[Tutor] reading file, adding to each line, writing file

[Alan Gauld]

"David" <ldl08 at gmx.net> wrote

> Here is my latest try, which works:
> 
> # add " -d" to each line of a textfile
> 
> infile = open("step3", 'r') 
> outfile = open("pyout","a") 
> 
> line = infile.readline()    # Invokes readline() method on file

line is now a string representing a line in the file.

> for i in line:

You are now iterating over every character in line

>    line2 = line[:-1] + " -d\n"

So you repeat this assignment for every character. 
No harm done its the same every time. Just very inefficient!

>    outfile.write(line2),     # trailing ',' omits newline character

But you are writing the same line for every character - did 
you find lots of duplicate lines in the output?

>    line = infile.readline()

And now you reset line to the next line so invalidating some 
of what I njust said. I doubt this actually works properly although 
it may appear to!

The use of the for loop is much simpler, avoiding all the 
readline confusion:

infile = open("step3", 'r')
outfile = open("pyout","a") 

for line in infile:   # no need for readline
    outfile.write(line.rstrip() + " -d\n")
      
infile.close()
outfile.close()
print "done!"

That's all you need!
for loops reading direct from the file are ideally suited to this 
kind of program and indeed any situation where you are 
handling a fixed size input. 
while loops are better for when you don't know where the 
end will be or even if there will be an end!

HTH,

Alan G.