2.7. Dynamic Typing

2.7.1. Duck typing

• The Unreasonable Effectiveness of Dynamic Typing for Practical Programs by Robert Smallshire

Syntax similarities:

data = {1}
isinstance(data, set)   # True
isinstance(data, dict)  # False

data = {1: 1}
isinstance(data, set)   # False
isinstance(data, dict)  # True

data = {}
isinstance(data, set)   # False
isinstance(data, dict)  # True

data = {1:1}

type(data)
# <class 'dict'>
data
# {1:1}

_ = data.pop(1)

type(data)
# <class 'dict'>
data
# {}

data = {1}

type(data)
# <class 'set'>
data
# {1}

_ = data.pop()

type(data)
# <class 'set'>
data
# set()

2.7.2. Everything is an object

• even function is an object!

2.7.3. Object properties

def add_numbers(a: int, b: float) -> float:
    """Function add numbers"""
    return a + b


print(add_numbers.__doc__)
# Function add numbers

print(add_numbers.__name__)
# add_numbers

print(add_numbers.__annotations__)
# {'a': <class 'int'>, 'b': <class 'float'>, 'return': <class 'float'>}

print(add_numbers.__class__)
# <class 'function'>

2.7.4. Object methods

def add_numbers(a, b):
    """Function add numbers"""
    return a + b


add_numbers(1, 2)
# 3

add_numbers.__call__(1, 2)
# 3

add_numbers()
# Traceback (most recent call last):
# TypeError: function() missing 2 required positional arguments: 'a' and 'b'

add_numbers.__call__()
# Traceback (most recent call last):
# TypeError: function() missing 2 required positional arguments: 'a' and 'b'

2.7.5. Injecting properties

def add_numbers(a, b):
    """Function add numbers"""
    return a + b


add_numbers.myattr = 10

print(add_numbers.myattr)
# 10

2.7.6. Injecting methods

def add_numbers(a, b):
    """Function add numbers"""
    return a + b


add_numbers.say_hello = lambda name: print(f'My name... {name}')

add_numbers.say_hello('José Jiménez')
# My name... José Jiménez

2.7.7. Proxy methods

One of the most common use of *args, **kwargs is for proxy methods:

class Point2D:
    def __init__(self, x, y):
        self.x = x
        self.y = y


class Point3D(Point2D):
    def __init__(self, *args, **kwargs):
        if 'z' in kwargs:
            z = kwargs.pop('z')
        else:
            *args, z = args

        super().__init__(*args, **kwargs)
        self.z = z

    def __str__(self):
        return f'Point3D(x={self.x}, y={self.y}, z={self.z})'


p1 = Point3D(x=1, y=2, z=3)
p2 = Point3D(1, 2, 3)
p3 = Point3D(1, 2, z=3)

print(p1)
# Point3D(x=1, y=2, z=3)

print(p2)
# Point3D(x=1, y=2, z=3)

print(p3)
# Point3D(x=1, y=2, z=3)

2.7.8. Container Class

• A.K.A. Placeholder class

Dynamically creating fields:

class Container:
    def __init__(self, **kwargs):
        for key, value in kwargs.items():
            setattr(self, key, value)


a = Container(firstname='Mark', lastname='Watney')
a.firstname          # 'Mark'
a.lastname           # 'Watney'

b = Container(species='Setosa')
b.species            # 'Setosa'

Dynamically creating fields:

class Astronaut:
    def __init__(self, lastname, **kwargs):
        self.lastname = lastname

        for key, value in kwargs.items():
            setattr(self, key, value)


mark = Astronaut(lastname='Watney', addresses=())
melissa = Astronaut(firstname='Melissa', lastname='Lewis', agency='NASA')

print(mark.lastname)   # Watney
print(melissa.firstname)  # Melissa

print(mark.__dict__)    # {'lastname': 'Watney', 'addresses': ()}
print(melissa.__dict__)    # {'lastname': 'Melissa', 'firstname': 'Lewis', 'agency': 'NASA'}

class Container:
    def __init__(self, **kwargs):
        self.__dict__ = kwargs


a = Container(firstname='Mark', lastname='Watney')
print(a.firstname)          # Mark
print(a.lastname)           # Watney

b = Container(species='Setosa')
print(b.species)             # Setosa

2.7.9. Example

DATA = [
    {"firstname": "Mark", "lastname": "Watney", "addresses": [
        {"street": "2101 E NASA Pkwy", "city": "Houston", "postcode": 77058, "region": "Texas", "country": "USA"},
        {"street": "", "city": "Kennedy Space Center", "postcode": 32899, "region": "Florida", "country": "USA"}]},

    {"firstname": "Melissa", "lastname": "Lewis", "addresses": [
        {"street": "4800 Oak Grove Dr", "city": "Pasadena", "postcode": 91109, "region": "California", "country": "USA"},
        {"street": "2825 E Ave P", "city": "Palmdale", "postcode": 93550, "region": "California", "country": "USA"}]},

    {"firstname": "Rick", "lastname": "Martinez", "addresses": []},

    {"firstname": "Alex", "lastname": "Vogel", "addresses": [
        {"street": "Linder Hoehe", "city": "Cologne", "postcode": 51147, "region": "North Rhine-Westphalia", "country": "Germany"}]}
]


class Container:
    def __init__(self, *args, **kwargs):
        for key, value in kwargs.items():
            setattr(self, key, value)

    def __repr__(self):
        name = self.__class__.__name__
        arguments = tuple(self.__dict__.values())
        return f'\n\n{name}{arguments}'


result = [Container(**data)
          for data in DATA]


print(result)
# [Container('Mark', 'Watney', [{'street': '2101 E NASA Pkwy', 'city': 'Houston', 'postcode': 77058, 'region': 'Texas', 'country': 'USA'}, {'street': '', 'city': 'Kennedy Space Center', 'postcode': 32899, 'region': 'Florida', 'country': 'USA'}]),
#  Container('Melissa', 'Lewis', [{'street': '4800 Oak Grove Dr', 'city': 'Pasadena', 'postcode': 91109, 'region': 'California', 'country': 'USA'}, {'street': '2825 E Ave P', 'city': 'Palmdale', 'postcode': 93550, 'region': 'California', 'country': 'USA'}]),
#  Container('Rick', 'Martinez', []),
#  Container('Alex', 'Vogel', [{'street': 'Linder Hoehe', 'city': 'Cologne', 'postcode': 51147, 'region': 'North Rhine-Westphalia', 'country': 'Germany'}])]