Python JAVA immutability

immutability

An immutable datatype, has the property that the value of an object never changes once constructed. The downside of immutability is that a new object must be created for every value.

Python immutable data type

• int, float, long, complex
• str
• bytes
• tuple
• frozen set

Python mutable data type

• byter array
• list
• set
• dict

Difference between i+=1 and i=i+1

	method	return value	immutable object	mutable object
i+=1	iadd	the object that was muted	two methods are the same(because the old instance can't be muted, so a new one has to be created)	change the object inplace and return the muted object
i=i+1	add	a new instance of sth	two methods are the same	create a new instance and return the new instance reference

Example

a = [1, 2, 3]
b = a
b += [1, 2, 3]
print a  #[1, 2, 3, 1, 2, 3]
print b  #[1, 2, 3, 1, 2, 3]

a = [1, 2, 3]
b = a
b = b + [1, 2, 3]
print a #[1, 2, 3]
print b #[1, 2, 3, 1, 2, 3]