument. It returns False if not. We'll be using this function in the second half of this book to do lots of things, but right now you should see how you can import it.

Using import is a way to get tons of free code other better (well, usually) programmers have written so you do not have to write it.

What You Should See

Just like your other scripts, run this one with two arguments, the file to copy from and the file to copy it to. If we use your test.txt file from before we get this:

$ python ex17.py test.txt copied.txt
Copying from test.txt to copied.txt
The input file is 81 bytes long
Does the output file exist? False
Ready, hit RETURN to continue, CTRL-C to abort.

Alright, all done.

$ cat copied.txt
To all the people out there.
I say I don't like my hair.
I need to shave it off.
$

It should work with any file. Try a bunch more and see what happens. Just be careful you do not blast an important file.

Warning

Did you see that trick I did with cat? It only works on Linux or OSX, sorry Windows people!

Extra Credit

  1. Go read up on Python's import statement, and start python to try it out. Try importing some things and see if you can get it right. It's alright if you do not.
  2. This script is really annoying. There's no need to ask you before doing the copy, and it prints too much out to the screen. Try to make it more friendly to use by removing features.
  3. See how short you can make the script. I could make this 1 line long.
  4. Notice at the end of the WYSS I used something called cat? It's an old command that "con*cat*enates" files together, but mostly it's just an easy way to print a file to the screen. Type man cat to read about it.
  5. Windows people, find the alternative to cat that Linux/OSX people have. Do not worry about man since there is nothing like that.
  6. Find out why you had to do output.close() in the code.

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Exercise 16: Reading And Writing Files

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Exercise 18: Names, Variables, Code, Functions

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