3.4.7. Default values

In pd.read_csv, the only mandatory argument is filepath_or_buffer, which is the name of the csv file (Fig. 3.5). Every other argument has a default value, and exists only to modify the default behaviour of the function.

Let’s examine this concept with a simpler function. We define the following function that calculates the ground reaction force based on the acceleration and mass of a person’s center of mass:

GRAVITATIONAL_CONSTANT = 9.81  # m/s2


def calculate_reaction_force(mass, acceleration):
    """
    Calculate the vertical reaction force based on the mass and acceleration.

    Parameters
    ----------
    mass : float
        Mass of the person in kg.
    acceleration : float
        Acceleration of the center of mass of the person, in m/s2.

    Returns
    -------
    float
        The ground reaction force, in N.

    """
    return (mass * acceleration) + (mass * GRAVITATIONAL_CONSTANT)


# Test the function
print(calculate_reaction_force(mass=60, acceleration=3.5))

798.6

Good practice: Constants

Note these conventions used for GRAVITATIONAL_CONSTANT:

No magic constants: Instead of writing plainly 9.81 in the function’s return statement, we defined the gravity using a name, and we used that name in the equation. We generally want to avoid “magical” constants dispersed around the code: we call them “magical” because after time, we tend to not remember what these values are for, other than making the function work “magically”. Using named constants is a good way to auto-document the code.

Top of the file: It is common practice to define all the constants once and at a same obvious place, which is the top of the file.

CAPITAL_CASE: In Python, a constant is simply a variable that we agree to never modify. To emphasize that it should never be modified, it is common practice to user CAPITAL_CASE to define constants, whereas we use lower_case to define standard variables.

Although this function works very well as is, we may want to generalize it for different gravitational constants, to simulate a similar task in a different gravity. In this case, the gravitational constant could become a parameter of the function:

def calculate_reaction_force(mass, acceleration, gravitational_constant):
    """
    Calculate the ground reaction force based on the mass and acceleration.

    Parameters
    ----------
    mass : float
        Mass of the person in kg.
    acceleration : float
        Acceleration of the center of mass of the person, in m/s2.
    gravitational_constant : float
        Gravitational constant, in m/s2.

    Returns
    -------
    float
        The ground reaction force, in N.

    """
    return (mass * acceleration) + (mass * gravitational_constant)


# Test the function:
print(
    calculate_reaction_force(
        mass=60, acceleration=3.5, gravitational_constant=9.81
    )
)

print(
    calculate_reaction_force(
        mass=60, acceleration=3.5, gravitational_constant=5.2
    )
)

798.6
522.0

However, since most of the time, the gravitational constant really is 9.81 m/s², the function would be clearer with a default value for gravitational_constant. This is done by using an equal = sign in the signature:

def calculate_reaction_force(mass, acceleration, gravitational_constant=9.81):
    """
    Calculate the ground reaction force based on the mass and acceleration.

    Parameters
    ----------
    mass : float
        Mass of the person in kg.
    acceleration : float
        Acceleration of the center of mass of the person, in m/s2.
    gravitational_constant : float
        Gravitational constant, in m/s2. Default is 9.81.

    Returns
    -------
    float
        The ground reaction force, in N.

    """
    return (mass * acceleration) + (mass * gravitational_constant)

Now, on Earth at sea level, the function works exactly as the original one, without having to specify a gravitational constant:

print(calculate_reaction_force(mass=60, acceleration=3.5))

798.6

but it also adds the possibility to use other gravitational constants, using the same function:

print(
    calculate_reaction_force(
        mass=60, acceleration=3.5, gravitational_constant=5.2
    )
)

522.0