Variables

A variable in Python is a pointer to some content.

Address

Technically, a variable in Python is a pointer, and it contains a memory address to an allocated memory zone containing an object.

x = 42
id(x)

139907129622096

In Python, when numbers are small (between -5 and 255 let say), the memory address are equal.

x = 42
y = 42
print(id(x))
print(id(y))

139907129622096
139907129622096

x = 4398473209847023984723981029836012983
y = 4398473209847023984723981029836012983
print(id(x))
print(id(y))

139906654312272
139906654314288

L = [1, 4]
print(id(L))

139906654305152

L = [1, 4]
M = [1, 4]
print(id(L))
print(id(M))

139907016431104
139906654343488

Types

Python is dynamically typed: each variable x has a type type(x).

type(2)

int

type([1, 2])

list

type((1, 2))

tuple

type((1,))

tuple

type({1, 2})

set

type(frozenset((1, 2)))

frozenset

Mutability vs Immutability

A variable is mutable when the content can be changed, and immutable otherwise.

x = 2

x = 2
x += 1

x = (1, 4)

x = (1, 4)
x[0] = 2

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

Cell In[16], line 2
      1 x = (1, 4)
----> 2 x[0] = 2


TypeError: 'tuple' object does not support item assignment

x = [1, 4]

• What do you think about this picture for the datastructure for a list?

The problem is about resizing the list. The pointer may change. The solution in CPython is to add a new level of indirection.

x = [1, 4]
x[1] = 5

Functions

For the function, it is the same.

def f(x):
    return x+1

type(f)

function

Attributes and methods

Each type, e.g. int is a class has attributes and methods obtained with dir(int). dir(2) gives attributes and methods of 2.

dir(int)

['__abs__',
 '__add__',
 '__and__',
 '__bool__',
 '__ceil__',
 '__class__',
 '__delattr__',
 '__dir__',
 '__divmod__',
 '__doc__',
 '__eq__',
 '__float__',
 '__floor__',
 '__floordiv__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__getnewargs__',
 '__gt__',
 '__hash__',
 '__index__',
 '__init__',
 '__init_subclass__',
 '__int__',
 '__invert__',
 '__le__',
 '__lshift__',
 '__lt__',
 '__mod__',
 '__mul__',
 '__ne__',
 '__neg__',
 '__new__',
 '__or__',
 '__pos__',
 '__pow__',
 '__radd__',
 '__rand__',
 '__rdivmod__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__rfloordiv__',
 '__rlshift__',
 '__rmod__',
 '__rmul__',
 '__ror__',
 '__round__',
 '__rpow__',
 '__rrshift__',
 '__rshift__',
 '__rsub__',
 '__rtruediv__',
 '__rxor__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__sub__',
 '__subclasshook__',
 '__truediv__',
 '__trunc__',
 '__xor__',
 'as_integer_ratio',
 'bit_length',
 'conjugate',
 'denominator',
 'from_bytes',
 'imag',
 'numerator',
 'real',
 'to_bytes']

dir(2)

['__abs__',
 '__add__',
 '__and__',
 '__bool__',
 '__ceil__',
 '__class__',
 '__delattr__',
 '__dir__',
 '__divmod__',
 '__doc__',
 '__eq__',
 '__float__',
 '__floor__',
 '__floordiv__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__getnewargs__',
 '__gt__',
 '__hash__',
 '__index__',
 '__init__',
 '__init_subclass__',
 '__int__',
 '__invert__',
 '__le__',
 '__lshift__',
 '__lt__',
 '__mod__',
 '__mul__',
 '__ne__',
 '__neg__',
 '__new__',
 '__or__',
 '__pos__',
 '__pow__',
 '__radd__',
 '__rand__',
 '__rdivmod__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__rfloordiv__',
 '__rlshift__',
 '__rmod__',
 '__rmul__',
 '__ror__',
 '__round__',
 '__rpow__',
 '__rrshift__',
 '__rshift__',
 '__rsub__',
 '__rtruediv__',
 '__rxor__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__sub__',
 '__subclasshook__',
 '__truediv__',
 '__trunc__',
 '__xor__',
 'as_integer_ratio',
 'bit_length',
 'conjugate',
 'denominator',
 'from_bytes',
 'imag',
 'numerator',
 'real',
 'to_bytes']

dir(f)

['__annotations__',
 '__call__',
 '__class__',
 '__closure__',
 '__code__',
 '__defaults__',
 '__delattr__',
 '__dict__',
 '__dir__',
 '__doc__',
 '__eq__',
 '__format__',
 '__ge__',
 '__get__',
 '__getattribute__',
 '__globals__',
 '__gt__',
 '__hash__',
 '__init__',
 '__init_subclass__',
 '__kwdefaults__',
 '__le__',
 '__lt__',
 '__module__',
 '__name__',
 '__ne__',
 '__new__',
 '__qualname__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__']

f.__code__

<code object f at 0x7f3ea9d110e0, file "/tmp/ipykernel_100995/1304018138.py", line 1>

Reference counters

import sys
lst = []
extra_ref = lst
sys.getrefcount(lst)

3

import sys
sys.getrefcount([])

Quiz: same object or not?

L = [[] for i in range(5)]
L[0].append(1)
L

[[1], [], [], [], []]

L = [[]] * 5
L[0].append(1)
L

[[1], [1], [1], [1], [1]]