How are variables structured in Python?

Variables in Python are not containers of data, but rather labels or names that refer to objects in memory.

When you assign a value to a variable, you are not copying that value, but creating a reference to an existing or new object.

Main concepts:

Variable names: Can contain letters (Latin), digits, and underscores. Cannot start with a digit. Case-sensitive.
Objects: Everything in Python is an object. Each object has a type, value, and identifier (memory address).
References: A variable stores a reference to an object's identifier.

Example:

# Create an int object with value 100 and assign its identifier to 'a'
a = 100 
# Variable 'b' now references the same int object with value 100
b = a
# Create a new int object with value 200 and assign its identifier to 'a'
a = 200

In this example, after a = 200, the variable a refers to the new object 200, while the variable b still refers to the object 100.

Dynamic typing: The type of a variable is determined by the type of the object it references and can change during program execution.

# Variable 'x' references an int object
x = 10 
# Now variable 'x' references a str object
x = "hello"

Garbage collection: When there are no more references to an object, Python automatically frees the memory occupied by that object using a garbage collection mechanism.
Immutable vs. Mutable objects:
- Immutable: Numbers, strings, tuples. Attempting to "modify" such an object actually creates a new object.
- Mutable: Lists, dictionaries, sets. Their contents can be changed without creating a new object.

# Immutable str object
s = "world"
# Create a new str object "hello world", 's' now references it
s = "hello " + s

# Mutable list object
l = [1, 2, 3]
# Modify the existing list object
l.append(4)

Thus, variables in Python are flexible and powerful tools for working with data, based on the concept of references to objects.