Python Dictionary

Python Dictionary: A dictionary maps a set of objects (keys) to another set of objects (values). A Python dictionary is a mapping of unique keys to values. In Python, Dictionaries are mutable, which means they can be changed. The values that the keys point to can be any Python value. And these are unordered. Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. An empty dictionary without any items is written with just two curly braces {}.



Example

thisdict = { "brand": "Lamborghini", "model": "GT", "year": 1963 } 
print(thisdict)

Output
{‘brand’: ‘Lamborghini’, ‘model’ : ‘GT’, ‘year’ : 1963 }

Python Dictionary

Accessing Items

You can access the items in the Python dictionary by referring it the key name, inside square brackets. And there is also a method called get() that will also help in accessing the element from a dictionary.

Example

x = thisdict["model"]

Output: Lamborghini

For accessing the elements we can also use the get()

Example

x = thisdict.get("model")

Output: Lamborghini

Change the Values

You can change the value of a specific item by referring to its key name

Example: change the year to 2019

thisdict = { "brand": "Lamborghini", "model":  "GT",  "year": 1963 } 
thisdict["year"] = 2019

Output
{‘brand’: ‘Lamborghini’, ‘model’: ‘GT’, ‘year’ = 2019}

Looping through Dictionary

You can loop through a dictionary by suing for a loop. The return value is the keys of the dictionary, but there are methods to return the values as well.
Example

for x in thisdict: 
print(x)

Output
brand
model
year

If you would like to print all the values in the Python dictionary, one by one

Example

for x in thisdict: 
print(thisdict[x])

Output
Lamborghini
GT
1963

Return Values

You can return the values by using values( )

Example

for x in thisdict.values():
 print(x)

Output
Lamborghini
GT
1963

Loop through Dictionary

You can also loop the dictionaries by using the items() function.

Example

for x, y in thisdict.items():
 print(x, y)

Output
brand Lamborghini
model   GT
year     1963

Checking ‘if’ key exists

You can also check whether a specific key exists in the dictionary.

Example

thisdict = { "brand": "Lamborghini", "model":  "GT", "year": 1963 } 
if "model" in thisdict: 
print("Yes, 'model' is one of the keys in the thisdict dictionary")

Output
Yes, ‘model’ is one of the keys in the thisdict dictionary

Dictionary Length

To determine how many items a dictionary has (key-value pairs), using len( ) method.

Example

print(len(thisdict))

Output: 3

Adding items to Dictionary

Adding a new item to the dictionary is done by using a new index key and assigning a value to it.

Example

thisdict = { "brand": "Lamborghini", "model":  "GT", "year": 1963 } 
thisdict["color"] = "red" 
print(thisdict)

Output
{‘brand’: ‘Lamborghini’, ‘model’: ‘GT’, ‘year’ : 1963, ‘color’ : ‘red’ }

Removing Items

There are several methods to remove items from dictionary. We can use the pop( ). All the items from a dictionary can be deleted at once by using clear() method. And the items in a Nested dictionary can also be deleted by using del keyword and providing specific nested key and a particular key to be deleted from that nested Dictionary.

Example

thisdict = { "brand": "Lamborghini", "model":  "GT", "year": 1963  } 
thisdict.pop("model") 
print(thisdict)

Output: {‘brand’: ‘Lamborghini’, ‘year’: 1963}

We can also use the popitem( ) to remove the last inserted item.

Example

thisdict = { "brand": "Lamborghini", "model":  "GT", "year": 1963 } 
thisdict.popitem( ) 
print(thisdict)

Output: {‘brand’: ‘Lamborghini’, ‘model’ : ‘GT’ }

We can also use the del keyword for removing the item with the specified key name.

Example

thisdict = { "brand": "Lamborghini", "model":  "GT",  "year": 1963 } 
del thisdict["model"] 
print(thisdict)

Output: {‘brand’: ‘Lamborghini’, ‘year’: 1963 }

Empties Dictionary

We can use clear( ) to empty the directory.

Example

thisdict = { "brand": "Lamborghini", "model":  "GT", "year": 1963 } 
thisdict.clear( ) 
print(thisdict)

Output: {}

Dictionary Constructor

We can also use dictionaries by using constructors.

Example

thisdict = dict(brand="Lamborghini", model="GT", year=1963) 
print(thisdict)

Output:{‘brand’: ‘Lamborghini’, ‘model’: ‘GT’, ‘year’: 1963}

Dictionary Methods

A method, in the context of object-oriented programming, is a procedure or a function in the class. As a part of the class, a method defines the behavior of class instance. A class can have more than one method.

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 a dictionary 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

Properties of Dictionary Keys

In Python Dictionary values will not be having any restrictions. They can be in either standard objects or user-defined objects. And it is not true for keys.

There are two main points to keep in mind regarding Keys. They are  as follows: 

1) More than one entry per key is not allowed, i.e. no duplicate key is allowed. If duplicate keys are allowed then last assignment keys will win.

Example

dict = {'Name': 'Safia', 'Age': 27, 'Name': 'Shaik'} 
print "dict['Name']: ", dict['Name']

Output: dict [‘Name’]: Mani

2) Keys must be immutable, which means you can use strings, numbers or tuples as dictionary keys but something like [‘key’] is not allowed.

Example

dict = {'Name': 'Safia', 'Age': 27, 'Name': 'Shaik'} 
print "dict['Name']: ", dict['Name']

Output
Traceback (most recent call last):File “test.py”, line 3,
in <module>dict = {[‘Name’]: ‘Zara’, ‘Age’: 7};
Type Error: unhashable type: ‘list’