"Subclassing and Composition – A Pythonic Tour of Trade-Offs", Hyne...

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class Base:
pass
class Sub(Base):
pass
Subclassing

class Base:
pass
class Sub(Base):
pass
Subclassing

@dataclass
class Base:
x: int
class Sub(Base):
y = 0
def compute(self):
self.y += self.x * 2

@dataclass
class Base:
x: int
def cool_new_feature(self):
self.y = "LOL"
class Sub(Base):
y = 0
def compute(self):
self.y ...

Composition

@dataclass
class A
b: B
y: int = 0
Composition

@dataclass
class A
b: B
y: int = 0
Composition

@dataclass
class A
b: B
y: int = 0
def compute(self):
self.y += self.b.x * 2
Composition

@dataclass
class A
b: B
y: int = 0
def compute(self):
self.y += self.b.x * 2
Composition

Goose Chase
Program Flow™

class A:
def method(self): ...
class B:
def method(self): ...
class C:
def method(self): ...
class You(A, B, C):
def foo(s...

class A:
def method(self):
self.bar()
class B(A):
def bar(self): ...
class C(B):
def bar(self): ...
class You(C):
def foo(...

class A:
def method(self):
self.bar()
class B(A):
def bar(self): ..
class C(B):
def bar(self): ..
.
.
class You(C):
def fo...

— Architecture Patterns with Python, page 131

— Architecture Patterns with Python, page 131

Repository

Tracking
Repository

class AbstractTrackingRepository(abc.ABC):
def init (self):
self.seen = set()

class AbstractTrackingRepository(abc.ABC):
def init (self):
self.seen = set()
@abc.abstractmethod
def _add(self, product):...

class AbstractTrackingRepository(abc.ABC):
def init (self):
self.seen = set()
def add_product(self, product):
self._add(pr...

class AbstractTrackingRepository(abc.ABC):
def init (self):
self.seen = set()
def add_product(self, product):
self._add(pr...

class FakeTrackingRepository(AbstractTrackingRepository):
def init (self, products):
super(). init ()
self._products = set...

class FakeTrackingRepository(AbstractTrackingRepository):
def init (self, products):
super(). init ()
self._products = set...

class FakeTrackingRepository(AbstractTrackingRepository):
def init (self, products):
super(). init ()
self._products = set...

class AbstractTrackingRepository(abc.ABC):
def init (self):
self.seen = set()
def add_product(self, product):
self._add(pr...

class AbstractTrackingRepository(abc.ABC):
def init (self):
self.seen = set()
def add_product(self, product):
self._add(pr...

class AbstractRepository(abc.ABC):
@abc.abstractmethod
def add(self, product: Product) -> None:
...
@abc.abstractmethod
de...

class AbstractRepository(abc.ABC):
@abc.abstractmethod
def add(self, product: Product) -> None:
...
@abc.abstractmethod
de...

y(abc.ABC):
Nominal subtyping
class AbstractRepositor (metaclass=abc.ABCMeta):
@abc.abstractmethod
def add(self, product: ...

@typing.runtime_checkable
class Repository(typing.Protocol):
def add(self, product: Product) -> None:
...
def get(self, sk...

@typing.runtime_checkable
class Repository(typing.Protocol):
def add(self, product: Product) -> None:
...
def get(self, sk...

@typing.runtime_checkable
class Repository(typing.Protocol):
def add(self, product: Product) -> None:
...
def get(self, sk...

class FakeRepository:
def init (self, products):
self._products = set(products)

class FakeRepository:
def init (self, products):
self._products = set(products)
def add(self, product: Product ) -> None:
...

class FakeRepository:
def init (self, products):
self._products = set(products)
def add(self, product: Product ) -> None:
...

class FakeRepository:
def init (self, products):
self._products = set(products)
def add(self, product: Product ) -> None:
...

class FakeRepository:
def init (self, products):
self._products = set(products)
def add(self, product: Product ) -> None:
...

class FakeRepository(AbstractRepository):
def init (self, products):
self._products = set(products)
def add(self, product:...

class FakeRepository:
def init (self, products):
self._products = set(products)
def add(self, product: Product ) -> None:
...

@AbstractRepository.register
class FakeRepository:
def init (self, products):
self._products = set(products)
def add(self,...

@dataclass
class TrackingRepository:
_repo: Repository
seen: set[Product] = field(default_factory=set)

@dataclass
class TrackingRepository:
_repo: Repository
seen: set[Product] = field(default_factory=set)
def add_product(sel...

@dataclass
class TrackingRepository:
_repo: Repository
seen: set[Product] = field(default_factory=set)
def add_product(sel...

@dataclass
class TrackingRepository:
_repo: Repository
seen: set[Product] = field(default_factory=set)
def add_product(sel...

Subclassing
requires knowledge + self-discipline

Subclassing
requires knowledge + self-discipline
Composition
mechanically enforces discipline

Namespaces are one honking great
idea – let’s do more of those!
— Uncle Timmy, Zen of Python, PEP 20

def decorator(cls):
cls.a = 42
return cls

def decorator(cls):
cls.a = 42
return cls
@decorator
class C
pass

def decorator(cls):
cls.a = 42
return cls
@decorator
class C
pass
assert hasattr(C, "a")

def decorator(cls):
cls.a = 42
return cls
@decorator
class C
pass
assert hasattr(C, "a")
== C = decorator(_C)

class Question(django.db.models.Model):
question_text = CharField(max_length=200)
pub_date = DateTimeField('date published...

class Question(django.db.models.Model):
question_text = CharField(max_length=200)
pub_date = DateTimeField('date published...

class Question(django.db.models.Model):
question_text = CharField(max_length=200)
pub_date = DateTimeField('date published...

[' annotations ', ' class ', ' dataclass_fields ', ' dataclass_params ',
' delattr ', ' dict ', ' dir ', ' doc ', ' eq ', ...

[' annotations ', ' class ', ' dataclass_fields ', ' dataclass_params ',
' delattr ', ' dict ', ' dir ', ' doc ', ' eq ', ...

['DoesNotExist', 'MultipleObjectsReturned', ' class ', ' delattr ', ' dict ',
' dir ', ' doc ', ' eq ', ' format ', ' ge '...

['DoesNotExist', 'MultipleObjectsReturned', ' class ', ' delattr ', ' dict ',
' dir ', ' doc ', ' eq ', ' format ', ' ge '...

class QuestionsRepository:
conn: sqlalchemy.engine.Connection
def get(self, id: int) -> Question:
row = conn.execute(
self...

-> Question
class QuestionsRepository:
conn: sqlalchemy.engine.Connection
def get(self, id: int) :
row = self.conn.execute...

class QuestionsRepository:
conn: sqlalchemy.engine.Connection
def get(self, id: int) -> Question:
row = self.conn.execute(...

['DoesNotExist', 'MultipleObjectsReturned', ' class ', ' delattr ', ' dict ',
getstate ', ' gt ', ' hash ', ' init ', ' in...

Readability

Readability
Control

Readability Convenience
Control

Readability Convenience
Control Performance

Seth Michael Larson
Working on urllib3 full-time for one week

Programs are meant to
be read by humans and
only incidentally for
computers to execute.
— Abelson & Sussman, SICP
Seth Mic...

class A:
b: B
def other_method(): ...
def method(self):
self.other_method()
self.b.even_other_method()
class B:
a: A
def e...

class A:
b: B
def other_method(): ...
def method(self):
self.other_method()
self.b.even_other_method()
class B:
a: A
def e...

class C:
a: A
b: B
def method(self):
self.a.other_method()
self.b.even_other_method()

Exceptions

class MyError(Exception):
pass
Exceptions

class MyError(Exception):
pass
class MyError(ValueError, KeyError):
pass
Exceptions

class MyError(Exception):
pass
class MyError(ValueError, KeyError):
pass
Exceptions

E-Mail Addresses

class Mailbox:
id: UUID
addr: str
pwd: str
E-Mail Addresses

class Mailbox:
id: UUID
addr: str
pwd: str
class Forwarder
id: UUID
addr: str
targets: list[str]
E-Mail Addresses

class Mailbox:
id: UUID
addr: str
pwd: str
class Forwarder
id: UUID
addr: str
targets: list[str]
E-Mail Addresses
EmailAdd...

class Mailbox:
id: UUID
addr: str
pwd: str
class Forwarder
id: UUID
addr: str
targets: list[str]
E-Mail Addresses
EmailAdd...

Duplication is far cheaper
than a wrong abstraction.
— Sandi Metz

Create one class, make fields optional

class AddrType(enum.Enum):
MAILBOX = "mailbox"
FORWARDER = "forwarder"
class EmailAddr
type: AddrType
id: UUID
addr: str
C...

class AddrType(enum.Enum):
MAILBOX = "mailbox"
FORWARDER = "forwarder"
class EmailAddr
type: AddrType
id: UUID
addr: str
#...

class AddrType(enum.Enum):
MAILBOX = "mailbox"
FORWARDER = "forwarder"
class EmailAddr
type: AddrType
id: UUID
addr: str
#...

Composition

class EmailAddr:
id: UUID
addr: str
Composition

class EmailAddr:
id: UUID
addr: str
class Mailbox
email: EmailAddr
pwd: str
Composition

class EmailAddr:
id: UUID
addr: str
class Mailbox
email: EmailAddr
pwd: str
class Forwarder
email: EmailAddr
targets: list...

class EmailAddr:
id: UUID
addr: str
class Mailbox
email: EmailAddr
pwd: str
class Forwarder
email: EmailAddr
targets: list...

class EmailAddr:
id: UUID
addr: str
class Mailbox
email: EmailAddr
pwd: str
class Forwarder
email: EmailAddr
targets: list...

Create a Common Base Class, Then Specialize

class EmailAddr:
id: UUID
addr: str
Create a Common Base Class, Then Specialize

class EmailAddr:
id: UUID
addr: str
class Mailbox(EmailAddr):
pwd: str
class Forwarder(EmailAddr):
targets: list[str]
Crea...

class EmailAddr:
id: UUID
addr: str
class Mailbox(EmailAddr):
pwd: str
class Forwarder(EmailAddr):
targets: list[str]
Crea...

type EmailAddr struct {
addr string
}
type Mailbox struct {
EmailAddr
pwd string
}

type EmailAddr struct {
addr string
}
type Mailbox struct {
EmailAddr
pwd string
}
func main() {
mbox := Mailbox{EmailAddr...

type EmailAddr struct {
addr string
}
type Mailbox struct {
EmailAddr
pwd string
}
func main() {
mbox := Mailbox{EmailAddr...

I never allow myself to have an
opinion on anything that I don’t
know the other side’s argument
better than they do.
— Cha...

OX.CX/SC

OX.CX/SC
@HYNEK
(@MASTODON.SOCIAL)

OX.CX/SC
@HYNEK
(@MASTODON.SOCIAL)
YOUTUBE.COM/
@THE_HYNEK

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