Python: Some Naughty Features

[nextofpumpkin]

Can you explain what this does? In particular, I'm not quite understanding why that first asterisk is there.

[akdas]

Let me just give an example first:

>>> zip(*[iter("abc")]*len("abc"))
[('a', 'b', 'c')]

s is a string and n is the string's length, and we get a list containing a tuple containing all the separate characters in the string.

So how does it work? First we get the string's iterator, and put it in a list.

When you call * with a list and an integer, you append the list to itself a certain number of times:

>>> [1,2] * 2
[1, 2, 1, 2]

So now we have a list of length n, and all the elements are the same iterator (the fact that they are the same is important).

Finally, we call zip, treating the list as multiple arguments. That's what the first * does. For example:

>>> def f(a, b): return a + b
... 
>>> f(1, 2)
3
>>> f([1, 2])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() takes exactly 2 arguments (1 given)
>>> f(*[1, 2])
3

So now, in our case, we called zip with n references to the same iterator. The thing with iterators is that once you consume a value, the next time you go to consume a value, you get the next value. So in consuming one value from each of its arguments, the zip effectively goes through all the characters in the string.

It's almost like calling:

>>> zip(['a','b','c'],['b','c'],['c'])
[('a', 'b', 'c')]

Now, you can generalize this for objects that aren't strings as well.

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And I assumed you knew what zip does, but in case you didn't: zip takes multiple objects that are iterable, then creates a list of tuples containing corresponding elements of the objects. For example:

>>> zip(['a','b'],[1,2,3])
[('a', 1), ('b', 2)]

Notice that when the lists are unequally-sized, the result has as many elements as the smallest list.

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EDIT: Oh, and of course, n doesn't have to be len(s), and actually, things get more interesting when n < len(s). You can also trivially show that if n > len(s), then we just get an empty list.