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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?

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  • 2
    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. Commented Jul 18, 2018 at 6:45

6 Answers 6

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477

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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4 Comments

I've got one question... [x for x in range(1, 10) if x % 2] is correct syntax. [x if x % 2 for x in range(1, 10)] - this is not, but [x if x%2 else x*100 for x in range(1, 10)] is again, correct syntax. How come?
@tkoomzaaskz in your second example, this is neither a ternary-if-else operator (missing the else), nor a filter (since it is on the EXP part of the list comprehension)
@tkoomzaaskz To further clarify, note that you can add a second if at the end: [x if x%2 else x*100 for x in range(1, 10) if not x%3] The first if is part of the ternary operator, the second if is part of the list comprehension syntax. The whole x if x%2 else x*100 is "on the same level" as a simple 2*x, it's the expression to evaluate on the left side of the for, when the filtering of if not x%3 has already taken place.
Hello, would a one-line statement be more performant than making it on two lines like for i in x: and then in the for loop if i == y: ?
32 
[x if x % 2 else x * 100 for x in range(1, 10) ]
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20

You can do that with list comprehension too:

A=[[x*100, x][x % 2 != 0] for x in range(1,11)]
print A
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2 Comments

Very nice. A boolean slice. Thanks you just gave me an easier to read solution.
You can also double assign like so: A,B = [10,11][a==19],[1,14][a==20]
15

Just another solution, hope some one may like it :

Using: [False, True][Expression]

>>> map(lambda x: [x*100, x][x % 2 != 0], range(1,10))
[1, 200, 3, 400, 5, 600, 7, 800, 9]
>>>
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4

#how you can squre a list of an array of negative and positive values

my_list=[-2,-3,0,1,5]
squred_values=[]
  

squred_values=[-i**2 if i<0 else i**2 for i in my_list]

#or

for i in my_list:
    if i<0:
        squred_values.append( -i**2)`enter code here`
    else:
        squred_values.append( i**2)
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0

two for in one list comprehension

ex = [['obi', 'is', '#alive'],['oge', 'is', 'beautiful'],
                ['Ade', 'the', '#comedian', 'de', '#rich'],['Jesus', 'wept']]
res = [j if j.startswith("#") else "_"+j for i in ex for j in i]
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1 Comment

I don't see what information this answer adds over the much older and explained existing answers...

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