How to check if string A is substring of string B?

Peer-reviewed by HireXTech Technical Panel • Updated for 2025/2026 hiring • Editorial standards

Interviewer Expectations for this Question

Core Competency

Assesses fundamental understanding of Python conventions, runtime behavior, and memory/performance considerations.

Evaluation Criteria

Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.

Comprehensive Model Answer Verified Solution

The most readable, pythonic, and performant way to check if substring A exists inside string B is using the in operator:

str_b = "HireXTech Interview Preparation"
str_a = "Interview"

if str_a in str_b:
    print(f"'{str_a}' was found!")

### Other String Search Methods:

str.find(sub): Returns the lowest index where the substring starts, or -1 if not found:

   idx = str_b.find("Tech")  # Returns 5

str.index(sub): Like find(), but raises a ValueError if the substring does not exist.
str.startswith() and str.endswith(): Checks prefix or suffix matching:

   str_b.startswith("HireX")  # True

Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.

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