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copied!<p>Roughly, <code>partial</code> does something like this (apart from keyword args support etc):</p> <pre><code>def partial(func, *part_args): def wrapper(*extra_args): args = list(part_args) args.extend(extra_args) return func(*args) return wrapper </code></pre> <p>So, by calling <code>partial(sum2, 4)</code> you create a new function (a callable, to be precise) that behaves like <code>sum2</code>, but has one positional argument less. That missing argument is always substituted by <code>4</code>, so that <code>partial(sum2, 4)(2) == sum2(4, 2)</code></p> <p>As for why it's needed, there's a variety of cases. Just for one, suppose you have to pass a function somewhere where it's expected to have 2 arguments:</p> <pre><code>class EventNotifier(object): def __init__(self): self._listeners = [] def add_listener(self, callback): ''' callback should accept two positional arguments, event and params ''' self._listeners.append(callback) # ... def notify(self, event, *params): for f in self._listeners: f(event, params) </code></pre> <p>But a function you already have needs access to some third <code>context</code> object to do its job:</p> <pre><code>def log_event(context, event, params): context.log_event("Something happened %s, %s", event, params) </code></pre> <p>So, there are several solutions:</p> <p>A custom object:</p> <pre><code>class Listener(object): def __init__(self, context): self._context = context def __call__(self, event, params): self._context.log_event("Something happened %s, %s", event, params) notifier.add_listener(Listener(context)) </code></pre> <p>Lambda:</p> <pre><code>log_listener = lambda event, params: log_event(context, event, params) notifier.add_listener(log_listener) </code></pre> <p>With partials:</p> <pre><code>context = get_context() # whatever notifier.add_listener(partial(log_event, context)) </code></pre> <p>Of those three, <code>partial</code> is the shortest and the fastest. (For a more complex case you might want a custom object though).</p>

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