Inline if else?

Question

Here is part of code. I need to understand what this code do :

if instruction == "JOINT" or instruction == "ROOT":
    parent = joint_stack[-1] if instruction == "JOINT" else None
    joint = BvhJoint(words[1], parent)
    self.joints[joint.name] = joint

How to understand this line special ->

    parent = joint_stack[-1] if instruction == "JOINT" else None

Is this code equal with JS code:

if (instruction == "JOINT") {
    parent = joint_stack[-1];
} else {

}

?

score 2 · Accepted Answer · answered May 31 '22 at 15:15

The equivalent JS code would be:

   if (instruction == "JOINT") { 
        parent = joint_stack[-1];
    } else {
        parent = null;
    }

or more idiomatically:

    parent = instruction == "JOINT" ? joint_stack[-1] : null;

This ternary expression in Python:

joint_stack[-1] if instruction == "JOINT" else None

is the same as the JS ternary expression:

instruction == "JOINT" ? joint_stack[-1] : null

Januka samaranyake · Answer 2 · 2022-05-31T15:19:22.840

1

 parent = joint_stack[-1] if instruction == "JOINT" else None

This will check the “instruction” is equal to “JOINT” if yes, last value of joint_stack array will be assigned to the parent, otherwise None

edited May 31 '22 at 15:19

answered May 31 '22 at 15:16

Januka samaranyake

2,228
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The condition it's checking is `instruction == "JOINT"`, not `joint_stack[-1] == "JOINT"`. – Samwise May 31 '22 at 15:17

Thomas Junk · Answer 3 · 2022-05-31T15:27:34.397

1

The JS equivalent would be

const parent = (instruction === "JOINT")?joint_stack[-1]:null;

The shortcut to conditional expression was added to Python 2.5. See Does Python have a ternary conditional operator?

edited May 31 '22 at 15:27

answered May 31 '22 at 15:17

Thomas Junk

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Inline if else?

3 Answers3