🧠 Python DeepCuts — 💡 Name Binding vs Assignment

In Python, variables don’t “hold” values like boxes — they’re names bound to objects in memory.

Understanding this distinction helps you reason about how Python handles references, mutability, and side effects.

🧩 Variables Are Just Names

When you assign a value to a variable, Python binds that name to an existing object — it doesn’t copy the data.

a = [1, 2, 3]
b = a

print("a is b:", a is b)   # True — both names refer to the same list

Now both a and b point to the same list in memory.

🔁 Rebinding vs Mutation

• Rebinding: assign a name to a new object
• Mutation: modify the contents of an existing object

a = [1, 2, 3]
b = a

a.append(4)      # mutate — same object
print(a, b)      # both change

a = [10, 20, 30] # rebind — a now points to a new list
print(a, b)      # only a changes

This is why “pass-by-assignment” is a better way to describe how Python handles arguments.

🧩 Immutable Objects Behave Differently

Integers, floats, strings, and tuples are immutable — any “change” creates a new object.

x = 10
y = x

x += 5           # creates a new int object
print(x, y)      # x: 15, y: 10
print(id(x), id(y))  # different IDs

Immutable objects can’t be modified in place, so every operation results in a new reference.

⚡ Mutability in Action: Lists vs Tuples

The += operator behaves differently for mutable and immutable types:

a = [1, 2]
b = a
a += [3, 4]
print(a is b)   # True — list mutated in place

x = (1, 2)
y = x
x += (3, 4)
print(x is y)   # False — new tuple created

Here’s the key:

• list.iadd() mutates
• tuple.add() creates a new object

🧭 Visualizing Name Binding

Think of names as arrows pointing to objects.

Multiple names can point to the same object — until one name is rebound.

data = {"key": [1, 2, 3]}
alias = data

alias["key"].append(4)
print(data, alias)       # both changed
print(data is alias)     # True

Both names reference the same dictionary, so mutation through one affects the other.

✅ Key Points

• ✅ Assignment binds names — it doesn’t copy data.
• ✅ Mutable objects change in place; immutable ones don’t.
• ✅ is checks identity; == checks value.
• ✅ Rebinding moves the label; mutation modifies the target.
• 🧠 Everything in Python is about names → objects → references.