python immutable and mutable
When beginning with python, you may probably get involved in the caos caused by the immutable and mutable concepts in python.
Yes, python has no such stuff as pointers as you love so much in C. But mutable type kind of behave like one (maybe only a little bit).
But we'll begin with immutable.
Number, string, tuple, these are immutable types.
a = 1
b = '123'
c = (1,2,3)
The immutable don't mean a,b,c is immutable.
so, it's valid we do:
a = 2
b = '234'
c = (1,2)
But what they refer to are immutable:
b[1] = '5'
c[0] = 8
do this, and you'll get the excpetion info: ' xx type don't support item assignment'
Yes, that's it. And you can think it like this:
a = 2
b = 2
this in fact create a stuff '2' , and a and b are just reference to it.
you can't change the '2'
a -------> 2
^
|
b --------- |
but you can change a or b to let them refer to different value, and that value will be created first to let a or b to refer to.
Then, mutable.
List, dict are mutable.
If you've understood immutable type. It's easy now to understand this.
With mutable type, you can modify the value your reference points to.
a = [1,2,3]
b = a
a.append(4)
now b is also [1,2,3,4]
This time, the [1,2,3] a and b refers to are modifiable.
So, pay attention to them.
With immutable type, you can only create a new value or some stuff to redirect your reference variable to.
With mutable, you can change those stuff