[Python] - Call by Object reference & Call by sharing

1. Call by value

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• call by value used to C programming
• call the function , parameter copied value and pass value arguement
• so, do not changed origin data within function

2. Call by reference (pointer)

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• call by reference used to Java programming
• parameter copied reference = pointer (value address)
• so, function can access origin data by address

3. Call by object reference( Call by sharing )

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Python's Object devided into 2 objects ( mutable object & immutable object )

 

immutable object

• tuple , characters , int.. which of immutable objects do not change.
• so, immutable objects, there is no real difference between call by sharing and call by value

mutable object

• list , set , dictionary , which can be changed
• so, within the function are visible to the caller, which may appear to differ from call by value semantics.
• because the object is not copied or cloned -> it is shared

 

# Python Call by Object reference
# Call by sharing

# immutable object
# For immutable objects, there is no real difference between call by sharing and call by value
a = 1

def immutable_object_test(num):
    print( 'before allocated : {} '.format(id(num)) )
    num = 10
    print( 'after allocated : {} '.format(id(num)) )

print( 'a : {}'.format(id(a)))
immutable_object_test(a)
print( 'result : {}'.format(a))
print('-'*10)

# mutable object
# if the objects are mutable, within the function are visible to the caller, which may appear to differ from call by value semantics.
# Mutations of a mutable object within the function are visible to the caller because the object is not copied or cloned—it is shared.
a = [1,2,3,4]
def mutable_object_test( temp ):
    print( 'before allocated : {} '.format(id(temp)) )
    temp.append(5)
    print( 'after allocated : {} '.format(id(temp)) )

print( 'a : {}'.format(id(a)))
mutable_object_test( a )
print( 'result : {}'.format(a))

def try_to_change_list_contents(the_list):
    print('got', the_list)
    the_list.append('four')
    print('changed to', the_list)

outer_list = ['one', 'two', 'three']

print('before, outer_list =', outer_list)
try_to_change_list_contents(outer_list)
print('after, outer_list =', outer_list)

# 결과
before, outer_list = ['one', 'two', 'three']
got ['one', 'two', 'three']
changed to ['one', 'two', 'three', 'four']
after, outer_list = ['one', 'two', 'three', 'four']

def try_to_change_list_reference(the_list):
    print('got', the_list)
    the_list = ['and', 'we', 'can', 'not', 'lie']
    print('set to', the_list)

outer_list = ['we', 'like', 'proper', 'English']

print('before, outer_list =', outer_list)
try_to_change_list_reference(outer_list)
print('after, outer_list =', outer_list)

# 결과
before, outer_list = ['we', 'like', 'proper', 'English']
got ['we', 'like', 'proper', 'English']
set to ['and', 'we', 'can', 'not', 'lie']
after, outer_list = ['we', 'like', 'proper', 'English']

def try_to_change_string_reference(the_string):
    print('got', the_string)
    the_string = 'In a kingdom by the sea'
    print('set to', the_string)

outer_string = 'It was many and many a year ago'

print('before, outer_string =', outer_string)
try_to_change_string_reference(outer_string)
print('after, outer_string =', outer_string)

# 결과
before, outer_string = It was many and many a year ago
got It was many and many a year ago
set to In a kingdom by the sea
after, outer_string = It was many and many a year ago