python - One-line list comprehension: if-else variants

Question

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?

Fow what it's worth, you have a "list comprehension", not a generator. The ultimate syntax is the same except that generators use () instead of []. — mgilson
possible duplicate of Is it possible to use 'else' in a python list comprehension? — Raphael Ahrens
It took me a while to get why if x % 2 eliminates even numbers (instead of keeping them) — it is because when x is even the x % 2 expression results in 0, which, in turn, evaluates to False, while any int except 0 evaluates to True. — user8554766

shx2 shx2 · Accepted Answer · 2013-06-26T13:18:58

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.

python - One-line list comprehension: if-else variants

5 Answers