Python Dictionaries

Adding Items

Adding an item to the dictionary is done by using a new index key and assigning a value to it: Example thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict["color"] = "red" print(thisdict)

Removing Items

There are several methods to remove items from a dictionary: Example The pop() method removes the item with the specified key name: thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict.pop("model") print(thisdict) Example The popitem() method removes the last inserted item (in versions before 3.7, a random item is removed instead): thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict.popitem() print(thisdict) Example The del keyword removes the item with the specified key name: thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } del thisdict["model"] print(thisdict) Example The del keyword can also delete the dictionary completely: thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } del thisdict print(thisdict) #this will cause an error because "thisdict" no longer exists. Example The clear() keyword empties the dictionary: thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict.clear ()print(thisdict)

Dictionary Length

To determine how many items (key-value pairs) a dictionary have, use the len() method. Example Print the number of items in the dictionary: print(len(thisdict))

Check if Key Exists

To determine if a specified key is present in a dictionary use the in keyword: Example Check if "model" is present in the dictionary: thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } if "model" in thisdict: print("Yes, 'model' is one of the keys in the thisdict dictionary")

Accessing Items

You can access the items of a dictionary by referring to its key name: Example Get the value of the "model" key: x = thisdict["model"] There is also a method called get() that will give you the same result: Example Get the value of the "model" key: x = thisdict.get("model")

Change Values

You can change the value of a specific item by referring to its key name: Example Change the "year" to 2018: thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict["year"] = 2018

Dictionary

A dictionary is a collection which is unordered, changeable and indexed. In Python dictionaries are written with curly brackets, and they have keys and values. Example: Create and print a dictionary: thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } print(thisdict)

The dict() Constructor

It is also possible to use the dict() constructor to make a dictionary: Example thisdict = dict(brand="Ford", model="Mustang", year=1964) # note that keywords are not string literals # note the use of equals rather than colon for the assignment print(thisdict)

Dictionary Methods

Python has a set of built-in methods that you can use on dictionaries. clear() Removes all the elements from the dictionary copy() Returns a copy of the dictionary fromkeys() Returns a dictionary with the specified keys and values get() Returns the value of the specified key items() Returns a list containing the a tuple for each key value pair keys() Returns a list containing the dictionary's keys pop() Removes the element with the specified key popitem() Removes the last inserted key-value pair set default() Returns the value of the specified key. If the key does not exist: insert the key, with the specified value update() Updates the dictionary with the specified key-value pairs values() Returns a list of all the values in the dictionary

Loop Through a Dictionary

You can loop through a dictionary by using a for loop. When looping through a dictionary, the return value are the keys of the dictionary, but there are methods to return the values as well. Example Print all key names in the dictionary, one by one: for x in thisdict: print(x) Example Print all values in the dictionary, one by one: for x in thisdict: print(thisdict[x]) Example You can also use the values() function to return values of a dictionary: for x in thisdict.values(): print(x) Example Loop through both keys and values, by using the items() function: for x, y in thisdict.items(): print(x, y)