Python Dictionary: A Data Collection Type (Part 3 of 3)

If you are to remember one thing from this article, it is to remember that a python dictionary stores data in “key to value” pairs. This means that for every key in the dictionary there is a corresponding value. This value can be any type of data object (another dictionary, list, tuple, integer, string, boolean, etc.); however, the keys must be unique and an immutable type (string, number, or tuple with immutable elements).

I like to think of dictionaries as a key and lock system. You have the one key that can unfasten the lock which can hold any number of things, but that key is very specific. This is the simplest analogy for me since you already are using the word “key” in the python definition.

The Python dictionary is unordered, meaning that you cannot access values through the index (as you might with a list or tuple), but you must access them through the key.

In this Article:

1. How to create
2. Accessing Elements
3. Adding/Deleting Elements

Creation Time!!

The python dictionary likes to be fancy. It has special keys that need to be called to return items, so it needs a fancy way to enclose the items inside it. It uses the curly brackets {}. You can almost see the progression here from the most basic to the most complex data type. The tuple is immutable collection of data and uses the most basic symbol the parentheses (). The list is a step up since it is mutable and can be changed and thus uses the brackets []. The last step for the most complex of the data types is the dictionary and uses the fanciest symbol the curly brackets {}.

The reason I bring this up is that that is how you can make a blank python dictionary, by using {}.

blank_dict = {}

OUTPUT:

A blank dictionary is useful if you want to use a function or for loop to fill the dictionary with data later on.

Next, the dictionary can be created using the data listed in key: value pairs. Like mentioned before, the keys must be of immutable types such as a string, integer or tuple, but the values can be of any type. In our first example, imagine Eleanor is studying different anatomical aspects of Eastern Bluebirds (a threatened species). She measures body length, wingspan, and weight. The first bird measures 16 cm, 25cm, 27g, respectively. Eleanor then places this information into her python dictionary.

bb_measurements = {‘length’: 16, ‘wingspan’: 25, ‘weight’: 27}

OUTPUT:

In this initial example, we are using strings as the immutable object for the dictionary keys; however, they could also be numbers or a tuple. I will show them below; however, they don’t make sense in terms of the current example, but I will still show the possibility here.

bb_measurements = {1: 16, 2: 25, 3: 27}

OUTPUT:

This is also a point where it can get a little confusing. Since you are able to use numbers as the keys in a dictionary, it can appear like you are using the index to reference the keys in the dictionary. However, this is not the case. If you really want the keys to be referenced by a number you have to input the keys as you do above. You cannot automatically refer to them as this like you would be able to with an index in a list.

bb_measurements = {(‘length’,’cm’): 16, (‘wingspan’, ‘cm’): 25, (‘weight’, ‘g’): 27}

OUTPUT:

Using a tuple as a key may be useful if you need to denote multiple parts to a key if you might then be putting the dictionary into a multi-indexed dataframe; however, the idiosyncrasies of this are beyond the scope of this article.

You can also create a dictionary with any type as the value, it doesn’t have to be restricted to numbers, but can be strings, boolean, lists, additional dictionaries, etc. Similar to lists, they do not have to all be the same either, thus you can have a value with a list, the next value as an integer, and the next as another dictionary. Making dictionaries extremely versatile.

Accessing Python Dictionary Elements

Now remember what the most important take home message is from this article, “dictionaries use key:value pairs”. Thus, if you are trying to access an element within the dictionary you must use the key to access your data. You cannot use their index as you might in a list or tuple. In our example with Eleanor, lets say she wants to access the weight of the Eastern Bluebird she measured, she would do the following:

bb_measurements[‘weight’]

OUTPUT:

Alternatively, she could also use the .get() method:

bb_measurements.get(‘weight’)

OUTPUT:

Either method works great. The only difference is that if you try to call a key that is not in the dictionary, you will receive a ValueError with the brackets, while you will receive “None” from the .get() method.

bb_measurements[‘hi’]bb_measurements.get(‘hi’)

OUTPUT:

The difference between the two results might be useful to you depending on what return you are looking for when you are using the dictionary in a function.

Adding Python Dictionary Elements

Eleanor also measured the length of the tarsus (basically the shinbone in a bird) in the Eastern Bluebirds and would like to add that measurement as well. She can easily do this by putting the key that she would like in brackets and then putting the value after the equals sign.

bb_measurements['tarsus'] = 6.3

OUTPUT:

If Eleanor realizes after looking back at her notes that the bluebird actually was 16.1 cm long not just 16, she can update the value in the dictionary by using the same process and the 16 will be replaced with a 16.1.

bb_measurements['length'] = 16.1

OUTPUT:

Deleting Python Dictionary Elements

Eleanor’s lab-mates have not been using tarsus length in their measurements. Thus, to maintain consistency in the lab’s data, Eleanor decides to remove the element from the python dictionary. She has 3 methods to choose from to achieve cohesiveness.

1. del
2. .pop()
3. .popitem()

If she wants to clear the whole dictionary and start over, she can use the .clear() method.

Eleanor can use the “del” keyword to remove a single item or delete the entire dictionary.

del bb_measurements['tarsus']

OUTPUT:

.pop() method will remove the item from the dictionary and return the value.

bb_measurements.pop('tarsus')

OUTPUT:

popitem() method will remove the item that was last added to the dictionary and return the key and value as a tuple.

bb_measurements.popitem()

OUTPUT:

Now Eleanor has her dictionary for her Eastern Bluebird all in order, my hope is that you do as well.