How does Python evaluate chained comparisons like `1 < x < 10`?
Chained comparison semantics
Python treats a < b < c as (a < b) and (b < c), with two important differences from writing it out by hand: b is evaluated only once, and evaluation short-circuits, so if the first comparison is False the third operand is never evaluated. It is not parsed as (a < b) < c as in C-like languages, where you would compare a boolean to c. Any mix of comparison operators can be chained, including ==, !=, in, is and not in, and the chain can be arbitrarily long.
Why this matters: single evaluation is semantically important when the middle operand is a function call, property or expensive expression, so 0 < compute() < limit calls compute() once. It also makes range checks readable, which is why I always prefer 0 <= i < n over i >= 0 and i < n. Bytecode-wise, CPython duplicates the middle value on the stack and jumps out early on a false result; you cannot reproduce that with a normal function because function arguments are all evaluated eagerly.
Common mistakes: expecting a != b != c to mean 'all three distinct' (it only compares adjacent pairs, so a == c is allowed); writing exotic chains such as x < y > z that are legal but confusing; and using chained comparisons with numpy arrays or pandas Series, where the implicit 'and' calls bool() on an array and raises 'truth value of an array is ambiguous'. For arrays use (a > 0) & (a < 10) or np.logical_and, and for SQLAlchemy columns use .between() or & for the same reason. These libraries cannot override 'and', which is what the chain desugars to.
Trade-off: chained comparisons are a readability win for numeric ranges, but I avoid chains mixing different operator kinds (a == b in c) because reviewers must mentally expand them. This behaviour is stable across all Python 3 versions.
Scenario Questions
0-2 years experience
- 1Does 3 > 2 > 1 give the same result as (3 > 2) > 1? Evaluate both and explain.
- 2Rewrite if x >= 0 and x <= 100 using idiomatic Python.
2-5 years experience
- 1In 0 < f() < 10, how many times is f called, and what happens to the second comparison if the first is False?
- 2A teammate writes if a != b != c to mean 'all three are different'. Is that correct? What would you write instead?
5-8 years experience
- 1Chained comparisons on a numpy array raise a ValueError. Why, and how do you write the element-wise range check?
- 2You are designing an Interval or Date class that should work in chained comparisons. Which rich comparison methods do you implement, and when do you return NotImplemented?
8+ years experience
- 1Explain at the bytecode level how CPython compiles a < b < c so b is evaluated once and the chain short-circuits, and why a helper function or lambda cannot emulate it.
- 2Would you allow chained comparisons in a query-builder DSL? Why does 0 < col < 10 fail with SQLAlchemy or pandas, and how do those libraries work around it?
Follow-up Questions
- Why can't you use chained comparisons on numpy arrays, and what do you use instead?
- Does a != b != c check that all three values are distinct?