高阶函数python演练 [英] higher order function python walkthrough
问题描述
def tracer(fn):
def traced(x):
print('Calling',fn,'(',x,')')
result=fn(x)
print('Got',result,'from',fn,'(',x,')')
return result
return traced
def fact(n):
if n ==0:
return 1
return n * fact(n-1)
new_fact = tracer(fact)
new_fact(2)
我在pythontutor.com上使用此代码可以更好地理解高阶函数,但是我很难理解为什么将第8步中的new_fact(2)映射为traced?换句话说,被跟踪的函数如何知道参数为2?
I used this code on pythontutor.com to better understand higher order functions, but i'm having difficulty understanding why new_fact(2) in step 8 is mapped to traced? In other words, how does the traced function know the argument is 2?
推荐答案
在Python中,函数也是对象.当您调用tracer()
函数时,它将返回嵌套的traced()
函数;实际上是在创建该函数的新副本:
In Python, functions are objects too. When you call the tracer()
function, it returns the nested traced()
function; it is in fact creating a new copy of that function:
return traced
您将返回的函数对象存储在new_fact
中,然后将其命名为:
You stored that returned function object in new_fact
, then called it:
>>> tracer(fact)
<function traced at 0x10644c320>
>>> new_fact = tracer(fact)
>>> new_fact
<function traced at 0x10644c398>
>>> new_fact(2)
('Calling', <function fact at 0x10644c230>, '(', 2, ')')
('Got', 2, 'from', <function fact at 0x10644c230>, '(', 2, ')')
2
您可以使用任何功能执行此操作;用另一个名称存储对函数的引用:
You can do this with any function; store a reference to a function in another name:
>>> def foo(): print 'foo'
...
>>> bar = foo
>>> bar()
foo
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