Immutable vs Mutable types

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Solving problem is about exposing yourself to as many situations as possible like Immutable vs Mutable types and practice these strategies over and over. With time, it becomes second nature and a natural way you approach any problems in general. Big or small, always start with a plan, use other strategies mentioned here till you are confident and ready to code the solution.
In this post, my aim is to share an overview the topic about Immutable vs Mutable types, which can be followed any time. Take easy to follow this discuss.

Immutable vs Mutable types

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:

class RoundFloat(float):
    def __new__(cls, val):
        return float.__new__(cls, round(val, 2))

Is this considered to be immutable because of the class structure / hierarchy?, meaning float is at the top of the class and is its own method call. Similar to this type of example (even though my book says dict is mutable):

class SortedKeyDict(dict):
    def __new__(cls, val):
        return dict.__new__(cls, val.clear())

Whereas something mutable has methods inside the class, with this type of example:

class SortedKeyDict_a(dict):
    def example(self):
        return self.keys()

Also, for the last class(SortedKeyDict_a), if I pass this type of set to it:

d = (('zheng-cai', 67), ('hui-jun', 68),('xin-yi', 2))

without calling the example method, it returns a dictionary. The SortedKeyDict with __new__ flags it as an error. I tried passing integers to the RoundFloat class with __new__ and it flagged no errors.

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]
Answered By: morningstar

Answer #2:

You have to understand that Python represents all its data as objects. Some of these objects like lists and dictionaries are mutable, meaning you can change their content without changing their identity. Other objects like integers, floats, strings and tuples are objects that can not be changed.
An easy way to understand that is if you have a look at an objects ID.

Below you see a string that is immutable. You can not change its content. It will raise a TypeError if you try to change it. Also, if we assign new content, a new object is created instead of the contents being modified.

>>> s = "abc"
>>>id(s)
4702124
>>> s[0]
'a'
>>> s[0] = "o"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> s = "xyz"
>>>id(s)
4800100
>>> s += "uvw"
>>>id(s)
4800500

You can do that with a list and it will not change the objects identity

>>> i = [1,2,3]
>>>id(i)
2146718700
>>> i[0]
1
>>> i[0] = 7
>>> id(i)
2146718700

To read more about Python’s data model you could have a look at the Python language reference:

Answered By: sebs

Answer #3:

Common immutable type:

  1. numbers: int(), float(), complex()
  2. immutable sequences: str(), tuple(), frozenset(), bytes()

Common mutable type (almost everything else):

  1. mutable sequences: list(), bytearray()
  2. set type: set()
  3. mapping type: dict()
  4. classes, class instances
  5. etc.

One trick to quickly test if a type is mutable or not, is to use id() built-in function.

Examples, using on integer,

>>> i = 1
>>> id(i)
***704
>>> i += 1
>>> i
2
>>> id(i)
***736 (different from ***704)

using on list,

>>> a = [1]
>>> id(a)
***416
>>> a.append(2)
>>> a
[1, 2]
>>> id(a)
***416 (same with the above id)
Answered By: Spectral

Answer #4:

First of all, whether a class has methods or what it’s class structure is has nothing to do with mutability.

ints and floats are immutable. If I do

a = 1
a += 5

It points the name a at a 1 somewhere in memory on the first line. On the second line, it looks up that 1, adds 5, gets 6, then points a at that 6 in memory — it didn’t change the 1 to a 6 in any way. The same logic applies to the following examples, using other immutable types:

b = 'some string'
b += 'some other string'
c = ('some', 'tuple')
c += ('some', 'other', 'tuple')

For mutable types, I can do thing that actallly change the value where it’s stored in memory. With:

d = [1, 2, 3]

I’ve created a list of the locations of 1, 2, and 3 in memory. If I then do

e = d

I just point e to the same list d points at. I can then do:

e += [4, 5]

And the list that both e and d points at will be updated to also have the locations of 4 and 5 in memory.

If I go back to an immutable type and do that with a tuple:

f = (1, 2, 3)
g = f
g += (4, 5)

Then f still only points to the original tuple — you’ve pointed g at an entirely new tuple.

Now, with your example of

class SortedKeyDict(dict):
    def __new__(cls, val):
        return dict.__new__(cls, val.clear())

Where you pass

d = (('zheng-cai', 67), ('hui-jun', 68),('xin-yi', 2))

(which is a tuple of tuples) as val, you’re getting an error because tuples don’t have a .clear() method — you’d have to pass dict(d) as val for it to work, in which case you’ll get an empty SortedKeyDict as a result.

Answered By: agf

Answer #5:

If you’re coming to Python from another language (except one that’s a lot like Python, like Ruby), and insist on understanding it in terms of that other language, here’s where people usually get confused:

>>> a = 1
>>> a = 2 # I thought int was immutable, but I just changed it?!

In Python, assignment is not mutation in Python.

In C++, if you write a = 2, you’re calling a.operator=(2), which will mutate the object stored in a. (And if there was no object stored in a, that’s an error.)

In Python, a = 2 does nothing to whatever was stored in a; it just means that 2 is now stored in a instead. (And if there was no object stored in a, that’s fine.)


Ultimately, this is part of an even deeper distinction.

A variable in a language like C++ is a typed location in memory. If a is an int, that means it’s 4 bytes somewhere that the compiler knows is supposed to be interpreted as an int. So, when you do a = 2, it changes what’s stored in those 4 bytes of memory from 0, 0, 0, 1 to 0, 0, 0, 2. If there’s another int variable somewhere else, it has its own 4 bytes.

A variable in a language like Python is a name for an object that has a life of its own. There’s an object for the number 1, and another object for the number 2. And a isn’t 4 bytes of memory that are represented as an int, it’s just a name that points at the 1 object. It doesn’t make sense for a = 2 to turn the number 1 into the number 2 (that would give any Python programmer way too much power to change the fundamental workings of the universe); what it does instead is just make a forget the 1 object and point at the 2 object instead.


So, if assignment isn’t a mutation, what is a mutation?

  • Calling a method that’s documented to mutate, like a.append(b). (Note that these methods almost always return None). Immutable types do not have any such methods, mutable types usually do.
  • Assigning to a part of the object, like a.spam = b or a[0] = b. Immutable types do not allow assignment to attributes or elements, mutable types usually allow one or the other.
  • Sometimes using augmented assignment, like a += b, sometimes not. Mutable types usually mutate the value; immutable types never do, and give you a copy instead (they calculate a + b, then assign the result to a).

But if assignment isn’t mutation, how is assigning to part of the object mutation? That’s where it gets tricky. a[0] = b does not mutate a[0] (again, unlike C++), but it does mutate a (unlike C++, except indirectly).

All of this is why it’s probably better not to try to put Python’s semantics in terms of a language you’re used to, and instead learn Python’s semantics on their own terms.

Answered By: abarnert

Answer #6:

Difference between Mutable and Immutable objects

Definitions

Mutable object: Object that can be changed after creating it.
Immutable object: Object that cannot be changed after creating it.

In python if you change the value of the immutable object it will create a new object.

Mutable Objects

Here are the objects in Python that are of mutable type:

  1. list
  2. Dictionary
  3. Set
  4. bytearray
  5. user defined classes

Immutable Objects

Here are the objects in Python that are of immutable type:

  1. int
  2. float
  3. decimal
  4. complex
  5. bool
  6. string
  7. tuple
  8. range
  9. frozenset
  10. bytes

Some Unanswered Questions

Questions: Is string an immutable type?
Answer: yes it is, but can you explain this:
Proof 1:

a = "Hello"
a +=" World"
print a

Output

"Hello World"

In the above example the string got once created as “Hello” then changed to “Hello World”. This implies that the string is of the mutable type. But it is not when we check its identity to see whether it is of a mutable type or not.

a = "Hello"
identity_a = id(a)
a += " World"
new_identity_a = id(a)
if identity_a != new_identity_a:
    print "String is Immutable"

Output

String is Immutable

Proof 2:

a = "Hello World"
a[0] = "M"

Output

TypeError 'str' object does not support item assignment

Questions: Is Tuple an immutable type?
Answer: yes, it is.
Proof 1:

tuple_a = (1,)
tuple_a[0] = (2,)
print a

Output

'tuple' object does not support item assignment
Answered By: Anand Tripathi

Answer #7:

Whether an object is mutable or not depends on its type. This doesn’t depend on whether or not it has certain methods, nor on the structure of the class hierarchy.

User-defined types (i.e. classes) are generally mutable. There are some exceptions, such as simple sub-classes of an immutable type. Other immutable types include some built-in types such as int, float, tuple and str, as well as some Python classes implemented in C.

A general explanation from the “Data Model” chapter in the Python Language Reference”:

The value of some objects can change. Objects whose value can change
are said to be mutable; objects whose value is unchangeable once they
are created are called immutable.

(The value of an immutable container
object that contains a reference to a mutable object can change when
the latter’s value is changed; however the container is still
considered immutable, because the collection of objects it contains
cannot be changed. So, immutability is not strictly the same as having
an unchangeable value, it is more subtle.)

An object’s mutability is
determined by its type; for instance, numbers, strings and tuples are
immutable, while dictionaries and lists are mutable.

Answered By: taleinat

Answer #8:

A mutable object has to have at least a method able to mutate the object. For example, the list object has the append method, which will actually mutate the object:

>>> a = [1,2,3]
>>> a.append('hello') # `a` has mutated but is still the same object
>>> a
[1, 2, 3, 'hello']

but the class float has no method to mutate a float object. You can do:

>>> b = 5.0
>>> b = b + 0.1
>>> b
5.1

but the = operand is not a method. It just make a bind between the variable and whatever is to the right of it, nothing else. It never changes or creates objects. It is a declaration of what the variable will point to, since now on.

When you do b = b + 0.1 the = operand binds the variable to a new float, wich is created with te result of 5 + 0.1.

When you assign a variable to an existent object, mutable or not, the = operand binds the variable to that object. And nothing more happens

In either case, the = just make the bind. It doesn’t change or create objects.

When you do a = 1.0, the = operand is not wich create the float, but the 1.0 part of the line. Actually when you write 1.0 it is a shorthand for float(1.0) a constructor call returning a float object. (That is the reason why if you type 1.0 and press enter you get the “echo” 1.0 printed below; that is the return value of the constructor function you called)

Now, if b is a float and you assign a = b, both variables are pointing to the same object, but actually the variables can’t comunicate betweem themselves, because the object is inmutable, and if you do b += 1, now b point to a new object, and a is still pointing to the oldone and cannot know what b is pointing to.

but if c is, let’s say, a list, and you assign a = c, now a and c can “comunicate”, because list is mutable, and if you do c.append('msg'), then just checking a you get the message.

(By the way, every object has an unique id number asociated to, wich you can get with id(x). So you can check if an object is the same or not checking if its unique id has changed.)

Answered By: nadapez

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