The Magic of Dynamic Typing

x = "10"
y = 5

# This will crash your program (Good!)
# print(x + y)  # TypeError: can only concatenate str (not "int") to str

# You must be explicit
print(int(x) + y)  # 15
print(x + str(y))  # "105"

class Book:
    def __len__(self):
        return 300  # Pages

class Movie:
    def __len__(self):
        return 120  # Minutes

# This function assumes 'item' is anything with a length
def describe_length(item):
    print(f"This item has length: {len(item)}")

# It works on built-ins
describe_length("Hello")      # This item has length: 5
describe_length([1, 2, 3])    # This item has length: 3

# And on our custom objects!
describe_length(Book())       # This item has length: 300
describe_length(Movie())      # This item has length: 120
    
# We didn't have to inherit from a "Lengthable" interface (like Java)

from typing import List, Optional

# Without hints: What is 'items'? List of ints? Strings?
def tough_old_way(items):
    return items[0].upper()

# With hints: We know 'users' is a list of strings
def modern_way(users: List[str]) -> str:
    if not users:
        return ""
    return users[0].upper()

# If you try this in VS Code, it will underline the error!
# modern_way([1, 2, 3])  # Error: List[int] is not compatible with List[str]

class Hero:
    pass

p = Hero()
p.name = "Batman"
p.power = 9000

print(p.__dict__)
# Output: {'name': 'Batman', 'power': 9000}

# This explains why we can add attributes at runtime!
p.sidekick = "Robin"
print(p.__dict__)
# Output: {'name': 'Batman', 'power': 9000, 'sidekick': 'Robin'}