How does comparison chaining (a < b < c) work? | Interview Question

Q: How does comparison chaining (a < b < c) work?

**Target Audience:** Beginners who have seen `0 **How to read this doc:** Plain words first, then a `📌 Interview term:` callout with the exact term. All outputs are from real runs; the **Verified** section quotes them in full. ## 1. Why This Even Matters — A Story First A bouncer checks two things: you must be at least 18, and your ticket must be for tonight. In maths class you would write the age rule as 18 ≤ age, and a range as 0 ≤ x < 10, read as "x is between 0 and 10". Most programming languages do not understand that second form: in C or JavaScript, `0 <= x < 10` first computes `0 <= x` (true or false), then compares *that boolean* with 10 — which is almost always true, and almost always a bug. Python decided to read it the way

The problem

Question presented to candidate: "In Python you can write 0 <= x < 10. What does the interpreter actually do with that, and why is 3 > 2 > 1 True while (3 > 2) > 1 is False?"

What a strong answer should cover:

a < b < c means (a < b) and (b < c), not (a < b) < c; every comparison operator chains this way, including ==, !=, is, is not, in and not in.
The middle operand is evaluated once, operands are evaluated left to right, and the chain short-circuits: c is never evaluated when a < b is false.
A chain only compares neighbours: 1 < 3 > 2 is legal and True, and says nothing about how 1 and 2 relate.
Gotchas from mixing operators: 1 in [1, 2] == True is False, False == False in [False] is True.
CPython detail: the chain keeps the middle value on the stack (SWAP/COPY in dis) and calls bool() on each intermediate result, which is why NumPy arrays raise "truth value is ambiguous" in a chain.

Code / implementation expected: Optional — a range check, plus a side-effect function showing each operand is evaluated once.

operators
chaining

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Solution

Target Audience: Beginners who have seen 0 **How to read this doc:** Plain words first, then a 📌 Interview term:` callout with the exact term. All outputs are from real runs; the Verified section quotes them in full.

1. Why This Even Matters — A Story First

A bouncer checks two things: you must be at least 18, and your ticket must be for tonight. In maths class you would write the age rule as 18 ≤ age, and a range as 0 ≤ x , =, ==, !=, plus the identity tests is/is notand the membership testsin/not in (see [==vsis](/interview-question/what-is-the-difference-between-and-is) and [how in` works](/interview-question/how-does-the-in-operator-work-in-python)).

The language reference states it this way: `x 2 > 1) # True : (3 > 2) and (2 > 1) print((3 > 2) > 1) # False : True > 1 is 1 > 1


The parentheses version is the C/JavaScript r

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5 min read · 10 sections

Run the code

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PythonChaining rules, the parentheses trap and mixed operators

Practice

PythonEvaluated once, short-circuit, and bool() on each step

Practice

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