Python - herougo/SoftwareEngineerKnowledgeRepository GitHub Wiki

Asyncio

• mixing asyncio with decorators: https://stackoverflow.com/questions/42043226/using-a-coroutine-as-decorator

def foo(f):
    async def wrapper(*args, **kwargs):
        return await f(*args, **kwargs)
    return wrapper

@foo
async def boo(*args, **kwargs):
    pass

• Quart: https://medium.com/snaptravel/how-we-optimized-service-performance-using-the-python-quart-asgi-framework-and-reduced-costs-by-1362dc365a0

  • Personal notes: CPU context switching: CPU saves the thread state and does something else.

SQLAlchemy

• there are 2 options for having your code interact with databases:
  • Gateway classes which act as the interface code and SQL queries (results in lots of code)
  • Object-Relational Mapping (ORM) use an OO paradigm for DB data manipulation (e.g. SQLAlchemy)
• this works

class Parent(Base):  # Booking
    __tablename__ = "parent"
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey("child.id"), nullable=True)

class Child(Base):  # BookingPatchEvent
    __tablename__ = "child"
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey("parent.id"))
    parent = relationship(Parent, uselist=False, foreign_keys=[parent_id])

• questions:
  • When you define a relationship, what happens behind the scenes when you access that attribute?
  • What if you overwrite the attribute with something else?

Metaclasses

Source: (optional) https://www.youtube.com/watch?v=NAQEj-c2CI8

• type of a class is "type" (e.g. type(MyClass))
  • "type" is the metaclass
• this creates a class

MyClass = type('MyClass', (BaseClass,), {'attr1': 0, 'method1': my_method})

• this ...

class Meta(type):
    def __new__(cls, class_name, bases, attrs):
        # called when the class itself is created (not instances of the class)
        print(attrs)
        result = super().__new__(cls, class_name, bases, attrs)
        return result

class Dog(metaclass=Meta):
    x = 5
    def hello(self):
        print('hi')

# attributes (x and hello) are printed without needing to create a Dog object

Code Quirks

Code Quirk 1

• this works

class A:
    a = 'hello'
    b = a
assert A().b == 'hello'