Mutable & Immutable
Mutable
  • Change the value of a mutable variable will not create a new id
  • list, byte array, sets, dict, class instances
    • #!/usr/bin/python

def changeMutable(v):
	v[2] = 100 # change 3rd element of v in main program
	v = [1, 2, 3, 4] # create a new object, not change v in main program
 
l = range(10)
 
changeMutable(l)
print l# [0, 1, 100, 3, 4, 5, 6, 7, 8, 9]
    Immutable
  • Change the value of an immutable variable will create a new id
  • int, float, complex, str, tuples, bytes, frozensets
    • #!/usr/bin/python

def changeNum(n):
	n *= 10 # create a new object, not change n in main program
 
# python create 10 in memory, all three variable points to the same memory
n = 10;
n2 = n;
n3 = 10;
print id(n), id(n2), id(n3);
 
changeNum(n)
print n# 10
 
# create a string
s = 'Hello '
print id(s)
# create a new string object
s = s+'World!'
print id(s)
 
print s# Hello World!
    Assignment
    #!/usr/bin/python

# create an object and assign the reference to l
l = range(10)
 
# assignment the reference of l to l2
l2 = l
 
# create a new object and assign the reference to l3
l3 = range(10) # mutable variable

# l and l2 point to same object, l3 is a different object
print id(l), id(l2), id(l3)
 
# create an immutable object
i = (1, 2, 3, 4)
 
# i and i2 point to same object
i2 = i
 
# create a new object
i3 = (1, 2, 3, 4)

print id(i), id(i2), id(i3)
    User defined class
  • user defined class can be used as both mutable and immutable
    • # mutable
class Vehicle(object):
    """Document String: Define a Vehicle class"""
 
    def __init__(self, brand):
        self._brand = brand;
 
    def __str__(self):
        return self._brand

    def __hash__(self):
        return hash(self._brand)

    def __eq__(self, other):
        if isinstance(other, self.__class__):
            return self._brand == other._brand
        return NotImplemented

def change(v):
    print('Inside function ...')
    print(id(v)) # 4350403216
    v._brand = 'Lincoln'

def main():
    v = Vehicle("Buick")
    print(id(v)) # 4350403216
    print(v) # Buick

    change(v)
    print('After call change function ...')
    print(id(v)) # 4350403216
    print(v) # Lincoln

if __name__ == '__main__':
    main()
			
    # immutable, need define __hash__ and __eq__
# dictionary use immutable data type as key
class Vehicle(object):
    """Document String: Define a Vehicle class"""
 
    def __init__(self, brand):
        self._brand = brand;
 
    def __str__(self):
        return self._brand

    def __hash__(self):
        return hash(self._brand)

    def __eq__(self, other):
        if isinstance(other, self.__class__):
            return self._brand == other._brand
        return NotImplemented

def main():
    v = Vehicle("Buick")
    container = {v: 100} # use Vehicle instance as key

    print(container[Vehicle("Buick")])

if __name__ == '__main__':
    main()
    Resource
  • CODEHABITUDE