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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?
833
Answer. Yes, an else clause can be used with an if in a list comprehension. The following code example shows the use of an else in a simple list comprehension.
Even factoring out the time it takes to lookup and load the append function, the list comprehension is still faster because the list is created in C, rather than built up one item at a time in Python.
Actually, list comprehension is much clearer and faster than filter+lambda, but you can use whichever you find easier. The first thing is the function call overhead: as soon as you use a Python function (whether created by def or lambda) it is likely that the filter will be slower than the list comprehension.
Because of differences in how Python implements for loops and list comprehension, list comprehensions are almost always faster than for loops when performing operations.
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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