For loops (novice)

Question

I recently started learning Python, and the concept of for loops is still a little confusing for me. I understand that it generally follows the format for x in y, where y is just some list.

The for-each loop for (int n: someArray) becomes for n in someArray,

And the for loop for (i = 0; i < 9; i-=2) can be represented by for i in range(0, 9, -2)

Suppose instead of a constant increment, I wanted i*=2, or even i*=i. Is this possible, or would I have to use a while loop instead?

Answer 1

As you say, a for loop iterates through the elements of a list. The list can contain anything you like, so you can construct a list beforehand that contains each step.

A for loop can also iterate over a "generator", which is a small piece of code instead of an actual list. In Python, range() is actually a generator (in Python 2.x though, range() returned a list while xrange() was the generator).

For example:

def doubler(x):
    while True:
        yield x
        x *= 2

for i in doubler(1):
    print i

The above for loop will print

1
2
4
8

and so on, until you press Ctrl+C.

Answer 2

You can use a generator expression to do this efficiently and with little excess code:

for i in (2**x for x in range(10)): #In Python 2.x, use `xrange()`.
    ...

Generator expressions work just like defining a manual generator (as in Greg Hewgill's answer), with a syntax similar to a list comprehension. They are evaluated lazily - meaning that they don't generate a list at the start of the operation, which can cause much better performance on large iterables.

So this generator works by waiting until it is asked for a value, then asking range(10) for a value, doubling that value, and passing it back to the for loop. It does this repeatedly until the range() generator yields no more values.