Why avoid while loops?

Question

I'm about 2 weeks deep in my study of Python as an introductory language. I've hit a point in Zed's "Learn Python the Hard Way" where he suggests:

Use a while-loop only to loop forever, and that means probably never. This only applies to Python, other languages are different.

I've googled all over this, referenced everything I can, but I can't find any reason in the world why this would be a convention in Python. What makes it different?

When I gave up programming 10 years ago, I was working in VB and would regularly be told to get rid of my For loops and use While loops instead. I was a hack (as I am today, though I wrote a LOT of code back then), so I just did what I was told without questioning it. Well, now I'm questioning it. Is this a speed issue? Is it just to avoid escape-less infinites?

Answer 1

The advice seems poor to me. When you're iterating over some kind of collection, it is usually better to use one of Python's iteration tools, but that doesn't mean that while is always wrong. There are lots of cases where you're not iterating over any kind of collection.

For example:

def gcd(m, n):     "Return the greatest common divisor of m and n."     while n != 0:         m, n = n, m % n     return m 

You could change this to:

def gcd(m, n):     "Return the greatest common divisor of m and n."     while True:         if n == 0:             return m         m, n = n, m % n 

but is that really an improvement? I think not.

Answer 2

It's because in the typical situation where you want to iterate, python can handle it for you. For example:

>>> mylist = [1,2,3,4,5,6,7] >>> for item in mylist: ...     print item ...  1 2 3 4 5 6 7 

Similar example with a dictionary:

>>> mydict = {1:'a', 2:'b', 3:'c', 4:'d'} >>> for key in mydict: ...     print "%d->%s" % (key, mydict[key]) ...  1->a 2->b 3->c 4->d 

Using a while loop just isn't necessary in a lot of common usage scenarios (and is not "pythonic").

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Here's one comparison from a mailing list post that I though was a good illustration, too:

> 2. I know Perl is different, but there's just no equivalent of while
> ($line = <A_FILE>) { } ?

Python's 'for' loop has built-in knowledge about "iterable" objects, and that includes files. Try using:

for line in file:     ... 

which should do the trick.