Control Flow Constructs

break ends the innermost enclosing loop immediately and control resumes after it. continue abandons only the rest of the current pass and moves on to the next item or condition test. Neither statement touches an outer loop.

enumerate() wraps any iterable and lazily yields (count, item) two-tuples. The start argument sets the first count, so start=1 numbers items from one. It shifts only the counter, never which items come out.

A list iterator holds an integer index and advances it on every pass. Deleting an element shifts everything after it down one slot, so the next index lands one element too far and the shifted item is never visited.

The else clause of a for or while loop runs when the loop ends normally: the iterable is exhausted or the while condition turns false. A break skips it, and an empty iterable still triggers it.

range(10, 0, -1) yields 10 down to 1: start is included and stop excluded, so 0 never appears. A range is empty whenever the step cannot carry start toward stop, as in range(0, 5, -1).

Python evaluates cond first. If it is truthy, only a is evaluated and becomes the value; otherwise only b is. The branch that is not chosen never runs, so its side effects and its exceptions never happen.

A dict literal evaluates every value eagerly while the dict is being built, so handler() would run the handler right there and store its return value. Storing the bare name keeps the function object, which you call after the lookup.

Conditions are tested top to bottom and the first truthy one wins: its block runs and the whole rest of the chain is skipped. If none is true, the optional else block runs.

:= is an assignment expression: it binds a name and also evaluates to the assigned value, so it can be used inside a condition or a call. Plain = is a statement, which produces no value at all.

The conditional expression binds looser than +, so Python reads it as ('Result: ' + status) if ok else 'fail'. When ok is falsy the prefix silently disappears. Parenthesize the conditional expression so it is a single operand of the concatenation.

A bare name in a case is a capture pattern: it always matches and binds the subject to that name, comparing nothing. To compare against a named constant you must write a dotted value pattern such as Color.RED.

case _: is the wildcard pattern: it matches any subject and binds nothing, so the name _ keeps whatever value it already had. Because it always succeeds, it must be the last case in the match statement.

A guard is an extra if expression written after a case pattern. It is checked only once that pattern has matched and bound its names; a falsy guard rejects the case and matching moves on to the next one.

It matches any sequence of at least two items - a list, tuple, range, deque and so on, but never a str or bytes. The first two items bind to a and b, and everything left over binds to rest as a new list.

Python's match statement has no fall-through. The first case whose pattern matches runs its block, and control then jumps past the whole statement, so there is nothing left to break out of.