Python high-order functions

Question

Given the following code, i cant understand why f and fib behaves differently. the example taken from Barkley cs61a course

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

when i load the code into the interpreter i got this output:

>>> 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

Aran-Fey · Accepted Answer · 2016-07-23 11:41:13Z

2

f.call_count is 1 because f calls fib, and then fib recursively calls itself to calculate the result. In this whole procedure, f is only called once.

But when you do fib = decor(fib), you're overwriting the fib function in the global scope, so from that moment on fib will recursively call the decorated fib.

answered Jul 23, 2016 at 11:41

Aran-Fey

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Comments

muhuk · Accepted Answer · 2016-07-23 11:45:06Z

You are overwriting fib and then f calls fib instead of itself:

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)

When I run this the output is:

=== 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

Note that fib is function at 0x7fa844639758 all the time.

is it means that fib is the same at the global scope ? i assume binding fib again to decor(fib) returns a new function which is not related to the 'old' fib, exactly as f does.... is it something specific to python ? make me think i miss some basic language understanding. seems obvious that rebind fib should return same as f

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