[Tutor] print to file probelm

[Peter Otten]

John Collins wrote:

> Hi,
> I'm a baffled beginner. The script below is one of a suite of scripts
> written by Simon Tatham a decade ago;
> http://www.chiark.greenend.org.uk/~sgtatham/polyhedra/
> http://www.chiark.greenend.org.uk/~sgtatham/polyhedra/polyhedra.tar.gz
> 
> OS:     WinXP
> Py Ver: 2.7
> Script: canvas.py
> 
> It reads from a file name in the command line and should print to a file
> name also specified in the command line. It inputs and runs fine but
> outputs to the command prompt *screen* while creating and *empty* output
> file of the name specified? I cannot figure out why?

> args = sys.argv[1:]
> 
> if len(args) > 0:
>      infile = open(args[0], "r")
>      args = args[1:]
> else:
>      infile = sys.stdin
> 
> if len(args) > 0:
>      outfile = open(args[0], "w")
>      args = args[1:]
> else:
>      outfile = sys.stdout

If you pass two args this will open two files, infile for reading, and 
outfile for writing. While infile is actually used...

> while 1:
>      s = infile.readline()
>      if s == "": break
       ...
> infile.close()

... outfile is not. The print statement

> print s

will always write to stdout. You can modify the print statement to

print >> outfile, s

or use

outfile.write(s + "\n")

instead. It is good style to close the file explicitly when you're done with

outfile.close()