Abz Rockers Abz Rockers - 1 year ago 75
Python Question

Python call by reference issue

As what I have understand on python, when you pass a variable on a function parameter it is already reference to the original variable. On my implementation when I try to equate a variable that I pass on the function it resulted empty list.

This is my code:

#on the main -------------
temp_obj = []
obj = [
{'name':'a1', 'level':0},
{'name':'a2', 'level':0},
{'name':'a3', 'level':1},
{'name':'a4', 'level':1},
{'name':'a5', 'level':2},
{'name':'a6', 'level':2},

the_result = myFunction(obj, temp_obj)

#above print would result to an empty list
#this is my problem

#end of main body -------------

def myFunction(obj, new_temp_obj):
inside_list = []
for x in obj[:]:
if x['level'] == 0:
obj.remove(x) #removing the element that was added to the inside_list

new_temp_obj = obj[:] #copying the remaining element

# the above print would result to
#[{'name': 'a3', 'level': 1}, {'name': 'a4', 'level': 1}, {'name': 'a5', 'level': 2}, {'name': 'a6', 'level': 2}]

return inside_list

Am I missing something or did I misunderstand the idea of python call by reference?

Answer Source

Python is not pass-by-reference. It is pass-by-object.

Consider the following two functions:

def f(mylist):
    mylist = []

def g(mylist):

Now let's say I call them.

mylist = [1]

mylist = [1] # reset the list

What would the output be?

If Python were pass-by-value, the functions would take a copy of the list, so modifying it would not affect the original list once you return out of the function. So in both cases, you would be printing the original list, [1].

If Python were pass-by-reference, the functions would accept a reference to the object and modifying it would modify the actual object that the reference references, so the first output would be [] and the second, [1,2].

If you run this example, you will find that the first output is [1] (the list if unaffected) and second output is [1,2] (the list is affected).


When you do new_temp_obj = obj[:], Python is constructing a new object obj[:] and giving it the name new_temp_obj.

If you were to append, Python would look for the thing called new_temp_obj and add elements to it. The argument you passed in tells it where to look for the list.

You are creating a totally new object at a totally new location in memory and simply giving it the same name,new_temp_obj

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