Python高阶函数

Question

给出下面的代码，我不明白为什么f和fib的行为不同。取自巴克利cs61a课程的例子

def fib(n):
    if n==1 or n==0:
        return n
    else:
        return fib(n-1) + fib(n-2)

def decor(f):
    def counted(*args):
        counted.call_count += 1
        return f(*args)
    counted.call_count = 0
    return counted

当我将代码加载到解释器中时，我得到了以下输出：

>>> fib(6)
8
>>> f = decor(fib)
>>> fib = decor(fib)
>>> # f and fib are both vars that represents a decorated fib function

>>> f(6)
8
>>> f.call_count # why 1 ???
1
>>> 
>>> fib(6)
8
>>> fib.call_count # 49 calls, that's fine
49

Answer 1

f.call_count是1，因为f调用fib，然后fib递归地调用自己来计算结果。在整个过程中，f只被调用一次。

但是当您执行fib = decor(fib)时，您将覆盖全局范围内的fib函数，因此从那时起，fib将递归地调用修饰后的fib。

Answer 2

您正在覆盖fib，然后f调用fib而不是它自己：

def fib(n):
    if n==1 or n==0:
        return n
    else:
        print("fib is: " + repr(fib))
        return fib(n-1) + fib(n-2)

def decor(f):
    def counted(*args):
        counted.call_count += 1
        return f(*args)
    counted.call_count = 0
    return counted



f = decor(fib)
fib = decor(fib)

print("=== f ===")
print(f(6))
print(f.call_count)

print("=== fib ===")
print(fib(6))
print(fib.call_count)

当我运行这个命令时，输出是：

=== f ===
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
8
1
=== fib ===
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
fib is: <function counted at 0x7fa844639758>
8
49

请注意，fib is始终在0x7fa844639758上运行。