Python's Default Mutable Argument Trap: Why It Trips Up Developers in Interviews

A subtle Python behavior involving default mutable arguments is one of the most common stumbling blocks in technical interviews, affecting candidates at all levels. When a mutable object like a list or dictionary is used as a default argument, Python evaluates it once at function definition time, not on each call, meaning all calls share the same object. This causes unexpected state accumulation across function calls, producing output that surprises most developers. The standard fix is to use None as the default and create a fresh mutable object inside the function body. Interviewers use this concept to assess a candidate's understanding of interpreter mechanics, manual state tracking, and Python best practices.