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

Answer 1

Just for an alternative, how about generalizing the iterate/increment operation to a lambda function so you can do something like this:

for i in seq(1, 9, lambda x: x*2):
    print i
...
1
2
4
8

Where seq is defined below:

#!/bin/python
from timeit import timeit

def seq(a, b, f):
    x = a;
    while x < b:
        yield x
        x = f(x)

def testSeq():
    l = tuple(seq(1, 100000000, lambda x: x*2))
    #print l

def testGen():
    l = tuple((2**x for x in range(27)))
    #print l

testSeq();
testGen();

print "seq", timeit('testSeq()', 'from __main__ import testSeq', number = 1000000)
print "gen", timeit('testGen()', 'from __main__ import testGen', number = 1000000)

The difference in performance isn't that much:

seq 7.98655080795
gen 6.19856786728

[EDIT]

To support reverse iteration and with a default argument...

def seq(a, b, f = None):
    x = a;
    if b > a:
        if f == None:
            f = lambda x: x+1
        while x < b:
            yield x
            x = f(x)
    else:
        if f == None:
            f = lambda x: x-1
        while x > b:
            yield x
            x = f(x)

for i in seq(8, 0, lambda x: x/2):
    print i

Note: This behaves differently to range/xrange in which the direction </> test is chosen by the iterator sign, rather than the difference between start and end values.