creating a new dict by updating an existing one

You essentially are doing this:

tmp = {
   'a': 1
}
result = tmp.update({
   'a': 2
})
del tmp

You could use:

dict({
   'a': 1
}, ** {
   'a': 2
})

and get a merged dictionary, however. Or, for a more practical looking version:

copy = dict(original, foo = 'bar')

Suggestion : 2

update() method updates the dictionary with elements from a dictionary object or an iterable object of key/value pairs.,The update() method updates the dictionary with the elements from another dictionary object or from an iterable of key/value pairs.,The update() method takes either a dictionary or an iterable object of key/value pairs (generally tuples).,Note: The update() method adds element(s) to the dictionary if the key is not in the dictionary. If the key is in the dictionary, it updates the key with the new value.

Example

marks = {
   'Physics': 67,
   'Maths': 87
}
internal_marks = {
   'Practical': 48
}

marks.update(internal_marks)

print(marks)

# Output: {
   'Physics': 67,
   'Maths': 87,
   'Practical': 48
}

Example

marks = {
   'Physics': 67,
   'Maths': 87
}
internal_marks = {
   'Practical': 48
}

marks.update(internal_marks)

print(marks)

# Output: {
   'Physics': 67,
   'Maths': 87,
   'Practical': 48
}

The syntax of update() is:

dict.update([other])

Example 1: Working of update()

d = {
   1: "one",
   2: "three"
}
d1 = {
   2: "two"
}

# updates the value of key 2
d.update(d1)

print(d)

d1 = {
   3: "three"
}

# adds element with key 3
d.update(d1)

print(d)

Example 2: update() When Tuple is Passed

dictionary = {
   'x': 2
}

dictionary.update([('y', 3), ('z', 0)])

print(dictionary)

Suggestion : 3

update() is a dictionary function in python used to update a dictionary by changing the values of existing keys or adding new keys.,In the above example, the new key TSLA with the value 80 is added to the dictionary using the dictionary function update(). Note that the update() function modifies the dict in-place.,Example: Add a new key to the dictionary using the update function,Dictionaries are quite useful and commonly used in python. It may happen that you require to add new keys or update an existing one in a dictionary. In this tutorial, we’ll look at how you can add or update items in a dictionary.

Example: Add a new key to the dictionary using subscript notation

# add item to a dictionary
# dictionary of a sample portfolio
shares = {
   'APPL': 100,
   'GOOG': 50
}
# print
print("Shares in your portfolio:", shares)

# add 80 shares of TSLA to the portfolio using subscript notation
shares['TSLA'] = 80
# print
print("Shares in your portfolio:", shares)

Output:

Shares in your portfolio: {
   'APPL': 100,
   'GOOG': 50
}
Shares in your portfolio: {
   'APPL': 100,
   'GOOG': 50,
   'TSLA': 80
}

Example: Update an existing key using the subscript notation

# update item in a dictionary
# dictionary of a sample portfolio
shares = {
   'APPL': 100,
   'GOOG': 50
}
# print
print("Shares in your portfolio:", shares)

# update the shares of 'GOOG'
to 150
shares['GOOG'] = 150
# print
print("Shares in your portfolio:", shares)

Example: Update an existing key using the update function

# add item to a dictionary
# dictionary of a sample portfolio
shares = {
   'APPL': 100,
   'GOOG': 50
}
# print
print("Shares in your portfolio:", shares)

# update the shares of 'GOOG'
to 150
shares.update({
   'GOOG': 150
})
# print
print("Shares in your portfolio:", shares)

Suggestion : 4

Last Updated : 04 Oct, 2021,GATE CS 2021 Syllabus

Output:

Original Dictionary: {
   'A': 'Geeks',
   'B': 'For'
}

Dictionary after updation: {
   'A': 'Geeks',
   'B': 'Geeks'
}

Output:

Key exist, value updated = 600 {
   'm': 600,
   'n': 100,
   't': 500
}

Suggestion : 5

You can append a new item to an existing dict object with elements in the following two ways.,4 Append in a Dictionary4.1 Assignment to Append Elements4.2 Update Method to Append Elements,Python allowed the dictionary object to be mutable. Hence, the add or update operations are permissible. You can push a new item or modify any existing with the help of the assignment operator.,You can call the dictionary’s update method to append a new element to it. We are extending the above example, see below:

Below is an example of a barren dictionary object which doesn’t have any elements.

#The empty Python dictionary object {}

Also, remember to use only immutable data types for the values while they can have duplicates.

# Define a blank dictionary with no elements
blank_dict = {}

# Define a dictionary with numeric keys
num_dict = {
   1: 'soccer',
   2: 'baseball'
}

# Define a dictionary with keys having different types
misc_dict = {
   'class': 'senior secondary',
   1: [1, 2, 3, 4, 5]
}

# Create a dictionary with dict() method
get_dict_from_func = dict({
   1: 'veg',
   2: 'non-veg'
})

# Create a dictionary from a sequence of tuples
make_dict_from_seq = dict([(1, 'jake'), (2, 'john')])

Both methods will work the same, but the former will return a KeyError while the latter returns None if the element is not available.

dict = {
   'Student Name': 'Berry',
   'Roll No.': 12,
   'Subject': 'English'
}
print("dict['Student Name']: ", dict['Student Name'])
print("dict['Roll No.']: ", dict['Roll No.'])
print("dict['Subject']: ", dict['Subject'])

Accessing an element with a non-existent key will return an error. Check out the below code.

dict = {
   'Student Name': 'Berry',
   'Roll No.': 12,
   'Subject': 'English'
}
print("dict['Name']: ", dict['Name'])

The above example uses the “Name” key which doesn’t exist. Running the same would produce a “KeyError.”

Traceback (most recent call last):
File "C:/Python/Python35/test_dict.py", line 2, in <module>
   print("dict['Name']: ", dict['Name'])
   KeyError: 'Name'

Suggestion : 6

In this example to update the dictionary by passing key/value pair. This method updates the dictionary.,It consists of only one parameterKey: This is the key pair that is to be searched in the dictionary.,Another example is to update the dictionary by key/value pair.,It consists of only one parameter.other: It is a list of key/value pairs.

Here is the Syntax of the update() method

dict.update([other])

Code:

dict = {
   'Africa': 200,
   'australia': 300,
   'England': 400
}
print("Country Name", dict)
dict.update({
   'China': 500
})
print("updated country name", dict)

Example:

dict = {
   'John': 200,
   'Micheal': 300,
   'Smith': 400
}
print("Name", dict)
dict.update({
   'Andrew': 500,
   'Hayden': 800
})
print("updated name", dict)

Here is the Syntax of the update function

dict.update([other])

Let’s take an example to check how to update values in the dictionary

dict1 = {
   'Germany': 200,
   'France': 300,
   'Paris': 400
}
print("Country name", dict1)
dict1.update({
   'France': 600
})
print("updated value", dict1)

Suggestion : 7

The update() method is also useful when you want to add the contents of one dictionary into another.,Say you have one dictionary, numbers, and a second dictionary, more_numbers.,This method is particularly helpful when you want to update more than one value inside a dictionary at the same time.,If you want to merge the contents of more_numbers with the contents of numbers, use the update() method.

Then, assign the variable to an empty set of curly braces, {}.

#create an empty dictionary
my_dictionary = {}

print(my_dictionary)

#to check the data type use the type() function
print(type(my_dictionary))

#output

#{}
#<class 'dict'>

It acts as a constructor and creates an empty dictionary:

#create an empty dictionary
my_dictionary = dict()

print(my_dictionary)

#to check the data type use the type() function
print(type(my_dictionary))

#output

#{}
#<class 'dict'>

The general syntax for this is the following:

dictionary_name = {
   key: value
}

You would achieve this by using dict() and passing the curly braces with the sequence of key-value pairs enclosed in them as an argument to the function.

#create a dictionary with dict()
my_information = dict({'name': 'Dionysia' ,'age': 28,'location': 'Athens'})

print(my_information)

#check data type
print(type(my_information))

#output

#{'name': 'Dionysia', 'age': 28, 'location': 'Athens'}
#<class 'dict'>

The general syntax for the method is the following:

dictionary_name = dict.fromkeys(sequence, value)

Suggestion : 8

We can combine an existing dictionary and a key-value pair using the double-asterisk ** unpacking operator. ,Section 1: Insert/Update Key in a DictionaryMethod 1: Create a New Key-Value Pair and Assign it to Dictionary | Subscript NotationMethod 2: Use The update() FunctionMethod 3: Using the Asterisk * Operator,Problem: Given a dictionary. How to update a key in it if the key does not exist?,To update a key in a dictionary if it doesn’t exist, you can check if it is present in the dictionary using the in keyword along with the if statement, and then update the key-value pair using subscript notation or update() method or the asterisk * operator.

Example:

device = {
      "brand": "Apple",
      "model": "iPhone 11",
   }

   <
   Some Method to Check
if key - value pairs "color": "red"
and "year": 2019 exists or not and then update / insert it in the dictionary >

   print(device)

Output:

{
   'brand': 'Apple',
   'model': 'iPhone 11',
   'color': 'red',
   'year': 2019
}

Let us have a look at the following program, which explains the syntax to create a new key-value pair and assign it to the dictionary:

device = {
   "brand": "Apple",
   "model": "iPhone 11",
}

device["year"] = 2019
print(device)

Let us have a look at the following code to understand the concept and usage of the ** operator to insert items in a dictionary.

device = {
   "brand": "Apple",
   "model": "iPhone 11",
}
device = {
   ** device,
   ** {
      "year": 2019
   }
}
print(device)

The following program explains how we can use the in keyword.

device = {
   "brand": "Apple",
   "model": "iPhone 11",
   "year": 2018
}

if "year" in device:
   print("key year is present!")
else:
   print("key year is not Present!")

if "color" in device:
   print("key color is present!")
else:
   print("key color is not present!")

Suggestion : 9

last modified July 29, 2022

1._

#!/usr/bin/python

capitals = {}

capitals["svk"] = "Bratislava"
capitals["deu"] = "Berlin"
capitals["dnk"] = "Copenhagen"

print(capitals)

The example creates a new empty dictionary and adds new keys and values.

$. / empty.py {
   'svk': 'Bratislava',
   'dnk': 'Copenhagen',
   'deu': 'Berlin'
}

3._

#!/usr/bin/python

capitals = dict()

capitals.update([('svk', 'Bratislava'), ('deu', 'Berlin'), ('dnk', 'Copenhagen')])

print(capitals)

5._

data = ['coins', 'pens', 'books', 'cups'];

items = dict.fromkeys(data, 0)

print(items)

items['coins'] = 13
items['pens'] = 4
items['books'] = 39
items['cups'] = 7

print(items)

The example creates a new dictionary from a list of values. Each element is initialized to zero. Later, each item is assigned a new integer value.

$. / from_keys.py {
   'coins': 0,
   'pens': 0,
   'books': 0,
   'cups': 0
} {
   'coins': 13,
   'pens': 4,
   'books': 39,
   'cups': 7
}