Elo ranking and Bradly-Terry

1. try/catch/else/finally

Encountered an interesting piece of code for try/catch

def no_env_var(var: str):
    try:
        # If you have this var, remove it
        if val := os.environ.get(var, None):
            del os.environ[var]
        yield
    finally:
        # Restore the var after this function
        if val:
            print("1", os.environ.get(var, None))
            os.environ[var] = val
            print("2", os.environ.get(var, None))

os.environ["abc"] = "123"
print("3",os.environ.get("abc",None))
with no_env_var("abc"):
    print("4",os.environ.get("abc",None))
print("5",os.environ.get("abc",None))

Couple of things to notice:

1. finally will always execute
2. try/catch/else is for exception case and no exception case.
3. The field in the code will make the finally execute after the field finish

So the output is

3 123    # Set inital var 
4 None   # inside the no_env_var scope
1 None   # In finally
2 123    # In finally, restore the var
5 123    # outside finally

2 Python Methods

1. normal method in a class takes self argument implicitly
2. staticmethod does NOT take self argument. Or you can add it explicitly, as a normal function
3. classmethold takes cls argument implicityly

    class Pizza(object):
        def __init__(self, size):
            self.size = size
        def get_size(self):
            return self.size
        @staticmethod
        def get_shape():
            return 'round'
        @classmethod
        def get_name(cls):
            return cls
    Pizza.get_size
    #<function __main__.Pizza.get_size(self)>
    Pizza.get_shape
    #<function __main__.Pizza.get_shape()>
    Pizza.get_name
    #<bound method Pizza.get_name of <class '__main__.Pizza'>>

1. Normal method has to run it with an object

     Pizza.get_size()
     #TypeError: Pizza.get_size() missing 1 required positional argument: 'self'
     Pizza(10).get_size()
     # 10
     Pizza.get_size(Pizza(10))
     # This is sth trick to implement. Also return 10 
   

2. staticmethod and classmethod can run with the class or an object

     Pizza.get_shape()
     # round
     Pizza(10).get_shape()
     # round
     Pizza.get_name()
     # __main__.Pizza
     Pizza(10).get_name()
     # __main__.Pizza
   

3. abstractmethod is used in base class and force to be modified. ```python import abc

class BasePizza(object): @abc.abstractmethod def get_radius(self): “"”Method that should do something.””” def get_size(self): # will not raise unless call get_size raise NotImplementedError BasePizza() # Will error even without calling get_radius #TypeError: Can’t instantiate abstract class BasePizza with abstract methods get_radius ```