looping through a file

Andrew Bennetts andrew-pythonlist at puzzling.org
Wed Apr 30 15:53:13 CEST 2003


On Wed, Apr 30, 2003 at 12:01:30PM +0000, Alex Martelli wrote:
[...]
> I think this will most likely solve your practical problems.  However,
> supposing for the sake of argument that InFile could be SO large that
> reading it all at once with readlines would be a problem (say, tens
> of megabytes), you could solve that too, for example, using again
> modern Python (2.2 or better in this case):
> 
> 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):
        if username == users_username and password == users_password:
            authorized = 1
            break
    else:
        authorized = 0
        
    
    InFile.close()

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

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)
    for username, password in izip(iterator, iterator):
        ...

So it's probably not worth defining a seperate function...

-Andrew.






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