How can I use a conditional expression (expression with if and else) in a list comprehension?

Question

I have a list comprehension that produces list of odd numbers of a given range:

[x for x in range(1, 10) if x % 2]

That makes a filter that removes the even numbers. Instead, I'd like to use conditional logic, so that even numbers are treated differently, but still contribute to the list. I tried this code, but it fails:

>>> [x for x in range(1, 10) if x % 2 else x * 100]
  File "<stdin>", line 1
    [x for x in range(1, 10) if x % 2 else x * 100]
                                         ^
SyntaxError: invalid syntax

I know that Python expressions allow a syntax like that:

1 if 0 is 0 else 3

How can I use it inside the list comprehension?

Answer 1

x if y else z is the syntax for the expression you're returning for each element. Thus you need:

[ x if x%2 else x*100 for x in range(1, 10) ]

The confusion arises from the fact you're using a filter in the first example, but not in the second. In the second example you're only mapping each value to another, using a ternary-operator expression.

With a filter, you need:

[ EXP for x in seq if COND ]

Without a filter you need:

[ EXP for x in seq ]

and in your second example, the expression is a "complex" one, which happens to involve an if-else.