Python: Variable Assigment

Python Variable Assigment

Python Variable Assigment

1 Overview

📘 Variable Assigment

In Python, variable assignment is the process where you use the assignment operator = to give a name (the variable) to a value (the object). Variables in Python do not have a fixed type; a variable can point to any kind of object, and you can reassign it to a different type later. This flexible behavior is called dynamic typing.

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2 Variable Assigment

In Python, everything is an object (we will better define this in future lessons). Each object has a type, a value, and a unique identity (its address in memory). When using the assignment operator =, we are attaching a name to an object.

For example:

x = 10  # creates an object of type int in memory with a value of 10 and assigns it to the variable name x
print(type(x))  # <class 'int'>
# check memory location
print(id(x))

Let’s see a more complex example:

a = 3
b = a
c = a

• An int with value 3 is stored in memory and assigned to the name a.
• Name b is assigned to the same object in memory as a.
• The same occurs with c; all three share the same memory address.

print(id(a))
print(id(b))
print(id(c))
# All outputs will be identical, showing they share the same address

If we reassign the variable a with a new value:

• A new object with that value is allocated in memory
• a now points to it
• b and c remain unaltered, pointing to the original memory address

a = "hello"
print(id(a))
print(id(b))
print(id(c))
# a now has a new id (address), b and c remain with the original id

If two or more names point to the same memory location and we modify the contents, the value is altered in all the variables:

x = [1, 2, 3]
y = x
x[^0] = 5
print(y)  # [5, 2, 3]

2.1 The is Keyword

Used for identity comparison.

• == is used to compare the values of two objects to check if they are equal.
• is checks if two variables refer to the same object in memory.

Example:

x = [1, 2, 3]
y = [1, 2, 3]
print(x == y)  # True, because values are equal
print(x is y)  # False, because they are different objects in memory

2.2 Memory Optimization of Immutables

Python may optimize memory use by reusing the same memory location for multiple variables:

• Occurs for some small ints, short strings, or short tuples.

Example:

a = "hello"
b = "hello"
print(a == b)  # True
print(a is b)  # True

But for longer strings:

a = "This is a longer string that won't be optimized"
b = "This is a longer string that won't be optimized"
print(a == b)  # True
print(a is b)  # False

Typical reuse occurs with ints between -5 and 256:

a = 5
b = 5
print(a is b)  # True

a = 1000
b = 1000
print(a is b)  # False

3 Garbage Collection (GC)

• The garbage collector is a built-in system in Python that frees memory from objects no longer in use.
• Works alongside reference counting.

3.1 Reference Counting

• The main memory management technique in Python.
• Objects have a count of references to them.
• When the count drops to zero, memory is deallocated.

Example:

a = [1, 2, 3]  # List created, reference count = 1
b = a          # Reference count increased to 2
del a          # Reference count decreased to 1
del b          # Reference count drops to 0, list memory deallocated