Python Dictionaries || Python Tutorial || Learn Python Programming

Python Dictionaries  ||  Python Tutorial  ||  Learn Python Programming

A standard data structure in computer science
is the “associative array” which is also called a “map.” In Python, this structure
is called a “dictionary.” Dictionaries are used when you have key/value pairs of
data – an input which is mapped to an output. If we had a dictionary that stored the quality
of YouTube videos, then the value for this lesson would be 17.8… Suppose we are creating a new social network
called “Friendface” where any life form or intelligent agent is allowed to join. Let us make our first post to Friendface…
We will use this post to show you how to create a dictionary. To begin, we will collect several pieces of
data for each post: The user id… message… language… datetime…
and the location from whence the post was made. Using dictionaries, we can store all of this
data in a single object. To create a dictionary, use the left brace,
then list the key name, a colon, and the value. If you have more than one key/value pair,
you separate them using a comma. Once you have finished entering your data,
use a right brace to finish the dictionary. We have created a dictionary called ‘post’
with 5 pieces of data. If you think of a dictionary as a map, there are 5 inputs, and 5 outputs.
In Python, inputs are called keys, and outputs are called values. Notice how the values have a variety of data
types in this dictionary: an integer, three strings… and a tuple of floats. Python is
flexible: you can use all kinds of data types for both keys and values. You are free to
mix and match data to suit your needs. If you use the ‘type’ function, you will
see ‘post’ is an instance of the ‘dict’ class. This suggests we can use the ‘dict’
constructor to create a dictionary, and this suggestion is correct. Reminder – we are using
Python version 3 in this video. Let’s create another Friendface post using
the ‘dict’ constructor. This time, we will only provide a few pieces of data…
You can see this made a dictionary. We add additional pieces of data by using
brackets. The key name goes inside the brackets, and
you use the equals sign to assign the value. Notice that in the constructor, you did not
have to put quotes around the key name, but when you add new data using brackets, you
DO use quotes. If you print the dictionary, you will see
all the data is there inside braces. Just as you use brackets to store new data
in a dictionary, you use brackets to access data as well. Example: to see the message
from the first post, use the key name ‘message’ inside brackets. But do not be careless. Let’s see what happens
if you try to access data that is not in the dictionary. When we created ‘post2’, we
did not assign a location. If we try to access this value, we get a Key Error. How do we avoid such catastrophes? Simple.
One way is to use the ‘in’ operator to first check if a key is in the dictionary. Another way is to try and retrieve the value,
but handle the possibility of a KeyError. To do this, type the “try” command followed
by a colon. Next, attempt to print the location.
If the dictionary does not have a ‘location’ key, it will raise a KeyError.
We handle this case by creating an “except” block. Type “except” and the possible
errors you would like to handle. Here, we want to handle the KeyError. If this occurs,
the following code block will execute. It works… There is another way to access data in a dictionary
and handle the possibility it does not have a certain key. First, display the directory
for the ‘post2’ dictionary. Here we see a list of methods available to us. We will
explore the ‘get’ method. To see what this does, use the help function. The ‘get’ method lets you try and get
the value for a specific key. If the dictionary does not contain data for that key, you can
specify a default value. We will use this method to get the ‘location’ from post2.
If this post does not have a location, let’s return None.
If you print the value, you see the method did return ‘None.’ Turn your attention back to the original post. A common task is to iterate over all the key/value
pairs in a dictionary. A straightforward way to do this is to loop
over all the keys, then get the value for each key. The “keys” method gives us an
object we can loop over that contains all the keys for the dictionary. If you are using Python version 2, the output
may look different. This is because we are using Python 3, and
in version 3, the print function became more powerful. The order of the data may be different for
you. Do not panic. Dictionaries are not ordered data. As long as you see all the data, everything
is under control. Another way to iterate over all the key/value
pairs in a dictionary is to use the “items” method. This will give you both the key and
the value in each step of the iteration. There are a variety of methods for removing
data from a dictionary. The “pop” and “popitem” methods allow
you to remove a single item from a dictionary, while the “clear” method will remove all
data. After you complete this video, be sure to experiment with these methods. Look at me… I am a dictionary…

19 thoughts on “Python Dictionaries || Python Tutorial || Learn Python Programming”

  1. Why can't all tutorials like yours?

    Most tutorials out there are too bloated with ego stuff, with too many things which don't belong there. YOUR tutorials, on the other hand, are concise, precise, straight to the point.

  2. I'm sorry, but, how is 1:45 possible, while in python lists are data types, but lists are not accepted as a value because they are mutable data types, which is one of the requirements for a data type to be accepted as a key?

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