How does Python interpret chained 'in' membership tests?

Python allows chaining of operators—including the membership operator ("in")—by design. This isn’t a special case in the documentation for "in"; it comes from the way Python handles comparison operators, which are defined to chain. For example, an expression like:

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a in b in c

is interpreted roughly as:

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(a in b) and (b in c)

This automatic chaining applies to other operators such as <, <=, >, >=, ==, and != as well. Even though the Python documentation for membership tests doesn’t explicitly call out chaining, the language’s grammar allows any operator in the comparison family to be chained.

Why Does the Grammar Allow This?

• Consistency with Comparisons: Chaining comparisons (like a < b < c) has long been a part of Python. Membership tests fall into a similar category, so they inherit the same chaining behavior.

• Reduced Redundancy: It can avoid repetition. Instead of writing:

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  (a in b) and (b in c) and (c in d)

  you can write:

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  a in b in c in d

Practical Use Cases

In practice, chained membership tests aren’t as common as chained numeric comparisons. They may occasionally come in handy when you want to ensure a sequence of membership conditions holds, but be aware that the semantics are strictly a left-to-right evaluation of each binary "in" test converted to logical "and" between them. For example, the expression:

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"test" in "testing" in "testing" in "testing"

is interpreted as:

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("test" in "testing") and ("testing" in "testing") and ("testing" in "testing")

Even though the second and third tests are not very interesting in this particular example, Python’s grammar allows it.

Parentheses and Order of Operations

If you try adding parentheses to change the associativity, you’ll likely break the intended chain because the chaining is built into the language’s grammar. The neat, implicit behavior only works when you write them in a single, undivided chain.

Summary

• Mechanism: Chained "in" tests work like chained comparisons.
• Docs: The behavior isn’t highlighted separately for "in", but it falls under the general operator chaining rules documented for comparisons.
• Use Case: It can reduce verbosity, though its practical applications are less common than for numerical comparisons.

This design is consistent with Python’s philosophy of offering concise syntax for common idioms, even if it occasionally leads to unexpected results when a typo (or unintentional chaining) occurs.