Edge Case Identification and Testing Questions

Focuses on systematically finding, reasoning about, and testing edge and corner cases to ensure the correctness and robustness of algorithms and code. Candidates should demonstrate how they clarify ambiguous requirements, enumerate problematic inputs such as empty or null values, single element and duplicate scenarios, negative and out of range values, off by one and boundary conditions, integer overflow and underflow, and very large inputs and scaling limits. Emphasize test driven thinking by mentally testing examples while coding, writing two to three concrete test cases before or after implementation, and creating unit and integration tests that exercise boundary conditions. Cover advanced test approaches when relevant such as property based testing and fuzz testing, techniques for reproducing and debugging edge case failures, and how optimizations or algorithmic changes preserve correctness. Interviewers look for a structured method to enumerate cases, prioritize based on likelihood and severity, and clearly communicate assumptions and test coverage.

EasyTechnical

0 practiced

You're given a function reverse_string(s: str) -> str in Python. Before implementing it, write three unit tests (describe input and expected output) that cover both common and edge cases: a typical ASCII string, an empty string, a single-character string, a Unicode string containing multi-byte characters and combining marks, and a very long string (e.g., length > 1e6). Explain why each test matters and any assumptions about encoding/normalization.

MediumTechnical

0 practiced

A service serializes user objects to JSON and later deserializes them. Provide a suite of unit and integration tests to check serialization/deserialization correctness: missing required fields, extra unknown fields, field type mismatches, field ordering, default values, and round-trip equality or normalization. Provide a small example schema (id:int, name:string, tags:list<string>) and three test cases that would catch subtle bugs.

MediumTechnical

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Requirement ambiguity: does our sorting routine need to be stable? Describe how you would clarify with stakeholders and enumerate test cases to prove stability or absence of it. Provide a concrete unit test where stability is required: sort by primary key while preserving original order for equal keys.

MediumTechnical

0 practiced

Design property-based tests for a banking transaction system where operations include deposit(amount), withdraw(amount), and transfer(from, to, amount). List invariants to assert (e.g., no negative balances, conservation of total balance when transfers occur) and describe how to generate sequences of operations that capture edge cases like overdrafts, zero amounts, and concurrent transfers.

HardTechnical

0 practiced

Mutation testing can reveal gaps in test suites by mutating source code and checking whether tests fail. Describe how you'd set up mutation testing for a core library, interpret mutation scores, handle equivalent mutants, and prioritize which mutants to investigate. How would you integrate mutation testing into CI considering runtime costs?

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