Command-Query Separation

A function should either do something (command) or answer something (query) — never both.

# dirty — modifies AND returns a value in the same function
def solve(stack, value):
    stack.append(value)
    return len(stack)  # query mixed with command

Refactor into two separate functions:

• push(stack, value) — command: pushes value, returns None
• size(stack) — query: returns the current length, no mutation

solve(stack, value, 'push') dispatches to push · solve(stack, 'size') dispatches to size.