Python Dictionary – Append / Add / Update key-value pairs

How to add or append new key value pairs to a dictionary or update existing keys’ values in python.

Python dictionary append key-value pair: In this article we will discuss about how we can add or append new key value pairs to a dictionary or update existing keys’ values. As we know Dictionary in python is one of the important datatype which is used to store data values in key : value pair.

Syntax of dictionary :

dictionary_name = {key1: value1, key2: value2}

where,

  • key1, key2… represents keys in a dictionary. These keys can be a string, integer, tuple, etc. But keys needs to be unique.
  • value1, value2… represents values in dictionary. These values can be numbers, strings, list or list within a list etc.
  • key and value is separated by : symbol.
  • key-value pair symbol is separated by , symbol.

For example,

my_dictionary = {"Name":"Satya", "Address":"Bhubaneswar", "Age":20, }

So, now let’s explore the concept how to add a key:value pair to dictionary in Python.

Method #1 : Add/Update using update( ) function :

python dictionary append key/value pair: In python there is an inbuilt member function update( ) which can be used to add or update a new key-value pair to the dictionary.

dictionary_name.update(Iterable_Sequence of key: value)

Actually, this update( ) function adds the key-value pair in the dictionary and if the key exists in the dictionary then it updates the value of that key.

If the key does not exist in the dictionary then it will add the key-value pair in the dictionary. Like this

#Program

CovidCase = {"January": 100000, "February": 110000, "March": 120000, "April": 130000}
print("Before adding new value")
print(CovidCase)
#Adding new key-value
#As the key does not exist so the key May and it's value will be added
CovidCase.update({"May": "140000"})
print("After adding new value")
print(CovidCase)
Output :
Before adding new value
{"January": 100000, "February": 110000, "March": 120000, "April": 130000}
After adding new value
{"January": 100000, "February": 110000, "March": 120000, "April": 130000, "May": 1400000}

If the key exists in the dictionary then it will update the key-value pair in the dictionary. Like this

#Program 

CovidCase = {"January": 100000, "February": 110000, "March": 120000, "April": 130000} 
print("Before adding new value") 
print(CovidCase) 
#Adding new key-value 
#As the key exists,so key January value will be updated
CovidCase.update({"January": "140000"}) 
print("After adding new value") 
print(CovidCase)
Output : 
Before adding new value 
{"January": 100000, "February": 110000, "March": 120000, "April": 130000} 
After adding new value 
{"January": 140000, "February": 110000, "March": 120000, "April": 130000, "May": 1400000}

Method #2 : Add/Update using Subscript [ ] notation :

In python there is a subscript [] operator which can also be used to create a new key-value pair just by assigning a value to that key.

If the key does not exist in the dictionary then it will add the key-value pair in the dictionary. Like this

#Program

CovidCase = {"January": 100000, "February": 110000, "March": 120000, "April": 130000}
print("Before adding new value")
print(CovidCase)
#Adding new key-value
#As the key does not exist so the key May and it's value will be added
CovidCase["May"]=140000
print("After adding new value")
print(CovidCase)
Output : 
Before adding new value 
{"January": 100000, "February": 110000, "March": 120000, "April": 130000} 
After adding new value 
{"January": 100000, "February": 110000, "March": 120000, "April": 130000, "May": 1400000}

If the key exists in the dictionary then it will update the key-value pair in the dictionary. Like this

#Program

CovidCase = {"January": 100000, "February": 110000, "March": 120000, "April": 130000} 
print("Before adding new value") 
print(CovidCase) 
#Adding new key-value
#As the key exists,so key January value will be updated
CovidCase["January"]=140000 
print("After adding new value") 
print(CovidCase)
Output : 
Before adding new value 
{"January": 100000, "February": 110000, "March": 120000, "April": 130000} 
After adding new value 
{"January": 140000, "February": 110000, "March": 120000, "April": 130000, "May": 1400000}

Method #3 : Append using append( ) function:

In python there is an inbuilt function append( ) to add values to the keys of the dictionary. As we will add only value to the keys, so we need to find out the keys to which we want to append the values.

#Program

CovidCase = {"January": [], "February": [], "March": []} 
print("Before adding new value") 
print(CovidCase) 
#Adding new key-value
#As the key exists,so key January value will be updated
CovidCase["January"].append(140000)
CovidCase["February"].append(150000)
CovidCase["March"].append(160000)
print("After adding new value") 
print(CovidCase)
Output : 
Before adding new value 
{"January": [ ], "February": [ ], "March": [ ]} 
After adding new value 
{"January": 140000, "February": 150000, "March": 160000}

Method #4 : Adding values to an existing key-value pair

With the help of update( ) function and subscript [ ] operator the key-value pair is added in dictionary only if that key does not exist in dictionary and if the key exists in the dictionary then the respective value of the key in  dictionary is updated. But if we do not want to replace the value of an existing key dictionary but we want to add a new value to a current value then also we can do it by creating a function.

#Program

def add_value(dict_obj, key, value):
    # Checking the key exists or not
    if key in dict_obj:
        # Key exist in dict.
        # Checking type of value of key is list or not
        if not isinstance(dict_obj[key], list):
            # If type not in list, then make it list
            dict_obj[key] = [dict_obj[key]]
        # Adding the value in list
        dict_obj[key].append(value)
    else:
        # if key is not in dictionary then add key-value pair
        dict_obj[key] = value


CovidCase = {"January": 140000, "February": 150000, "March": 160000}
#Before adding
print(CovidCase)
#After adding
add_value(CovidCase, "February", 170000)
print(CovidCase)
Output : 
Before adding new value 
{"January": [ ], "February": [ ], "March": [ ]} 
After adding new value 
{"January": 140000, "February": [150000, 170000] "March": 160000}

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