Immutable vs Mutable types

Question

I\'m confused on what an immutable type is. I know the float object is considered to be immutable, with this type of example from my book:

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Answer 1

What? Floats are immutable? But can't I do

x = 5.0
x += 7.0
print x # 12.0

Doesn't that "mut" x?

Well you agree strings are immutable right? But you can do the same thing.

s = 'foo'
s += 'bar'
print s # foobar

The value of the variable changes, but it changes by changing what the variable refers to. A mutable type can change that way, and it can also change "in place".

Here is the difference.

x = something # immutable type
print x
func(x)
print x # prints the same thing

x = something # mutable type
print x
func(x)
print x # might print something different

x = something # immutable type
y = x
print x
# some statement that operates on y
print x # prints the same thing

x = something # mutable type
y = x
print x
# some statement that operates on y
print x # might print something different

Concrete examples

x = 'foo'
y = x
print x # foo
y += 'bar'
print x # foo

x = [1, 2, 3]
y = x
print x # [1, 2, 3]
y += [3, 2, 1]
print x # [1, 2, 3, 3, 2, 1]

def func(val):
    val += 'bar'

x = 'foo'
print x # foo
func(x)
print x # foo

def func(val):
    val += [3, 2, 1]

x = [1, 2, 3]
print x # [1, 2, 3]
func(x)
print x # [1, 2, 3, 3, 2, 1]