Variable Assignment¶

Basic Assignment¶

1. Simple Assignment¶

a = 543
print(f"{a = }")        # a = 543
print(f"{id(a) = }")    # Memory address

2. Type Changes¶

a = 543      # Integer
print(type(a))  # <class 'int'>

a = 'boy'    # String
print(type(a))  # <class 'str'>

3. What Happens¶

Each assignment: 1. Creates/retrieves object 2. Binds name to object 3. Increments reference count

x = 42
# 1. Integer object 42 created/retrieved
# 2. Name 'x' bound to object
# 3. Refcount of 42 incremented

Multiple Assignment¶

1. Simultaneous¶

a, b = 1, 2
print(f"{a = }, {b = }")  # a = 1, b = 2

Process:

# Step 1: Create tuple (1, 2)
# Step 2: Unpack to a, b

2. Unpacking¶

# With starred expressions
numbers = list(range(10))
first, *middle, last = numbers
print(f"{first = }")   # first = 0
print(f"{middle = }")  # middle = [1,2,3,4,5,6,7,8]
print(f"{last = }")    # last = 9

3. Throwaway Values¶

# Using _ for unused variables
_, status_code, _ = ("HTTP", 200, "OK")
print(f"Status: {status_code}")  # Status: 200

Chained Assignment¶

1. Basic Chaining¶

a = b = c = 1
print(f"{a = }, {b = }, {c = }")  # All are 1

2. Execution Order¶

# Evaluated right-to-left
# But assigned left-to-right
x = 42
original_id = id(x)

# All names point to same object
y = z = x
print(id(x) == id(y) == id(z))  # True

3. Not Equivalent¶

# a = b = c = 1
# Is NOT exactly:
# c = 1; b = c; a = b

# Because:
a = b = c = [1, 2, 3]
# All refer to SAME list

# vs
c = [1, 2, 3]
b = c.copy()
a = b.copy()
# Three DIFFERENT lists

Identity Check¶

1. Same Object¶

x = 42
y = 42
print(f"{x is y = }")  # May be True (interning)
print(f"{id(x) == id(y) = }")  # May be True

2. Different Objects¶

x = [1, 2, 3]
y = [1, 2, 3]
print(f"{x is y = }")  # False
print(f"{x == y = }")  # True

Assignment Patterns¶

1. Swapping¶

# Pythonic swap
a, b = 10, 20
a, b = b, a
print(f"{a = }, {b = }")  # a = 20, b = 10

2. Walrus Operator¶

# Python 3.8+
data = [1, 2, 3, 4, 5]
if (n := len(data)) > 3:
    print(f"Large dataset: {n} items")

3. Conditional¶

# Ternary assignment
x = 10
result = "positive" if x > 0 else "non-positive"
print(result)  # positive

Common Mistakes¶

1. Tuple Creation¶

# Automatic tuple packing
a = 1, 2, 3
print(type(a))  # <class 'tuple'>

# Explicit
b = (1, 2, 3)
print(type(b))  # <class 'tuple'>

2. List vs Tuple¶

# List unpacking
[a, b, c] = [1, 2, 3]

# Tuple unpacking (more common)
a, b, c = 1, 2, 3

# Both work the same