How do references in functions work?

问题

First I wrote the first sample of code and it didn't work correctly. I prefer the first sample, but only the second one works correctly. I don't know why the first sample doesn't change the original array but second does. Where is the difference?

First sample:

import heapq

def heap_sort(tab):
    heap = []
    for i in tab:
        heapq.heappush(heap, i)
    tab = [heapq.heappop(heap) for _ in xrange(len(heap))]

temp_tab = [4, 3, 5, 1]
heap_sort(temp_tab)
print temp_tab

Prints:

[4, 3, 5, 1]

Second sample:

import heapq

def heap_sort(tab):
    heap = []
    for i in tab:
        heapq.heappush(heap, i)
    for i, _ in enumerate(tab):
        tab[i] = heapq.heappop(heap)

temp_tab = [4, 3, 5, 1]
heap_sort(temp_tab)
print temp_tab

Prints:

[1, 3, 4, 5]

回答1:

You could also use [:], that will change the original object passed in:

def heap_sort(tab):
    heap = []
    for i in tab:
        heapq.heappush(heap, i)
    tab[:] = [heapq.heappop(heap) for _ in xrange(len(heap))]

So instead of reassigning the name tab to a new object you are actually updating the original tab object.

You could also use a generator expression instead of building the whole list:

tab[:] = (heapq.heappop(heap) for _ in xrange(len(heap)))

回答2:

Because you're just reassigning a new name called tab inside the function, it doesn't affect the global name tab you've defined. So, change your function to actually return the value, will work:

import heapq

def heap_sort(tab):
    heap = []
    for i in tab:
        heapq.heappush(heap, i)
    # return the supposed tab value
    return [heapq.heappop(heap) for _ in xrange(len(heap))]

tab = [4, 3, 5, 1]
# assign the tab to the returned value
tab = heap_sort(tab)
print tab
[1, 3, 4, 5]

For your reference, read How do I pass a variable by reference? will help you understand how the referencing works in Python.

回答3:

Try this:

>>> def heap_sort(tab):
    heap=[]
    for i in tab:
        heapq.heappush(heap,i)
    heapq.heapify(heap)
    return heap

>>> t=heap_sort(t)
>>> print(t)
[1, 3, 5, 4]

来源：https://stackoverflow.com/questions/29884268/how-do-references-in-functions-work

标签

python

python-2.7

reference