One-line list comprehension: if-else variants

It's more about python list comprehension syntax. I've got a list comprehension that produces list of odd numbers of a given range:

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

This makes a filter - I've got a source list, where I remove even numbers (if x % 2). I'd like to use something like if-then-else here. Following code 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

There is a python expression like if-else:

    1 if 0 is 0 else 3

How to use it inside a list comprehension?

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.


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