functools.partial usage

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`partial(multiply, 3)` creates a new callable with `a` pre-filled as 3. Calling `triple(7)` then passes `b=7`, so the full call is `multiply(3, 7)` which returns 21. The trick is that `partial` fills positional arguments from the left - so `triple` is effectively `lambda b: multiply(3, b)`. This pattern is useful whenever you want to specialize a general function without rewriting it.