Raising errors

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Raising errors#

When writing good code, it is beneficial to raise exceptions yourself if you are provided with incorrect input.

Consider the example method calculate_weight below, which calculates the weight of an object, given that we know its density and volume:

def calculate_weight(density, volume):
    """
    Calculates weight of an object, given its density and volume.

    Args:
        density (int): Density of an object.
        volume (int): Volume of an object.

    Returns:
        int: Weight of an object.
    """
    return density * volume

calculate_weight(3, 10)

30

calculate_weight(-4, 5)

-20

If we provide invalid input such as negative density or volume, the method will still give us an answer, because it is following a formula. However, using that incorrect calculation can be harmful in applications. Therefore, we can raise exceptions ourselves using the keyword raise:

def calculate_weight(density, volume):
    """
    Calculates weight of an object, given its density and volume.

    Args:
        density (int): Density of an object.
        volume (int): Volume of an object.

    Returns:
        int: Weight of an object.

    Raises:
        ValueError: if density or volume are negative values.
    """
    if density < 0:
        raise ValueError(
            "Invalid density. Density parameter can only take positive numbers"
        )
    if volume < 0:
        raise ValueError(
            "Invalid volume. Volume parameter can only take positive numbers"
        )
    return density * volume

calculate_weight(75, 5)

375

calculate_weight(-4, 50)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[13], line 1
----> 1 calculate_weight(-4, 50)

Cell In[11], line 16, in calculate_weight(density, volume)
      2 """
      3 Calculates weight of an object, given its density and volume.
      4 
   (...)
     13     ValueError: if density or volume are negative values.
     14 """
     15 if density < 0:
---> 16     raise ValueError(
     17         "Invalid density. Density parameter can only take positive numbers"
     18     )
     19 if volume < 0:
     20     raise ValueError(
     21         "Invalid volume. Volume parameter can only take positive numbers"
     22     )

ValueError: Invalid density. Density parameter can only take positive numbers

Exercise#

You are given the method calculate_area again to calculate the shuttering area of a concrete rectangular column. The shuttering is a temporary arrangement done for vertical surfaces to support the wet concrete till it attains the desired strength (source: Civilread.com).

If incorrect output is provided to it, the method will return -1. Instead of that, we would like to raise a ValueError. Modify the method to raise this exception on incorrect output. Furthermore, update the documentation of the method to account for this change.

def calculate_area(height, length, breadth):
    """
    Calculates shuttering area of a concrete rectangular column.

    Args:
        height (int): Height of the column.
        length (int): Length of the column.
        breadth (int): Breadth of the column.

    Returns:
        int: The area or -1 if the input is invalid.
    """
    if height <= 0 or length <= 0 or breadth <= 0:
        return -1
    else:
        return (2 * breadth + 2 * height) * length

Solution

def calculate_area(height, length, breadth):
    """
    Calculates shuttering area of a concrete rectangular column.

    Args:
        height (int): Height of the column.
        length (int): Length of the column.
        breadth (int): Breadth of the column.

    Returns:
        int: The area.
    Raises:
        ValueError: if height, length or breadth are non-positive values.
    """
    if height <= 0 or length <= 0 or breadth <= 0:
        raise ValueError("Invalid height or length or breadth provided.")
    else:
        return (2 * breadth + 2 * height) * length
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