I assumed sequence types in Python were value types. It turns out they’re reference types (Meaning that the value of a variable won’t be copied when assigned to a new variable, but referenced). So now I’m wondering, what are the value types in Python? That is, what types in Python can I assign to new variables without worrying that the variable was referenced?

All values in Python are references. What you need to worry about is if a type is mutable. The basic numeric and string types, as well as tuple and frozenset are immutable; names that are bound to an object of one of those types can only be rebound, not mutated.

>>> t = 1, 2, 3
>>> t[1] = 42
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

Coming from iOS development using strongly typed Swift language, Python reference was a bit confusing so I decided to do a little comparison. Here is the summary: –

  • When assigning a variable to python say a = 10 you are simply pointing/referencing the the object in this case 10 which is stored in a memory. So if that object changes then the value of a variable a also changes but changing a does not change the object 10, This behave similar to Swift Primitive value types such as Int.

To make this clear here is an example: –

 # "a" points to an object in this case 10
a = 10

# "b" points to the same object which a points but does not point to a variable a.
b = a 

# Now if we change "a" to point to another object in memory say 20. 
a = 20

# "b" still points to the old object 10 in other words
# "b == 10" but "a == 20", This is because "b" was never pointing to the variable "a" 
# even though we assigned it as "b = a" instead it was pointing to the object 10
#  which is # the same as writing b = 10. 

Let’s check with a more complex data structure List

list1 = [10,20,30,40]
list2 = list1 #[10,20,30,40]

list1 = [3,4] 

# list1 ==> [3,4]
# list2 ==> [10,20,30,40]

Again that behave the same to Swift and other similar languages. Here comes the huge difference Let’s try changing value at a certain index ( This gets more tricky)

list1 = [10,20,30,40]
list2 = list1 #[10,20,30,40]

# change value of list 1 at a certain index say index 0
list1[0] = 500

# If you check again the values of list1 and list2 you will be surprised. 
#list1 ==> [500,20,30,40]
#list2 ==> [500,20,30,40]

They both change because they were all pointing to the same object so changing the object changes all list1 and list2. This is very confusing from other Languages such as Swift. In Swift List/Array are value types meaning they are not referenced instead they are copied around, However in python it is another story, changing a value at a certain index results in changing that value for all properties which references that object just like in the example above. This is very important to keep in mind for folks coming from Swift or other similar languages.

So how do we copy in python?

  • If you want to copy the list in python then you have to explicitly do so as shown on the example below: –
list1 = [10,20,30,40]
list2 = list(list1)

# list1 ==> [10,20,30,40]
# list2 ==> [10,20,30,40]

Doing so will avoid undesired effects when list1 changes list2 will remain the same.

As an example

list1[0] = 500
#list1 ==> [500,20,30,40] # Changed
#list2 ==> [10,20,30,40] # Unchanged

The answer above is correct, but I object to the semantics of “reference”.

C-like languages treat variables as fixed buckets, in which values are placed. When you call a function, a new set of buckets are created, and the values are copied into them. Sometimes, a bucket is pass-by-reference, and actually becomes an alias for the caller’s bucket.

Python, on the other hand, treats variables as mere labels (names) for values (objects). When you call a function, a new set of labels are created and slapped onto those same objects.

It doesn’t make sense to mention “references” in the context of Python, because in every other language, “reference” implies an alternative to “value”. Python has no such duality; it just passes around and assigns objects. Nothing is referred to.

Nitpicky, perhaps, but the terminology causes no end of confusion for C++ programmers, who e.g. hear Python passes by references and don’t understand how to reassign the caller’s names.