Python Dictionaries are used to store data in a key-value pair. Dictionaries are changeable and it doesn’t allow duplicate keys.
You can check if a key exists in a dictionary using the If statement and dict.keys() method.
You may need to check if a key is present in a dictionary before adding a new key-value pair to the dictionary. In this tutorial, you’ll learn the different methods to check if a key exists in the dictionary.
If You’re in Hurry…
You can use the below code snippet to check if a key exists in the dictionary.
mydict = {
"one": "1",
"two": "2",
"three": "3",
"four": "4",
}
key="one"
if key in mydict:
print("Key exists")
else:
print("Key does not exist")Output
Key existsIf You Want to Understand Details, Read on…
In this tutorial, you’ll learn the different methods available to check if keys exist in a dictionary, and also you’ll learn how to check the keys in different use-cases.
The methods demonstrated in this tutorial also answers the question of how to check if Key exists in the Ordered Dictionary.
Now, let’s start.
Table of Contents
Sample Dictionary
This is the sample dictionary you’ll use in this tutorial. It contains the keys one, two, three, four.
Note: all the keys are in the lower case. This is useful to learn how to check if a key exists in the dictionary in a case-sensitive way.
mydict = {
"one": "1",
"two": "2",
"three": "3",
"four": "4",
}Check If Key Exists in Dictionary
There are three methods available in python to check if a key exists in the dictionary.
- Using
dictionary.keys() - Using
IfAndIN - Using
has_keys()(deprecated in Python 3)
Using Keys()
You can check if a key exists in a dictionary using the keys() method and IN operator.
The keys() method will return a list of keys available in the dictionary and IF, IN statement will check if the passed key is available in the list.
If the key exists, it returns True else, it returns False .
Snippet
key="one"
if key in mydict.keys():
print("Key exists")
else:
print("Key does not exist")Output
Key existsThis is how you can check if a key exists using the dictionary.keys() method.
Using If and In
You can check if the key exists in the dictionary using IF and IN. When using the dictionary directly with IF and IN, it’ll check if the key exists in the dictionary.
If a key exists, It returns True else it returns False.
Snippet
key="one"
if key in mydict:
print("Key exists")
else:
print("Key does not exist")Output
Key existsThis is how you can check if a key is available in the dictionary using the IF and IN.
Using has_key()
You can use the has_key() method to check if a key is available in a dictionary.
This is deprecated in python 3. If you are using an older version of python than python 3, then you can use this method.
You can check the python version in cmd using the below command.
import sys
print(sys.version)Output
3.9.2 (default, Sep 4 2020, 00:03:40) [MSC v.1916 32 bit (Intel)]Snippet
if mydict.has_key("One"):
print("Key exists")
else:
print("Key does not exist")This tutorial uses the version Python 3. Hence, the below error is thrown as output.
Output
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-4-29ad973f8a0b> in <module>
----> 1 if mydict.has_key("One"):
2 print("Key exists")
3 else:
4 print("Key does not exist")
AttributeError: 'dict' object has no attribute 'has_key'These are the different methods available in python to check if keys are available in the dictionary.
Now, you’ll learn about the different ways to use this.
Check If Key and Value Exists in Dictionary python
You can check if key and value exist in the dictionary using the dict.items().
The items() returns a key-value pair available in the dictionary as tuples in the list.
You can pass the key-value pair as a tuple in IF statement shown below to check if the pair exists in the dictionary.
Note: This is case-sensitive. Hence pass the Key value pair in a case-sensitive way.
Snippet
if ("one", "1") in mydict.items():
print("Key and Value exists")
else:
print("Key value pair doesn't exists")Where,
("one", "1")– Key value pair to be checkedmydict.items()– Dictionaryitems()method which will yield key value pairs as tuples.
If the key-value pair exists, then the IF statement will be executed. If it doesn’t exist, then the Else statement will be executed.
Output
Key and Value existsSnippet 2
Pass a key-value pair that doesn’t exist in the dictionary. Then you’ll see the Else statement executed as shown below.
if ("one", "ONE") in mydict.items():
print("Key and Value exists")
else:
print("Key value pair doesn't exists")Output
Key value pair doesn't existsThis is how you can check if a Key value pair is available in the dictionary.
Check If Multiple Keys Exists in Dictionary
You can check if multiple keys exist in the dictionary in one statement. You can use the all() along with the list comprehension to check if multiple or all keys exists in the dictionary.
Snippet
if all (key in mydict for key in ("one","two")):
print("All Keys exists in the dictionary")
else:
print("All doesn't exist")Where,
for key in ("one","two")– Keys to check in the dictionary will be iteratedkey in mydict– During the each iteration of the for loop, each key will be checked if it exists in the mydict- Then, it’ll return a list which contains
TrueorFalsebased on theINcheck. - At-last, the
all()method checks the list. If it contains onlyTrue, then it means all the passed keys are available in the dictionary. If it contains at-least oneFalse, then it returns false.
Output
All Keys exists in the dictionarySnippet 2
To check the negative scenario where all the passed keys are not available in the dictionary.
if all (key in mydict for key in ("one","two", "Five")):
print("All Keys exists in the dictionary")
else:
print("All Keys doesn't exist")Output
All Keys doesn't existThis is how you can check if multiple keys exist in a dictionary in one statement.
Check If Key Exists in Dictionary Case Insensitive
By default, the IF, IN method is case-sensitive. When you check if a key exists in the dictionary using IF, IN method, you should pass the proper case.
You can check if a key exists in a dictionary in a case insensitive way, then you need to create a set by converting all the keys to either lower case or upper case.
In the example, you’ll convert all the keys to lower case using lower() method. Then, you’ll also convert the key to be checked to lower case.
Then you can pass the key to be checked and the set with lower case keys to IF, IN statement to check if a key exists in a case-insensitive way.
Snippet
mydict = {
"one": "1",
"two": "2",
"three": "3",
"four": "4",
}
set_with_keys = set(key.lower() for key in mydict)
key_to_check = "One"
if key_to_check.lower() in set_with_keys:
print("Key exists")
else:
print("Key doesn't exists")Where,
set(key.lower() for key in mydict)– Creating a set with all keys in lower casekey_to_check.lower()– Converting key to be checked to lower caseif key_to_check.lower() in set_with_keys:– Checking if key exists in a case insensitive way
When you execute the below code, you’ll see the below output.
Output
Key existsThis is how you can check if a key exists in a dictionary in a case-insensitive way.
Check If Key Exists in List of Dictionaries
A list of dictionaries are dictionaries that are available in a list.
You can check if a key exists in the list of dictionaries by using the any() method and list comprehension available in python.
Snippet
list_of_dict = [{"one": "1"}, {"two": "2"}, {"three": "3"}]
if any("one" in keys for keys in list_of_dict):
print("Key exists in List of Dictionary")
else:
print("Key doesn't exists in List of Dictionary")Where,
list_of_dict– list that contains three dictionary objects"one" in keys for keys in list_of_dict– using list comprehension and generating the list of keys in the list of dictionaries. Then checking ifoneis available in the list. It’ll return a list which containsTruewhere the key is one andFalsewhere the key is not one.any()– Checks the list of True or False returned by list comprehension. If it contains at-least oneTrue, the key one is available in the list of dictionary. Then it returnsTrueand executes theIFblock. Otherwise, it executes theElseblock.
Output
Key exists in List of DictionarySnippet 2
To demonstrate the unavailable key check in the list of dictionaries.
list_of_dict = [{"one": "1"}, {"two": "2"}, {"three": "3"}]
if any("five" in keys for keys in list_of_dict):
print("Key exists in List of Dictionary")
else:
print("Key doesn't exists in List of Dictionary")Output
Key doesn't exists in List of DictionaryThis is how you can check if a key exists in the list of dictionaries.
Check If Key Exists and Has Value
You can check if a key exists and has a specific value in the dictionary by using the dictionary.items() method.
The items() method will return a list of tuples with the available key pairs in the dictionary. Then you can use IF and IN with a tuple to check if it exists in the dictionary as shown below.
If the key-value pair is exactly available in the list of tuples, then the IF block will be executed. Otherwise, Else block will be executed.
Snippet
if ("one", "1") in mydict.items():
print("Key and Value exists")
else:
print("Key value pair doesn't exists")Output
Key and Value existsThis is how you can check if a key exists and has a value in the dictionary.
Check If Key Exists in JSON Dictionary
JSON string means Javascript Object Notation and it is stores values in a key-value pair.
You can create a dictionary using the JSON string using the json.loads() method.
Then you can normally use the IF and IN to check if the key exists in the JSON dictionary as shown below.
Snippet
json_str = """{"one": 1, "two": 2, "three": 3}"""
jsn_dict = json.loads(json_str)
if "three" in jsn_dict:
print("Key exists in JSON Dict")
else:
print("Key does not exist in JSON Dict")Output
Key exists in JSON DictSnippet 2
This is to demonstrate the checking existence of the non-existent key.
json_str = """{"one": 1, "two": 2, "three": 3}"""
jsn_dict = json.loads(json_str)
if "five" in jsn_dict:
print("Key exists in JSON Dict")
else:
print("Key does not exist in JSON Dict")Output
Key does not exist in JSON DictThis is how you can check if a key exists in the JSON dictionary.
Check if Key Exists in Dictionary Time complexity
You’ve seen different methods available to check if a key exists in the dictionary. Now, you’ll learn about the time complexity of using the different methods available to check if a key exists in the dictionary.
Time Complexity Table
| Operation | Average Case | Amortized Worst Case |
| k in d | O(1) | O(n) |
| Get Item | O(1) | O(n) |
| Iteration[3] | O(n) | O(n) |
Based on the above table, using the IF and IN statements are the best and fastest way to check if a key exists in the dictionary.
To know more about the time complexity of the dictionaries, refer to this link and scroll down to the end to see about the dictionary.
Conclusion
To summarize, you’ve learned the methods available to check if the key is available in the dictionary. You’ve also learned how to use those methods in different scenarios to check if the key is available in the dictionary. Additionally, you’ve also seen the time complexity of using each method.
If you have any questions, comment below.