Structured token ladders

Beginner Defensive APIs: validation, sanitization & exceptions

Created by Pavel · 29.04.2026 at 19:10 UTC

Unary-free arithmetic ladders alternate operand, operator, operand, … with odd length: [10, '+', 2, '*', 3] has five tokens. Checking parity and digit/operator roles before casting to floats prevents ValueError explosions halfway through evaluation—better error messages, less partial state.

CSV and JSON pipelines use the same discipline: verify column count and dtype expectations before vectorised math.

Formal grammars generalise ladders; for small teaching kernels, explicit index parity is enough.

str.isdigit reference: [1].

Sources

[1]https://docs.python.org/3/library/stdtypes.html#str.isdigit Return to text

University approvals: 0

Tasks

Question 1

For a unary-free infix ladder starting and ending with operands, the number of tokens is always:

Hint

Operand/operator alternation with operands on both ends.

Even

Odd

Unconstrained

Exactly four

Question 2

Why validate token structure before float(tok) on operands?

Hint

Catch shape errors before coercion.

float() never raises

You fail fast with clearer errors and avoid partial evaluation

Python requires structural validation for all lists

pandas forbids mixed-type lists

Question 3

valid_ladder(parts, ops) is True iff len(parts) >= 3, odd length, even indices are digit-only strings, odd indices are in set ops.

Hint

Index parity encodes operand vs operator slots.

def valid_ladder(parts, ops):
    pass

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Card Info

Topic: Defensive APIs: validation, sanitization & exceptions
Difficulty: Beginner
Completed: 0 users

Creator

Pavel