Exercise 1



Please complete this exercise by 16:00 on Wednesday the 6th of November 2019 (day before the next practical session).

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You can start working on your copy of Exercise 1 by accepting the GitHub Classroom assignment.

You can also take a look at the open course copy of Exercise 1 in the course GitHub repository (does not require logging in). Note that you should not try to make changes to this copy of the exercise, but rather only to the copy available via GitHub Classroom.


Assert statements

Assertions are a way to assert, or ensure, that the values being used in your scripts are going to be suitable for what the code does. It is common to use assert statements with functions as they are a good way to ensure the correct functionality of a function and guide the user to use function as intended. Read more about assertions from Geo-Python week 6 good coding practices.

One good example how to use assertions inside a function is to ensure that the values passed into the function are of correct type. It is also common to test value ranges with assert, such as test that values are positive. Consider following example that combines these two checks:

# A function for summing positive values
def sum_positive_values(value1, value2):
    """Sums positive values together."""

    # Check that the input values are of correct type (i.e. integers or floats)
    # We can check if the type of the input value can be found from a list of "correct" data types
    # --------------------------------------------------------------------------------------------

    # value1 -parameter
    assert type(value1) in [int, float], "Input value for 'value1' needs to be integer or floating point number! Found: %s" % type(value1)

    # value2 -parameter
    assert type(value2) in [int, float], "Input value for 'value2' needs to be integer or floating point number! Found: %s" % type(value2)

    # Check that the input values are positive
    # ----------------------------------------
    assert value1 > 0, "'value1' needs to be higher than 0! Found: %s" % value1
    assert value2 > 0, "'value2' needs to be higher than 0! Found: %s" % value2

    # If all the tests were passed, do the calculation and return the output
    return value1 + value2

This example demonstrates how it is possible to check and control that the input values are appropriate for the function, and guide the user how to use the function correctly with informative error messages.

Iterating multiple lists simultaneously

In Python a function called zip() makes it easy to iterate over multiple lists at the same time. Consider following example:

# Create lists
In [1]: dog_list = ['Blackie', 'Musti', 'Svarte']

In [2]: age_list = [4.5, 2, 15]

# Iterate over the lists using zip() to print an informative message
In [3]: for dog, age in zip(dog_list, age_list):
   ...:     print(dog, 'is', age, 'years old.')
Blackie is 4.5 years old.
Musti is 2 years old.
Svarte is 15 years old.

This example demonstrates how it was possible to take two lists (could be even more lists) and access the values from them using the same index number.


This approach assumes that the length of the lists are identical. If not, you will most probably get IndexError because the list index is out of range.