6  Identity & References

Now that we’ve seen all of the built-in types we can take a second look at mutability and explore what Python is doing under the hood, so that we are less likely to be surprised by the behavior.

Names & Mutability Revisited

Remember that when we do an assignment, we are associating a name with an object, a value in memory.

It is the object that has a type, not the name.

# a name is bound to the result of the expression
x = 1 + 1
# the name is re-assigned, we aren't changing data
x = x + 1
# this is why we can re-assign to a different type
x = "hello"

Immutable Types

• str
• tuple
• frozenset
• scalars: int, float, complex, bool, None

For immutable types, this is the only option, any changes require reassignment.

Mutable Types

• list
• dict
• set

On the other hand, mutable values can be changed in place.

x = [1, 2, 3]
x.append(4)  # no re-assignment needed!
print(x)

Object

All types in Python share an internal representation as an object (PyObject in C).

ll = [1, 2, 3, 4]
yy = ll           # increase ref count

object

Field	Example	Purpose
id	393239323	uniquely identify object within Python interpreter
refcount	2	count how many names currently point to this object
type	list	type of object
data	0x80000000	memory address where the actual data is stored
length	4	Only present on collection types, stores pre-computed length.

Notice that name is not stored on the object! Why not?

Shared references

Multiple names can refer to the same object in memory, this is noticable when the objects in question are mutable.

x = [1, 2, 3]
y = x
y.append(4)
print(f"{y=}")
# spooky action at a distance
print(f"{x=}")

y=[1, 2, 3, 4]
x=[1, 2, 3, 4]

For immutables, any change causes reassignment:

a = 3
b = a
a *= 2         # reassignment!
print(f"{a=} {b=}")

a=6 b=3

Garbage Collection

Python is a garbage collected language.

We don’t free our own memory, Python does instead.

Behind the scenes, Python stores a reference counter on each object. How many names/objects reference the object.

When reference count drops to zero, Python can reclaim the memory.

Identity

The built-in id(...) function returns the identity of an object, which is an integer value guaranteed to be unique and constant for lifetime of object

In the official (“CPython”) Interpreter we are using in this class, it is the address of the memory location storing the object.

x = "Orange" 
print(id(x))  # Unique integer-value for the object pointed by x

4375914096

y = "Apple" 
print(id(y)) 

4393888736

fruit1 = ("Apples", 4)
fruit2 = ("Apples", 4)
fruit3 = fruit2
print(f"Fruit1 id = {id(fruit1)} \n Fruit2 id = {id(fruit2)}")
print(f"Fruit3 id= {id(fruit3)}")

Fruit1 id = 4394173120 
 Fruit2 id = 4394173184
Fruit3 id= 4394173184

fruit1 is fruit2

False

Equality vs. Identity

Two different ways of testing if objects are the “same”:

• Equality operator (==): Returns true if two objects are equal (i.e., have the same value)
• Identity operator (is): Returns true if two objects identities are the same.

a is b means id(a) == id(b)

a = [1, 2, 3]
b = [1, 2, 3]
print("a == b", a == b)

print(id(a))
print(id(b))
print("a is b", a is b)  # The id values are different

a == b True
4394339968
4394170176
a is b False

print(id(None))

4340290104

def f():
    pass
id(f())

4340290104

is None

If you ever need to check if a value is None, you’d use is None or is not None

list / string mutability revisited

# list d
d = [1, 2, 3]
print(id(d))
d.append(4)
print(d)
print(id(d))

4394340224
[1, 2, 3, 4]
4394340224

# str D
s = "Hello"
print(id(s))
s += " World"
print(s)

# did s change?
print(id(s))

4393893152
Hello World
4394351472

Aside: Object Creation Quirk

Each time you generate a new value in your script by running an expression, Python creates a new object (i.e., a chunk of memory) to represent that value.

– Learning Python 2013

Not quite! CPython does not guarantee this, and in fact sometimes caches & reuses immutable objects for efficiency.

a = 100000000
b = 100000000

# Two different objects, two different ids.
print(a is b)

False

a = 100
b = 100

# However, for small integer objects, CPython caches them
# this means that a and b point to the same object
print(a is b)

True

# CPython does the same for short strings
str1 = "MPCS"
str2 = "MPCS"
print(id(str1), id(str2))
str1 is str2

4393805760 4393805760

True

In practice this is just a quirk of the CPython interpreter, since the objects are immutable it isn’t important to know that they share memory in some cases.

copy & deepcopy

If y = x does not make a copy, how can we get one?

We’ve seen the .copy() method on a few of our types. Which ones?

We can also use the copy module:

x = [1, 2, 3]
y = x.copy()

print(id(x))
print(id(y))

x.append(4)
print(x, y)

4394171776
4394172224
[1, 2, 3, 4] [1, 2, 3]

# shallow copy example (nested mutables are not copied)

x = [[1, 2], [3, 4]]
y = x.copy()  # or copy.copy(x)

print("x is y", x is y)
print("x[0] is y[0]", x[0] is y[0])
print("x[1] is y[1]", x[1] is y[1])

# print(x, y)
x[0].append(5)
print(x, "\n", y)

x is y False
x[0] is y[0] True
x[1] is y[1] True
[[1, 2, 5], [3, 4]] 
 [[1, 2, 5], [3, 4]]

# deep copy (nested mutables are copied)
import copy

# copy.copy(obj) --> same as obj.copy()
z = copy.deepcopy(x)
print("x[0] is z[0]", x[0] is z[0])

x[0] is z[0] False