TIL: Using `__post_init__` with Python Dataclasses

dataclass `__post_init__`

Today I learnt that you can use __post_init__ to initiate fields that depend upon other fields within a dataclass.

Within a project I'm currently working on I have the following dataclass:

from dataclasses import dataclass

@dataclass
class FitnessProfile:
    """Represents an athlete's fitness profile.

    Attributes:
        name: A str of the athlete's name.
        time_trial_distance: An int of the time trial distance in meters.
        time_trial_time: An int of the time taken to complete the 2km time trial.

    Properties:
        max_aerobic_speed: A float representing MAS in meters per second.
    """

    name: str
    time_trial_distance: int
    time_trial_time: int

    @property
    def max_aerobic_speed(self) -> float:
        """Maximal Aerobic Speed in m/s (rounded to 2 decimal places)."""
        if self.time_trial_time != 0:
            return round(self.time_trial_distance / self.time_trial_time, 2)
        else:
            return 0

Within the FitnessProfile I'm using a property to dynamically calculate max_aerobic_speed which depends upon the time_trial_distance and time_trial_time fields in the dataclass.

>>> from post_init import FitnessProfile
>>> profile = FitnessProfile("John Smith", 2000, 480,)
>>> profile
FitnessProfile(name='John Smith', time_trial_distance=2000, time_trial_time=480)
>>> profile.max_aerobic_speed
4.17

Using a property means that if either time_trial_distance or time_trial_time are updated after an instance is instantiated then max_aerobic_speed will use the updated values to provide an updated value.

# current profile
>>> profile
FitnessProfile(name='John Smith', time_trial_distance=2000, time_trial_time=480)

# update time_trial_time
>>> profile.time_trial_time = 450

# updated profile with new time_trial_time
>>> profile
FitnessProfile(name='John Smith', time_trial_distance=2000, time_trial_time=450)

# max_aerobic_speed using updated time_trial_time
>>> profile.max_aerobic_speed
4.44

However, within the current project I'm not going to be updating fields within FitnessProfile once the dataclass instance has been instantiated. Therefore an alternative is to use the __post_init__ to initialise the max_aerobic_speed field.

from dataclasses import dataclass, field

@dataclass
class FitnessProfile:
    """Represents an athlete's fitness profile.

    Attributes:
        name: A str of the athlete's name.
        time_trial_distance: An int of the time trial distance in meters.
        time_trial_time: An int of the time taken to complete the 2km time trial.
        max_aerobic_speed: A float representing MAS in meters per second.
    """

    name: str
    time_trial_distance: int
    time_trial_time: int
    max_aerobic_speed: float = field(init=False)

    def __post_init__(self):
        self.max_aerobic_speed = round(self.time_trial_distance / self.time_trial_time, 2)

Using the updated FitnessProfile:

>>> from post_init import FitnessProfile
>>> profile = FitnessProfile("John Smith", 2000, 480)
>>> profile
FitnessProfile(name='John Smith', time_trial_distance=2000, time_trial_time=480, max_aerobic_speed=4.17)

This is fine for my use case but be aware that if I update either time_trial_distance or time_trial_time then max_aerobic_speed won't be updated using the new field values:

# initial profile
>>> profile = FitnessProfile("John Smith", 2000, 480)
>>> profile
FitnessProfile(name='John Smith', time_trial_distance=2000, time_trial_time=480, max_aerobic_speed=4.17)

# update time_trial_time
>>> profile.time_trial_time = 450

# updated profile with new time_trial_time
>>> profile
FitnessProfile(name='John Smith', time_trial_distance=2000, time_trial_time=450, max_aerobic_speed=4.17)

# unchanged max_aerobic_speed field
>>> profile.max_aerobic_speed
4.17

dataclass __post_init__

dataclass `__post_init__`