converting to list comprehension

Question

I have this code:

result = []
for x in [10, 20, 30]:
    for y in [2, 3, 4]:
        if y > 0:
            result.append(x ** y)

result

[100, 1000, 10000, 400, 8000, 160000, 900, 27000, 810000]

I'm trying to convert it to list comprehension with no luck(new in python)

This is my attempt:

print [ x ** y if y > 0 for x in [10, 20, 30] for y in [2, 3, 4]]

But there is a problem in the statement, any help will be most appropriated.

error:

  File "<stdin>", line 1
    print [ x ** y if y > 0 for x in [10, 20, 30] for y in [2, 3, 4]]
                              ^
SyntaxError: invalid syntax

Answer 1

The filtering condition has to be at the end, like this

print [x ** y for x in [10, 20, 30] for y in [2, 3, 4] if y > 0]

because the grammar for list comprehension is defined like this

list_display        ::=  "[" [expression_list | list_comprehension] "]"
list_comprehension  ::=  expression list_for
list_for            ::=  "for" target_list "in" old_expression_list [list_iter]
list_iter           ::=  list_for | list_if
list_if             ::=  "if" old_expression [list_iter]

So only expressions can come before the for..in and if statement can come only after that.

In your case, expression is satisfied by x ** y and then list_for is satisfied by for x in [10, 20, 30] and then another list_for is satisfied by for x in [10, 20, 30] and finally the list_if is satisfied by if y > 0. It is of the form

[ expression list_for list_for list_if ]

BTW, you can do the same with itertools.product, like this

from itertools import product
print [num**power for num,power in product([10, 20, 30], [2, 3, 4]) if power > 0]

Answer 2

You need the if statement at the end of the list comprehension

print [ x ** y for x in [10, 20, 30] for y in [2, 3, 4] if y > 0]

Answer 3

Remember this Syntax for Comprehension. '{}' for dict comprehension

[ expression for target1 in iterable1 if condition1
for target2 in iterable2 if condition2 ...
for targetN in iterableN if conditionN ]