functools partial 如何完成它的工作? [英] How does functools partial do what it does?
问题描述
我无法理解 functools 中的部分是如何工作的.我在这里有以下代码:
<预><代码>>>>总和 = lambda x, y : x + y>>>总和(1, 2)3>>>incr = lambda y : sum(1, y)>>>增量(2)3>>>def sum2(x, y):返回 x + y>>>incr2 = functools.partial(sum2, 1)>>>incr2(4)5现在排队
incr = lambda y : sum(1, y)
我知道我传递给 incr
的任何参数都将作为 y
传递给 lambda
它将返回 sum(1, y)
即 1 + y
.
我明白.但是我没看懂这个incr2(4)
.
4
如何在部分函数中作为 x
传递?对我来说,4
应该取代 sum2
.x
和 4
是什么关系?
粗略地说,partial
做了这样的事情(除了关键字 args 支持等):
def partial(func, *part_args):定义包装器(*extra_args):args = 列表(part_args)args.extend(extra_args)返回函数(*参数)返回包装器
因此,通过调用 partial(sum2, 4)
,您可以创建一个新函数(准确地说是一个可调用的),其行为类似于 sum2
,但具有一个位置参数较少的.缺少的参数总是被 4
替换,所以 partial(sum2, 4)(2) == sum2(4, 2)
至于为什么需要它,有多种情况.仅举一例,假设您必须将一个函数传递到预期有 2 个参数的地方:
class EventNotifier(object):def __init__(self):self._listeners = []def add_listener(self, callback):''' 回调应该接受两个位置参数,事件和参数 '''self._listeners.append(回调)# ...定义通知(自我,事件,*参数):对于 self._listeners 中的 f:f(事件,参数)
但是你已经拥有的函数需要访问第三个 context
对象才能完成它的工作:
def log_event(context, event, params):context.log_event("发生了 %s, %s", event, params)
所以,有几种解决方案:
自定义对象:
类监听器(对象):def __init__(self, context):self._context = 上下文def __call__(self, event, params):self._context.log_event("发生了 %s, %s", event, params)notifier.add_listener(监听器(上下文))
Lambda:
log_listener = lambda 事件,参数:log_event(context, event, params)notifier.add_listener(log_listener)
带有分音:
context = get_context() # 随便notifier.add_listener(部分(日志事件,上下文))
在这三个中,partial
是最短和最快的.(对于更复杂的情况,您可能需要自定义对象).
I am not able to get my head on how the partial works in functools. I have the following code from here:
>>> sum = lambda x, y : x + y
>>> sum(1, 2)
3
>>> incr = lambda y : sum(1, y)
>>> incr(2)
3
>>> def sum2(x, y):
return x + y
>>> incr2 = functools.partial(sum2, 1)
>>> incr2(4)
5
Now in the line
incr = lambda y : sum(1, y)
I get that whatever argument I pass to incr
it will be passed as y
to lambda
which will return sum(1, y)
i.e 1 + y
.
I understand that. But I didn't understand this incr2(4)
.
How does the 4
gets passed as x
in partial function? To me, 4
should replace the sum2
. What is the relation between x
and 4
?
Roughly, partial
does something like this (apart from keyword args support etc):
def partial(func, *part_args):
def wrapper(*extra_args):
args = list(part_args)
args.extend(extra_args)
return func(*args)
return wrapper
So, by calling partial(sum2, 4)
you create a new function (a callable, to be precise) that behaves like sum2
, but has one positional argument less. That missing argument is always substituted by 4
, so that partial(sum2, 4)(2) == sum2(4, 2)
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:
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)
But a function you already have needs access to some third context
object to do its job:
def log_event(context, event, params):
context.log_event("Something happened %s, %s", event, params)
So, there are several solutions:
A custom object:
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))
Lambda:
log_listener = lambda event, params: log_event(context, event, params)
notifier.add_listener(log_listener)
With partials:
context = get_context() # whatever
notifier.add_listener(partial(log_event, context))
Of those three, partial
is the shortest and the fastest.
(For a more complex case you might want a custom object though).
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