[Tutor] More Pythonic?

[Steven D'Aprano]

On Wed, Sep 09, 2015 at 09:05:23AM -0400, richard kappler wrote:
> Would either or both of these work, if both, which is the better or more
> Pythonic way to do it, and why?

The first works, but isn't really the best way to do it:

>  #######################
> 
> import whatIsNeeded
> 
> writefile = open("writefile", 'a')
> 
> with open(readfile, 'r') as f:
>     for line in f:
>         if keyword in line:
>             do stuff
>             f1.write(line)
>         else:
>             f1.write(line)
> 
> writefile.close()
> 
> ######################

Better would be this:

with open("writefile", 'a') as outfile:
    with open("readfile", 'r') as infile:
        for line in infile:
            if keyword in line:
                do stuff
            outfile.write(line)
    
(I think your intention is to always write the lines into the output 
file, but there are enough typos in your version that I can't be 
completely sure.)



This, on the other hand, is certainly not what you want:

> import whatIsNeeded
> 
> with open(readfile, 'r') as f:
>     for line in f:
>         try:
>             if keyword in line:
>                 do stuff
>         except:
>             do nothing

Why are you ignoring *all errors*? That will make it impossible (or at 
least very hard) to cancel the script with Ctrl-C, and it will cover 
up programming errors. Apart from a very few expert uses, you should 
never use a bare except. If you really want to "ignore all errors", use 
`except Exception`, but even that is not good practice. You should list 
and catch only the precise errors that you know you wish to ignore and 
can safely handle. Everything else indicates a bug that needs fixing.

By the way, "do nothing" in Python is spelled "pass".


>         with open(writefile, 'a') as f1:
>             f1.write(line)

And this will repeatedly open the file, append one line, then close it 
again. Almost certainly not what you want -- it's wasteful and 
potentially expensive.


> ######################
> 
> or something else altogether?
> 
> I'm thinking the first way is better as it only opens the files once
> whereas it seems to me the second script would open and close the writefile
> once per iteration, and the do nothing in the except seems just wrong to
> me. Is my thinking on target here?

Spot on target.



-- 
Steve