if-else sentence in one-line-python

Question

I have some code:

a_part = [2001, 12000]
b_part = [1001, 2000]
c_part = [11, 1000]
d_part = [1, 10]
data = range(1, 12000)
labels = [a_part, b_part, c_part, d_part]
sizes = []

# ---

for part in labels:
    sum = 0
    for each in data:
        sum += each if each >= part[0] and each <= part[1] else 0
        # error
        # sum += each if each >= part[0] and each <= part[1] 
    sizes.append(sum)
print(sizes)

And I rewrote this to be more Pythonic:

sizes = [sum(x for x in data if low<=x<=high) for low,high in labels]
# error
# sizes = [sum(x for x in data if low<=x<=high else 0) for low else 0,high in labels]
print(sizes)

I found that in the first snippet I can't leave out else 0 while the second example can't contain else 0 .

What is the difference else 0 makes between these examples?

Answer 1

You have two very different things here.

In the first you have an expression, and are using a conditional expression to produce the value; that requires an else because an expression always needs to produce something .

For example, if you wrote:

sum += each if each >= part[0] and each <= part[1]  # removing "else 0"

then what would be added to the sum if the test was false?

In the second you have a generator expression , and the if is part of the possible parts (called comp_if in the grammar ), next to (nested) for loops. Like an if ...: statement, it filters what elements in the sequence are used, and that doesn't need to produce a value in the false case; you would not be filtering otherwise.

To bring that back to your example:

sum(x for x in data if low<=x<=high)

when the if test is false, that x is just omitted from the loop and not summed. You'd do the same thing in the first example with:

if each >= part[0] and each <= part[1]:
    # only add to `sum` if true
    sum += each

Answer 2

This is not the same syntax at all. The first one is a ternary operator (like (a ? b : c) in C). It wouldn't make any sense not to have the else clause here. The second one is a list comprehension and the purpose of the if clause is to filter the elements of the iterable.

Answer 3

x if y else z

This is an inline-if expression which returns a value. It must always return a value, it cannot not return a value, hence it must contain an else clause.

[x for x in y if z]

This is a list comprehension. The if here acts as a filter for the loop. It's equivalent to for x in y: if z: ... .

To have an else in there, you put the inline-if expression in place of x :

[x if y else z for foo in bar]