Python - Using "if" and "for" in variable assignment

Such for statement is intended as syntactic sugar to a greater for. These three constructions are almost equivalent:

mylist = []
for x in range(3):
    mylist.append(x)

mylist = list(x for x in range(3))

mylist = [x for x in range(3)]

However the example with range(3) is useless without transforming x somehow, like:

mylist = [x*x for x in range(3)] # will produce [0, 1, 4]

Please see the docs. The construction inside list() call in my example is something different. Is called generator comprehension.

The if is a you point out just a more compact way of doing an if.

The compact for is called a list comprehension. The documentation describes it better than I do :)

As per your example alist = [x for x in range(3)] is the equivalent of

alist = []
for x in range(3):
    alist.append(x)

The list comprehension can be mixed with conditionals, as in this example where we'd get all numbers 0-9

alist = [x for x in range(10)]
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

if we only want every second number we can use modulo for each loop:

alist = [x for x in range(10) if x % 2]
# [1, 3, 5, 7, 9]

which would be equal to

alist = []
for x in range(10):
    if x % 2:
        alist.append(x)

alist = [x for x in range(3)]

What you are looking at is list comprehension, which consists of the following:

1. An Input Sequence.
2. A Variable representing members of the input sequence.
3. An Optional Predicate expression.
4. An Output Expression producing elements of the output list from members of the Input Sequence that satisfy the predicate.

More on this

So you can do things like this:

a_list = [1, ‘4’, 9, ‘a’, 0, 4]

ints = [ i for i in a_list if type(e) == types.IntType ]
# print ints
# [ 1, 9, 0, 4 ]

Python documentation

They're called list comprehensions. Python has examples in their documentation.