Adding code#
Let's create some code to be tested. Imagine we want to model the speed of a swallow carrying a cargo. We know from movies that there are multiple swallow species and that they are not all migratories. So first, we could create an enum to account for the species:
from enum import Enum
class SwallowSpecies(str, Enum):
AFRICAN = "african" # non-migratory
EUROPEAN = "european" # migratory
Next, we want to define our swallow and its cargo, so let's define a class:
class Swallow:
def __init__(self, species: str, cargo_weight: float = 0) -> None:
if cargo_weight < 0:
raise ValueError("Cargo weight cannot be negative")
if species.upper() != "AFRICAN" and species.upper() != "EUROPEAN":
raise ValueError('Species must be either "african" or "european"')
self.species = SwallowSpecies[species.upper()]
self._cargo_weight = cargo_weight
We made _cargo private because we don't want negative weights to be set after initialization. How to make sure of that? Let's prevent it in the setter:
class Swallow:
...
@property
def cargo_weight(self) -> float:
return self._cargo_weight
@cargo_weight.setter
def cargo_weight(self, value: float) -> None:
if value < 0:
raise ValueError("Cargo weight cannot be negative")
self._cargo_weight = value
Right, now we cannot change the cargo weight. Let's finally add methods to compute the speed or tell users whether the species is migratory:
class Swallow:
...
def get_speed(self) -> float:
if self.cargo_weight >= 0.45:
# the swallow is going backward, one pound is too heavy
return -60.0 / (1 + self._cargo_weight)
return 60.0 / (1 + self._cargo_weight)
def is_migratory(self) -> bool:
return self.species == SwallowSpecies.AFRICAN
Now that we have code, we can write tests that can verify it behaves as expected!
The code could not be simpler, why test it?
What if you decided to add an Asian swallow that is migratory. You might modify SwallowSpecies and then the Swallow.__init__ but forget to update Swallow.is_migratory. How would you know about the problem before running into it? By running tests automatically! Never underestimate our propension to forget things!
