looping through a file

[Alex Martelli]

Andrew Bennetts wrote:
    ...
>> InFile = open('users.dat', 'r')
>> 
>> for line in InFile:
>>     file_username = line.strip()
>>     file_password = InFile.next().strip()
>>     if file_username==users_username and file_password==users_password:
>>         authorized = 1
>>         break
>> else:
>>     authorized = 0
>> 
>> InFile.close()
> 
> I thought there might be something in 2.3's itertools module to help with
> this -- I was hoping you'd be able to write something like:
> 
>     InFile = open('users.dat', 'r')
>     
>     for username, password in multistep(InFile, 2):

Careful!!!  You'd still need to strip them before you compare them,
else the trailing \n or any other whitespace will make the comparisons
fail.  imap(str.strip, multistep(InFile, 2)) might work here (if
multistep existed).

> Where "multistep" would probably have a better name, and be defined like:
> 
>     def multistep(iterable, count):
>         # XXX: What should happen when len(iterable) % count != 0 ?

I think the last trailing part X with 0<=len(X)<count should be dropped.
Otherwise, unpacking assigments such as the above would fail -- and,
"padding with None's" isn't in fashion any more;-).

>         iterator = iter(iterable)
>         while 1:
>             values = []
>             for i in range(count):
>                 values.append(iterator.next())
>             yield values

I think I'd prefer expressing this loop (naming issues apart) as:

   while True: return [iterator.next() for i in range(count)]

> 
> I couldn't see anything in itertools to save me writing that out.  But I
> just figured out how to do it with itertools -- izip can do it:
> 
>     def multistep(iterable, count):
>         return itertools.izip(*((iter(iterable),)*count))
> 
> I guess you could write this directly:
> 
>     iterator = iter(InFile)

Unneeded, btw: iter(inFile) is inFile (I also exploit that in my
less-elegant snippet above where I call inFile.next(), secure that
this will consume a line).

>     for username, password in izip(iterator, iterator):
>         ...
> 
> So it's probably not worth defining a seperate function...

I think it is (if anything can still be added to new module itertools
in 2.3 -- I'm not sure ANY functionality can be added after beta-1), as
it's frequently needed.  But, if it isn't, the "trick" with the
iterator explicitly repeated is good and IMHO worth keeping in mind.

But one does still need the .strip call...;-)


Alex