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❮ Previous Next ❯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) Run example »Accessing Items
You can access the items of a dictionary by referring to its key name, inside square brackets:
Example
Get the value of the "model" key:
x = thisdict["model"] Run example »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") Run example »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 Run example »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) Run example »Example
Print all values in the dictionary, one by one:
for x in thisdict: print(thisdict[x]) Run example »Example
You can also use the values() function to return values of a dictionary:
for x in thisdict.values(): print(x) Run example »Example
Loop through both keys and values, by using the items() function:
for x, y in thisdict.items(): print(x, y) Run example »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") Run example »Dictionary Length
To determine how many items (key-value pairs) a dictionary has, use the len() method.
Example
Print the number of items in the dictionary:
print(len(thisdict)) Run example »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) Run example »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) Run example »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) Run example »Example
The del keyword removes the item with the specified key name:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } del thisdict["model"] print(thisdict) Run example »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. Run example »Example
The clear() keyword empties the dictionary:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict.clear() print(thisdict) Run example »Copy a Dictionary
You cannot copy a dictionary simply by typing dict2 = dict1, because: dict2 will only be a reference to dict1, and changes made in dict1 will automatically also be made in dict2.
There are ways to make a copy, one way is to use the built-in Dictionary method copy().
Example
Make a copy of a dictionary with the copy() method:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } mydict = thisdict.copy() print(mydict) Run example »Another way to make a copy is to use the built-in method dict().
Example
Make a copy of a dictionary with the dict() method:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } mydict = dict(thisdict) print(mydict) Run example »Nested Dictionaries
A dictionary can also contain many dictionaries, this is called nested dictionaries.
Example
Create a dictionary that contain three dictionaries:
myfamily = { "child1" : { "name" : "Emil", "year" : 2004 }, "child2" : { "name" : "Tobias", "year" : 2007 }, "child3" : { "name" : "Linus", "year" : 2011 } } Run example »Or, if you want to nest three dictionaries that already exists as dictionaries:
Example
Create three dictionaries, than create one dictionary that will contain the other three dictionaries:
child1 = { "name" : "Emil", "year" : 2004 } child2 = { "name" : "Tobias", "year" : 2007 } child3 = { "name" : "Linus", "year" : 2011 } myfamily = { "child1" : child1, "child2" : child2, "child3" : child3 } Run example »The dict() Constructor
It is also possible to use the dict() constructor to make a new 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) Run example »Dictionary Methods
Python has a set of built-in methods that you can use on dictionaries.
| Method | Description |
|---|---|
| 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 |
| setdefault() | 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 |
Test Yourself With Exercises
Exercise:
Use the get method to print the value of the "model" key of the car dictionary.
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
print()
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