Python dictionary is an ordered collection (starting from Python 3.7 ) of items. It stores elements in key/value pairs. Here, keys are unique identifiers that are associated with each value .
Let's see an example,
If we want to store information about countries and their capitals, we can create a dictionary with country names as keys and capitals as values .
| Keys | Values |
|---|---|
| Nepal | Kathmandu |
| Italy | Rome |
| England | London |
Create a dictionary in Python
Here's how we can create a dictionary in Python.
capital_city = {"Nepal": "Kathmandu", "Italy": "Rome", "England": "London"}
print(capital_city)
Output
{'Nepal': 'Kathmandu', 'Italy': 'Rome', 'England': 'London'}
In the above example, we have created a dictionary named capital_city . Here,
-
Keys
are
"Nepal","Italy","England" -
Values
are
"Kathmandu","Rome","London"
Note : Here, keys and values both are of string type. We can also have keys and values of different data types.
Example 1: Python Dictionary
# dictionary with keys and values of different data types
numbers = {1: "One", 2: "Two", 3: "Three"}
print(numbers)
Output
[3: "Three", 1: "One", 2: "Two"]
In the above example, we have created a dictionary named numbers . Here, keys are of integer type and values are of string type.
Add Elements to a Python Dictionary
We can add elements to a dictionary using the name of the dictionary with
[]
. For example,
capital_city = {"Nepal": "Kathmandu", "England": "London"}
print("Initial Dictionary: ",capital_city)
capital_city["Japan"] = "Tokyo"
print("Updated Dictionary: ",capital_city)
Output
Initial Dictionary: {'Nepal': 'Kathmandu', 'England': 'London'}
Updated Dictionary: {'Nepal': 'Kathmandu', 'England': 'London', 'Japan': 'Tokyo'}
In the above example, we have created a dictionary named capital_city . Notice the line,
capital_city["Japan"] = "Tokyo"
Here, we have added a new element to
capital_city
with
key
:
Japan
and
value
:
Tokyo
.
Change Value of Dictionary
We can also use
[]
to change the value associated with a particular key. For example,
student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}
print("Initial Dictionary: ", student_id)
student_id[112] = "Stan"
print("Updated Dictionary: ", student_id)
Output
Initial Dictionary: {111: 'Eric', 112: 'Kyle', 113: 'Butters'}
Updated Dictionary: {111: 'Eric', 112: 'Stan', 113: 'Butters'}
In the above example, we have created a dictionary named
student_id
. Initially, the value associated with the key
112
is
"Kyle"
. Now, notice the line,
student_id[112] = "Stan"
Here, we have changed the value associated with the key
112
to
"Stan"
.
Accessing Elements from Dictionary
In Python, we use the keys to access their corresponding values. For example,
student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}
print(student_id[111]) # prints Eric
print(student_id[113]) # prints Butters
Here, we have used the keys to access their corresponding values.
If we try to access the value of a key that doesn't exist, we'll get an error. For example,
student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}
print(student_id[211])
# Output: KeyError: 211
Removing elements from Dictionary
We use the
del
statement to remove an element from the dictionary. For example,
student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}
print("Initial Dictionary: ", student_id)
del student_id[111]
print("Updated Dictionary ", student_id)
Output
Initial Dictionary: {111: 'Eric', 112: 'Kyle', 113: 'Butters'}
Updated Dictionary {112: 'Kyle', 113: 'Butters'}
Here, we have created a dictionary named student_id . Notice the code,
del student_id[111]
The
del
statement removes the element associated with the key
111
.
We can also delete the whole dictionary using the
del
statement,
student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}
# delete student_id dictionary
del student_id
print(student_id)
# Output: NameError: name 'student_id' is not defined
We are getting an error message because we have deleted the student_id dictionary and student_id doesn't exist anymore.
Python Dictionary Methods
Methods that are available with a dictionary are tabulated below. Some of them have already been used in the above examples.
| Function | Description |
|---|---|
| all() |
Return
True
if all keys of the dictionary are True (or if the dictionary is empty).
|
| any() |
Return
True
if any key of the dictionary is true. If the dictionary is empty, return
False
.
|
| len() | Return the length (the number of items) in the dictionary. |
| sorted() | Return a new sorted list of keys in the dictionary. |
| clear() | Removes all items from the dictionary. |
| keys() | Returns a new object of the dictionary's keys. |
| values() | Returns a new object of the dictionary's values |
Dictionary Membership Test
We can test if a
key
is in a dictionary or not using the keyword
in
. Notice that the membership test is only for the
keys
and not for the
values
.
# Membership Test for Dictionary Keys
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
# Output: True
print(1 in squares) # prints True
print(2 not in squares) # prints True
# membership tests for key only not value
print(49 in squares) # prints false
Output
True True False
Iterating Through a Dictionary
We can iterate through each key in a dictionary using a
for
loop.
# Iterating through a Dictionary
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
for i in squares:
print(squares[i])
Output
1 9 25 49 81
Here, we have iterated through each
key
in the
squares
dictionary using the
for
loop.
