Question 1

Why does a mutable default argument cause bugs in Python?

Accepted Answer

Default argument values are evaluated once, when the function is defined, not each time it is called. For immutable defaults such as numbers, strings or None this is invisible. For a list, dictionary or set it means every call relying on the default shares one object, and any mutation persists into subsequent calls.

The symptom is a function that appears to accumulate state it was never asked to keep. A function that appends to a list defaulting to empty returns one item on the first call, two on the second, and so on. The bug is particularly awkward because it only appears when the caller omits the argument, so tests that always pass a value will never catch it.

The standard fix is to default to None and create the real object inside the body. That moves construction to call time, giving each invocation its own object, and it makes the intent explicit to a reader.

It is worth understanding that this is a consequence of functions being ordinary objects whose defaults are evaluated at definition, rather than a flaw. The same mechanism is occasionally used deliberately, for instance to bind a loop variable at definition time or as a cheap cache. Those uses are legitimate but deserve a comment, because a reader will otherwise assume a mistake.

The same rule applies anywhere defaults are evaluated early, including class attribute definitions. A mutable class attribute is shared by every instance, which produces the same surprise one level up and is a common source of confusion when objects appear to affect each other.

Linters flag mutable defaults in most standard configurations, which is the cheapest available protection against a bug that is easy to introduce and hard to notice in review.

Question 2

What does "immutable" mean and why does it help?

Accepted Answer

An immutable value cannot be changed after it is created. Operations that appear to modify it return a new value instead, leaving the original intact. Why this helps, and the reasons compound: You

Question 3

What is the difference between a shallow copy and a deep copy?

Accepted Answer

A shallow copy creates a new container holding references to the same inner objects. A deep copy recursively creates new copies of everything inside. The distinction only matters when the structure

Question 4

What are some common errors in Python and how can I fix them?

Accepted Answer

Understanding Python troubleshooting common errors is vital for effective coding. Common issues include SyntaxError, TypeError, and NameError, each requiring specific fixes.

Question 5

Why are null references called a billion-dollar mistake?

Accepted Answer

Because Tony Hoare, who introduced the null reference into ALGOL W in 1965, later described it exactly that way — apologising publicly for a design decision that has produced an enormous quantity of

Question 6

What is mocking in tests, and when does it go wrong?

Accepted Answer

Replacing a real dependency with a controlled stand-in, so a test can run fast, deterministically, and without touching a network, database or clock. It is essential and it is the most commonly

Question 7

Why does TypeScript say Object is possibly undefined?

Accepted Answer

The compiler is telling you that a value type includes undefined and you are using it as though it does not. With strict null checking enabled, undefined and null are not silently assignable to other

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