Question 1

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 contains mutable objects, which is exactly when it bites.

Copying a flat list of numbers shallowly is completely safe, because numbers are immutable and cannot be changed through either reference. Copying a list of lists shallowly gives you a new outer list whose elements are the original inner lists. Modifying an inner list through the copy changes the original, because there is only one inner list with two references to it.

That produces the classic confusing bug: you deliberately copied the data to avoid mutating the input, the copy appeared to work, and the caller sees their data change anyway.

A deep copy avoids it by duplicating every level. The cost is real: it is slower, uses proportionally more memory, and it must handle shared references and cycles, which it does by tracking objects it has already copied so a structure referencing itself does not recurse forever.

Choosing between them is a question about intent rather than safety. If you will only read the nested data, a shallow copy is cheaper and correct. If you will modify anything below the top level, you need a deep copy or you need to copy the specific parts you intend to change.

A third option is often better than either: avoid mutation entirely. Producing new values rather than modifying existing ones removes the question, and is why immutable data structures are popular in codebases where this class of bug has caused real problems.

The same distinction exists in most languages, though the syntax and defaults differ.

Question 2

How does garbage collection actually work?

Accepted Answer

By determining which objects are still reachable from your running program and reclaiming everything else. The key insight is that it does not detect garbage — it detects live data, and treats the

Question 3

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

Question 4

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 5

When should you use an array rather than a linked list?

Accepted Answer

Almost always — and the textbook answer, which favours linked lists for insertion and deletion, is misleading on modern hardware because it ignores how memory actually behaves. The theoretical

Question 6

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 7

What is the difference between COPY and ADD in a Dockerfile?

Accepted Answer

Both instructions move files from the build context into the image, and for ordinary files they behave identically. The difference is that ADD has two extra behaviours that trigger automatically, and

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