Classes

The types discussed in the previous chapter are only useful in conjunction with compatible classes. A class is is compatible with a particular type if all attributes and methods present in the type (defined by the respective protocol class) are also present in the class itself. For example, a City class may look something like this:

city.py

from dataclasses import dataclass


@dataclass
class City:
   name: str
   founded: int
   latitude: float
   longitude: float
   elevation: float

As the City class has latitude and longitude attributes, an instance of it is also an instance of the Location type:

$ python -i city.py
>>> from pysmo import Location
>>> nyc = City(name='New York', founded='1624', \
               latitude=40.7, longitude=-74, elevation=10)
>>> isinstance(nyc, Location)
True

This example also illustrates that a class may contain additional attributes and methods that are not part of the type definition. In fact, a class may even match multiple types!

Warning

In cases where a class contains additional attributes or methods, we strongly suggest to never access these in e.g. a function using pysmo types as input. For example, the code below will run if the city input is an instance of the City class (because it has the name attribute). However, the name attribute may not be present in another class that matches the Location type, resulting in a runtime error when this function is called.

# Do not do this!
def print_coordinates(city: Location) -> None:
   print(f"The coordinates of {city.name} are: {city.latitude}, {city.longitude}")