Dictionaries are an important way to store data in Python. They let us store information as key-value pairs which are indexed by key name rather than an index like in Python Lists. In this guide, we will cover everything you need to know about Python dictionaries. If you are interested in other data collections in Python, such as lists, sets, and tuples, . you can find out more about them here Let's begin by defining a dictionary. We use curly brackets, just like in Python sets - but we define both keys and values, which is slightly different than sets: dictionary = { "name" : "Johnny", "age" : 152 } While these methods of creating dictionaries are great, we are not limited by them. If you are familiar with Python lists, you can turn them straight into dictionaries using the function: dict() dictionary = dict([['name', 'Johnny'], ['age', 152]]) print(dictionary) # { "name" : "Johnny", "age" : 152 } Or, we can use the function to create a dictionary from variables: dict dictionary = dict(name="Johnny", age=153) print(dictionary) # { "name" : "Johnny", "age" : 152 } Accessing Python dictionary values can be accessed using the square bracket notation: dictionary = { "name" : "Johnny", "age" : 152 } print(dictionary["name"]) # Johnny Or, if you want, you can get this information using the method: get dictionary = { "name" : "Johnny", "age" : 152 } print(dictionary.get("name")) # Johnny Python dictionaries are also mutable, so using the square bracket notation above, we can update values: dictionary = { "name" : "Johnny", "age" : 152 } dictionary["age"] = 153 print(dictionary["age"]) # 153 Or, you can use the method, based on your preference: update() dictionary = { "name" : "Johnny", "age" : 152 } dictionary.update({"age" : 153}) print(dictionary["age"]) # 153 Dictionaries can also be multi-dimensional. So this is also a valid dictionary: dictionary = { "name" : { "firstName" : "Johnny", "lastName" : "Simpson" }, "age": 152 } Finally, you can get the length of a dictionary (i.e. the number of key-value pairs) using the function: len() dictionary = { "name" : "Johnny", "age" : 152 } dictionary.update("age", 153) print(len(dictionary)) # 2 Python Dictionary Methods We've covered both and so far, there are a bunch of other methods which are useful too. Here's a full list of them: get update - deletes all items from a python dictionary. dict.clear() - makes a copy of a dictionary, which has the same value, but a different reference. dict.copy() - removes the last key-value pair from the dictionary. dict.popitem() - removes the key-value pair with a key of "keyItem". dict.pop("keyItem") - updates the dictionary with keys and values from, overwriting any existing ones. dict.update(newDictionary) - will return the value for the item , and if it doesn't exist, will create a new key value pair of dict.setdefault("key", "default") key {'key' : 'default'} - takes two sets of data for both and , and creates a new dictionary based on them. dict.fromkeys(keys, values) keys values - returns an iterable set of tuples for each key-value pair. The returned data is known as a . dict.items() view object - returns an iterable set of keys for the dictionary. The returned data is known as a . dict.keys() view object - returns an iterable set of values for the dictionary. The returned data is known as a . dict.values() view object View Objects in Python Dictionaries You might notice that the last three methods, , , and , all return a . A view object is a dynamic way type of object which will update automatically should the dictionary be updated. They are also iterable. Let's look at a quick example, using : items() keys() values() view object dict.items() dictionary = dict(name="Johnny", age=153) getDictionaryItems = dictionary.items() for x in setDictionary: print(x) # Returns # ('name', 'Johnny') # ('age', 153) returns a list of tuples, so we can easily iterate over them using a loop. dict.items() for View Objects also support membership checks. For example, we can check if a certain key exists in a dictionary using the method, along with the and operators: keys() in not in dictionary = dict(name="Johnny", age=153) getDictionaryItems = dictionary.keys() print("name" in getDictionaryItems) # True, as name is a key in dictionary print("name" not in getDictionaryItems) # False, as name is a key in dictionary Deleting Items from Dictionaries in Python There are a number of ways to delete items from a dictionary, as shown in the methods listed above. The main ones are , , and . Let's look at a few examples, so it's clear how they work. To remove a specific item from a dictionary, we use . In the example below, I remove the dictionary item with the key : popitem() pop() clear() pop favouriteDrink dictionary = dict(name="Johnny", age=153, favouriteDrink="tea") dictionary.pop("favouriteDrink") print(dictionary) # {'name': 'Johnny', 'age': 153} If methods aren't your thing, you can just use the keyword to remove in the same way: del favouriteDrink dictionary = dict(name="Johnny", age=153, favouriteDrink="tea") del dictionary["favouriteDrink"] print(dictionary) # {'name': 'Johnny', 'age': 153} If we only wanted to remove the last item, without specifying it, we can use : popitem dictionary = dict(name="Johnny", age=153, favouriteDrink="tea") dictionary.popitem() print(dictionary) # {'name': 'Johnny', 'age': 153} Or if we wanted to clear the dictionary completely, we can use : clear() dictionary = dict(name="Johnny", age=153, favouriteDrink="tea") dictionary.clear() print(dictionary) # {} Using setdefault with Python Dictionaries The method on Python dictionaries either returns a value for a key if it exists or creates that key-value pair if it doesn't. The first argument is the key, and the second is the value you wish to set the key to should it not exist - for example, . If you do not set the second argument, it defaults to the string . setdefault dict.setdefault("age", 152) None Here is an example, which leads to a new key-value pair being added to the object dictionary below: dictionary = { "name" : "Johnny", "age": 152 } dictionary.setdefault("age", 152) # Does nothing but returns age, as age exists on dictionary dictionary.setdefault("favouriteDrink", "tea") # Adds `favouriteDrink` to dictionary print(dictionary) # {'name': 'Johnny', 'age': 152, 'favouriteDrink': 'tea'} Creating a New Dictionary from two lists of data in Python If you wish to create a new dictionary in python from a set of data, we can use the method. Here, we can define a list, set, or tuple of data for keys, and a single value for the value that each key should be. For example, below I've defined a tuple and a single value of . When combined with , we get a brand new dictionary: dict.fromkeys 1 fromkeys myKeys = ("name", "age", "favouriteDrink") myValues = 1 newDict = dict.fromkeys(myKeys, myValues) print(newDict) # {'name': 1, 'age': 1, 'favouriteDrink': 1} Iterating Over a Dictionaries Keys An easy way to iterate over dictionaries keys, along with the method, is to simply call , which returns an iterable for the keys in your dictionary. This essentially performs the same task as : dict.keys() iter() dict.keys() most of the time dictionary = dict(name="Johnny", age=153, favouriteDrink="tea") dictKeys = iter(dictionary) for x in dictKeys: print(x) # Returns name age favouriteDrink The difference between and , is you alter the keys after calling iter. To show you what I mean, below I add a new key after calling and try to iterate over with a loop. This produces an error, as the keys changed during iteration: iter dict.keys() cannot iter for dictionary = dict(name="Johnny", age=153, favouriteDrink="tea") dictKeys = iter(dictionary) dictionary['birthday'] = "Monday" for x in dictKeys: print(x) # RuntimeError: dictionary changed size during iteration Meanwhile, no error will be produced for , since produces a view object which is a dynamic live view of the dictionaries contents: dict.keys() dict.keys() dictionary = dict(name="Johnny", age=153, favouriteDrink="tea") dictKeys = dictionary.keys() dictionary['birthday'] = "Monday" for x in dictKeys: print(x) # Returns name age favouriteDrink birthday Iterating Over a Dictionaries Keys in Reverse Order Another useful function you might find yourself using is . This function takes dictionary keys and returns an iterable list of them in reverse order. As with , you will produce errors if you try to update the dictionary while iterating over it in form. reversed iter reversed Once reversed, the keys can be iterated upon to do some function of your choosing. Here is an example, which returns the string value of each key in reverse order for : dictionary dictionary = dict(name="Johnny", age=153, favouriteDrink="tea") reversedDict = reversed(dictionary) for x in reversedDict: print(x) # Returns favouriteDrink age name Checking Membership in a Python Dictionary Just like in sets and other data collections in Python, we can also check if something exists as a key in a dictionary. For this, we can use the or operators. , dictionary membership is checked by key, not value. in not in Remember dictionary = { "name" : "Johnny", "age" : 152 } print('name' in dictionary) # True print('name' not in dictionary) # False Conclusion That's everything for dictionaries - I hope you've enjoyed this Python tutorial. To learn more about engineering in general, . Otherwise, click the links below to learn more about the different types of data collections in Python: you can check out my other work here Python Data Collections Python Data Collections: Lists Python Data Collections: Tuples Python Data Collections: Sets Python Data Collections: Dictionaries Also published . here
