To determine how many items a dictionary has, use the len() function:
// Example
Print the number of items in the dictionary:
print(len(thisdict))
Dictionary Items - Data Types
The values in dictionary items can be of any data type:
// Example
// String, int, boolean, and list data types:
thisdict = {
"brand": "Ford",
"electric": False,
"year": 1964,
"colors": ["red", "white", "blue"]
}
type()
From Python's perspective, dictionaries are defined as objects with the data type 'dict':
<class 'dict'>
// Example
// Print the data type of a dictionary:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
print(type(thisdict))
Python Collections (Arrays)
There are four collection data types in the Python programming language:
List is a collection which is ordered and changeable. Allows duplicate members.
Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
Set is a collection which is unordered, unchangeable*, and unindexed. No duplicate members.
Dictionary is a collection which is ordered** and changeable. No duplicate members.
*Set items are unchangeable, but you can remove and/or add items whenever you like.
**As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.
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:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
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")
Get Keys
The keys() method will return a list of all the keys in the dictionary.
// Example
// Get a list of the keys:
x = thisdict.keys()
The list of the keys is a view of the dictionary, meaning that any changes done to the dictionary will be reflected in the keys list.
// Example
// Add a new item to the original dictionary, and see that the keys list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
x = car.keys()
print(x) #before the change
car["color"] = "white"
print(x)
#after the change
Get Values
The values() method will return a list of all the values in the dictionary.
// Example
// Get a list of the values:
x = thisdict.values()
The list of the values is a view of the dictionary, meaning that any changes done to the dictionary will be reflected in the values list.
// Example
// Make a change in the original dictionary, and see that the values list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
x = car.values()
print(x) #before the change
car["year"] = 2020
print(x)
#after the change
// Example
// Add a new item to the original dictionary, and see that the values list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
x = car.values()
print(x)
#before the change
car["color"] = "red"
print(x)
#after the change
Get Items
The items() method will return each item in a dictionary, as tuples in a list.
// Example
// Get a list of the key:value pairs
x = thisdict.items()
The returned list is a view of the items of the dictionary, meaning that any changes done to the dictionary will be reflected in the items list.
// Example
Make a change in the original dictionary, and see that the items list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
x = car.items()
print(x) #before the change
car["year"] = 2020
print(x) #after the change
// Example
// Add a new item to the original dictionary, and see that the items list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
x = car.items()
print(x) #before the change
car["color"] = "red"
print(x) #after the change
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")
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
Update Dictionary
The update() method will update the dictionary with the items from the given argument.
The argument must be a dictionary, or an iterable object with key:value pairs.
// Example
// Update the "year" of the car by using the update() method:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.update({"year": 2020})
Adding Items
Adding an item to the dictionary is done by using a new index key and assigning a value to it:
The update() method will update the dictionary with the items from a given argument. If the item does not exist, the item will be added.
The argument must be a dictionary, or an iterable object with key:value pairs.
// Example
// Add a color item to the dictionary by using the update() method:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.update({"color": "red"})
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)
// Some code 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.
// Some codeExample
The clear() method empties the dictionary:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.clear()
print(thisdict)
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() method to return values of a dictionary:
for x in thisdict.values():
print(x)
// Example
You can use the keys() method to return the keys of a dictionary:
for x in thisdict.keys():
print(x)
// Example
// Loop through both keys and values, by using the items() method:
for x, y in thisdict.items():
print(x, y)
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)
Another way to make a copy is to use the built-in function dict().
// Example
/ Make a copy of a dictionary with the dict() function:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict = dict(thisdict)
print(mydict)
Nested Dictionaries
A dictionary can contain dictionaries, this is called nested dictionaries.
