Dictionaries and Functions

What we have shown you so far exists similarly in many other programming languages. Now, we’ll show you something that is more specific to python: dictionaries.

Dictionaries¶

Much like a dictionary, a dictionary works by allowing us to find content associated with a specific keyword. In python, these are referred to as keys and items. As an example, we first create an empty dictionary. Then, we create a dictionary named lectures and immediately add a selection of first-semester physics lectures. Subsequently, in the ninth line, we add the lectures for the second semester. Finally, we output several items. (More on that after the code example.)

empty_dict = dict()  # generates an empty dict
lectures = {
    "Semester 01": [
        "Physics I",
        "Introduction to Programming",
        "Advanced Mathematics I",
    ]
}
lectures["Semester 02"] = ["Physics II", "Advanced Mathematics II"]
print(f"{lectures = }")
print(f"{lectures.keys() = }")
print(f"{lectures.items() = }")
print(f"{lectures["Semester 02"] = }")

In the code, you see a line that has been split, but the code works exactly the same way as if we were to write it like this.

lectures = {"Semester 01": ["Physics I", "Introduction to Programming", "Advanced Mathematics I"]}

It’s just that it’s more readable, since the line isn’t as long.

dictionary entries can be added either during the creation of the dictionary or at a later stage. When displayed, dictionaries don’t look quite as neat as lists or individual variables. Enclosed in curly braces {}, we see the keys first, followed by the associated items. We obtain the lists containing all keys and items using the last two outputs. To retrieve the item associated with a specific key, we use square brackets (as with lists) and insert our key inside them. The output we get here is the list of lectures from the second semester.

The list of keys is very useful when we want to work with the items. To demonstrate how this works, we use for loops to print out the lectures we have listed for the semesters.

for key in lectures.keys():
    print(f"In {key}, I attended the lectures")
    for lecture in lectures[key]:
        print(f"- {lecture}")

lectures.keys() gives us the list of keys from the dictionary, and we can use a for loop to iterate over them. lectures[key] provides the item associated with that key. In this example, the list of lectures for the respective semester is shown. We then run another for loop over this list. This allows us to display the individual lectures.

Can you modify the code so that the lectures match the ones you attended?

Writing your own functions¶

Finally, we’ll show you how to write your own functions. We’ve already shown you using loops how to repeat lines of code in a specific place. Functions allow you to reuse parts of your code in different places over and over again, without having to rewrite them constantly. We have frequently used one function provided by Python in these lessons: print. In the workshop itself, we will show you plenty of other functions, but here is how you can write any number of your own:

def function_name(argument_0, argument_1, argument_2):
    # Code to be executed
    # for example
    result = argument_1 * (argument_2 - argument_0)
    return result

First, let’s talk about the syntax. It starts with the keyword def (short for “define”), followed by the function name, function_name. You can choose this name (mostly) freely, though it is best to avoid names that already exist in python and that you might want to use elsewhere. Also, as with variable names, it helps if the name clearly explains what the function does. The parentheses () contain the arguments you want to pass to the function. These can be variables or even other functions. The lines that execute when the function is called are indented. Finally, if you wish, you can return a value from the function. To do this, we use the keyword return followed by the value we want to return.

Now, let’s see how to call the function and how to access the return value.

a = 2
b = 3
c = 4
return_value = function_name(a, b, c)
print(f"{return_value = }")
print(f"{function_name(a, b, c) = }")

Something must be passed for each argument. We can then either output the return value directly or store it in a variable. The latter often makes more sense, as the stored value can be used for further processing.

One last example is a physical force.

def coulomb_force(q1, q2, r):
    return 1 / (4 * 3.14 * 8.854e-12) * q1 * q2 / r

print(f"F_C = {coulomb_force(1.602e-19, -1.602e-19, 0.01)}")

We reduce the number of lines by doing the calculation within the return statement. You are familiar with the formula from your second semester; π is approximated as 3.14. We then use print to display the force between a positive and a negative elementary charge separated by a distance of 1 cm. As you can see, the output is also in e notation, just like the format used for some of the values we specified.

That concludes the lessons in this introduction to python. We’ll continue with the Toolbox Workshop.

Complete code¶

At the end of each lesson, you will find the complete code for that lesson. As this is the last lesson, you’ll find all the code here.

"""
This file contains the code for the python introduction of the Toolbox Workshop.
site: https://toolbox.pep-dortmund.org/introduction/python/

The included section is: Dictionaries and functions
"""

print("Lesson 4")
print("Dictionaries")

empty_dict = dict()  # generates an empty dict
lectures = {
    "Semester 01": [
        "Physics I",
        "Introduction to Programming",
        "Advanced Mathematics I",
    ]
}
# lectures = {"Semester 01": ["Physics I", "Introduction to Programming", "Advanced Mathematics I"]}
lectures["Semester 02"] = ["Physics II", "Advanced Mathematics II"]
print(f"{lectures = }")
print(f"{lectures.keys() = }")
print(f"{lectures.items() = }")
print(f"{lectures["Semester 02"] = }")

for key in lectures.keys():
    print(f"In {key}, I attended the lectures")
    for lecture in lectures[key]:
        print(f"- {lecture}")

print("Functions")


def function_name(argument_0, argument_1, argument_2):
    # Code to be executed
    # for example
    result = argument_1 * (argument_2 - argument_0)
    return result


a = 2
b = 3
c = 4
return_value = function_name(a, b, c)
print(f"{return_value = }")
print(f"{function_name(a, b, c) = }")


def coulomb_force(q1, q2, r):
    return 1 / (4 * 22/7 * 8.854e-12) * q1 * q2 / r


print(f"F_C = {coulomb_force(1.602e-19, -1.602e-19, 0.01):.2e}N")