Data-Driven Engineering

Python 5️⃣ Dictionary Notebook | Google Colab | Dictionary Intro

A dictionary is a set with 🔐 key:value pairs. It is designed to lookup values based on the 🔑 key to return a 🔒 value. An example is a dictionary of words (key) with a definition for each word (value).

💡 Create Dictionary

Like the list and set and unlike a tuple, a dict of length 1 is defined without an extra trailing comma {'i':i}. If key values are repeated, the last definition of the key:value pair is used (e.g. {'i':2,'i':3} result is {'i':3}).

Create a dictionary with values {'i':1,'x':2.7,'e':3.8e3}.

    {'i': 1, 'x': 2.7, 'e': 3800.0}

Another way to define the dictionary is with the dict() function.

📝 Print Dictionary

Print the dictionary and verify the object type as a dict with the type() function.

    {'i': 1, 'x': 2.7, 'e': 3800.0}

    dict

🔓 Access Value with 🔑 Key

Read the value of a dictionary item using the key.

    1

Set the value of a dictionary item also using the key.

    2

📑 Unpack Dictionary

Each key of a dictionary is accessed when d is used as an iterator in a for loop.

    i
    x
    e

Use d.items() to create a generator to separate the key:value pairs.

    i 2
    x 2.7
    e 3800.0

🪆 Nested Dictionary

A dictionary z can be a sub-element of another dictionary d as a nested dictionary.

Reference the sub-elements by including an additional square bracket at the end to designate the item such as d['z']['a'].

    11

🧦 Copy Dictionary

The copy function creates a shallow copy of dictionary d as a new dictionary d2. A shallow copy is a duplication of the first layer of a dictionary but the lower layers are copied as references.

When d2['x'] is set to 3, there is no corresponding change to d['x'].

    {'x': 1, 'y': 2, 'z': {'a': 11, 'b': 12}}
    {'x': 3, 'y': 2, 'z': {'a': 11, 'b': 12}}

However, when d2['z']['a'] is changed to 13, there is a corresponding change to d['z']['a'] because both share the same reference to dictionary z.

    {'x': 1, 'y': 2, 'z': {'a': 13, 'b': 12}}
    {'x': 3, 'y': 2, 'z': {'a': 13, 'b': 12}}

A deep copy creates a completely independent dictionary for all layers. Use the function copy.deepcopy() to create a deep copy. The value of d['z']['a'] doesn't change when d3['z']['a'] is set to 14.

    {'x': 1, 'y': 2, 'z': {'a': 13, 'b': 12}}
    {'x': 1, 'y': 2, 'z': {'a': 14, 'b': 12}}

🔑 Dictionary Attributes and Methods with dir

Use the dir() function to list all attributes (constants, properties) and methods (functions) that are available with an object.

A dict has the following methods (functions):

• clear - clear all entries: d.clear()
• copy - create a shallow copy of the dict: d4=d.copy()
• fromkeys - create a new dictionary from listed keys: d4=d.fromkeys(['x','y'])
• get - return value for key, else return None: d.get('x')
• items - return a set-like object with a view of dictionary items: d.items()
• keys - return a set-like object with a view of dictionary keys: d.keys()
• pop - remove a key:value from dict: d.pop('x')
• popitem - remove last element from dict: d.popitem()
• setdefault - insert key with a value of None if not in dict: d.setdefault('x')
• update - update dict with key:value pairs from another dict: d.update(d2)
• values - return a dict_values object that provides a view of the dictionary values: d.values()

Because a dict is not ordered, there is no particular index for each key:value pair. The order that items are added is tracked and pop removes the most recently added key:value pair. There are also no sort functions. Convert the keys to a list to sort the dict keys.

💻 Exercise 5A

Print the values in dictionary d one at a time using a for loop that iterates through the keys.

💻 Exercise 5B

Modify 'x':0 and add 'b':23 as key:value pairs to dictionary d. Print the elements of the dictionary to verify the change and addition.

✅ Knowledge Check

1. What is the output of the code: d = {'i':2,'i':3} ?

2.: How would you increment the value of key 'i' by 1 in the dictionary d? Select two correct answers.