Dictionary

Dictionaries in Python are mutable and unordered collection data structure. Mutable meaning it can be modified after creating and unordered meaning it does not keep track of it's elements.

Dictionaries are something special, usually called key-value data structure because of the way you store data inside them. You will certanly do a lot of work involving dictionaries in your python applications and its a very powerful concept, so let's get started.

Lesson Overview

In this lesson you will learn:

• How to create a dictionary
• How to manipulate elements
• How to iterate over dict

Dictionary

Creating Dictionary

We create a dictionary in Python using curly braces {}. It consists of two values; key and value. Key is what you use to access the element and value is what that element holds. Let's see an example:

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Keys can be any imutable type like strings, integers, floats, tuples, etc... Values can be practicly anything, from simple strings and numbers to complex lists, other dictionaries or any combinations which you want and need. To separate elements (key-value pairs), use comma ,.

Accessing Elements

Accessing an element in a dictionary is done with [] syntax but unlike indexes we used before, now we use key of the element to retrive it.

If the key does not exists, Python will raise KeyError. We can get around that by using .get() method which is safer to use as it does not raise an error, but returns None (by default) if the key does not exists.

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Updating Elements

Updating elements is as simple as assigment of a new value to existing key.

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Adding Elements

Adding new elements to dictionary is done just like updating but we use key that is not already in the dictionary.

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Removing Elements

To remove and return the value of the element from dictionary you can use .pop() method which takes in a single argument; key.

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You can also use delkeyword to remove a key-value pair from the dictionary.

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For clearing the whole dictionary content you can use .clear() method.

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Iterating Over Elements

Its common to iterate over dictionaries to check if it contains some specific values or simply using elements one by one. Dictionaries have two methods that help with this.

Let's go over what happens when you iterate over a dictionary by itself.

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We would expect to get both keys and values, but you can see, that is not the case. This only returnes keys present in the dictionary.

You can also use .keys() method to get the same result.

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While it can be useful, we usually want to grab values, or both keys and values. To grab just values contained we can use .values() method which does exactly that.

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To grab both keys and values, we can use .items() method. This method returnes 2 values which we can unpack directly to key and value pairs (these are just variables, so you can name them whatever you want).

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📚Learn more about Python dicts

Read this built-in: dict article from Real Python and pay special attention to sections dict Operators and dict Methods.

Remember, just read through, you do not have to know it all.

Exercise

Complete [TODO] Exercise 08 — WorkingTitle to practice dictionaries.

Assignment

In this assignment, we will completely eliminate list index tracking by refactoring our inventory into a dictionary, mapping each product name directly to its price and stock.

1. Open main.py in our project directory.
2. Replace inventory list with an empty dictionary {}.
3. When reading users input, assign the item directly to dictionary inventory.

   inventory[name] = {"price": price, "stock": stock}

4. In customer order, look up if item is in the inventory directly with if item_name in inventory, verify stock availability with inventory[item_name]["stock"] and update stock count in place with inventory[item_name]["stock"] -= 1 and if stock reaches 0 then delete the item from the inventory using del inventory[item_name].
5. Keep order as a list of tuples, and update the unsold_items variable to check between keys of the dictionary and tuple elements - use inventory.keys() to get a tuple-like object of dictionary keys that you can use in this case to substract order elements from keys of dictionary.
6. Test your application, make sure it works, commit and push to Github.

What's Next

We are now done with basic data structures in Python, next we move on to something even more fun and useful, kind of a super power in Python: coprehensions