in tests for the existence of a key in a dict:
d = {"key1": 10, "key2": 23}
if "key1" in d:
print("this will execute")
if "nonexistent key" in d:
print("this will not")
Use dict.get() to provide a default value when the key does not exist:
d = {}
for i in range(100):
key = i % 10
d[key] = d.get(key, 0) + 1
To provide a default value for every key, either use dict.setdefault() on each assignment:
d = {}
for i in range(100):
d[i % 10] = d.setdefault(i % 10, 0) + 1
...or better, use defaultdict from the collections module:
from collections import defaultdict
d = defaultdict(int)
for i in range(100):
d[i % 10] += 1
Top answer 1 of 16
5715
in tests for the existence of a key in a dict:
d = {"key1": 10, "key2": 23}
if "key1" in d:
print("this will execute")
if "nonexistent key" in d:
print("this will not")
Use dict.get() to provide a default value when the key does not exist:
d = {}
for i in range(100):
key = i % 10
d[key] = d.get(key, 0) + 1
To provide a default value for every key, either use dict.setdefault() on each assignment:
d = {}
for i in range(100):
d[i % 10] = d.setdefault(i % 10, 0) + 1
...or better, use defaultdict from the collections module:
from collections import defaultdict
d = defaultdict(int)
for i in range(100):
d[i % 10] += 1
2 of 16
1937
Use key in my_dict directly instead of key in my_dict.keys():
if 'key1' in my_dict:
print("blah")
else:
print("boo")
That will be much faster as it uses the dictionary's O(1) hashing as opposed to doing an O(n) linear search on a list of keys.
GeeksforGeeks
geeksforgeeks.org › python › python-check-given-key-already-exists-in-a-dictionary
Check if a Key Exists in a Python Dictionary - GeeksforGeeks
July 11, 2025 - If you accidentally assign a duplicate key value, the new value will overwrite the old one. To check if given Key exists in dictionary, you can use either in operator or get() method. Both are easy are use and work efficiently.
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Educative
educative.io › answers › how-to-check-if-a-key-exists-in-a-python-dictionary
How to check if a key exists in a Python dictionary
The code snippet below illustrates the usage of the if-in statement to check for the existence of a key in a dictionary: Fruits = {'a': "Apple", 'b':"Banana", 'c':"Carrot"} ... The has_key() method returns true if a given key is available in ...
W3Schools
w3schools.com › python › gloss_python_check_if_dictionary_item_exists.asp
Python Check if Key Exists in Dictionary
Python Examples Python Compiler Python Exercises Python Quiz Python Challenges Python Server Python Syllabus Python Study Plan Python Interview Q&A Python Bootcamp Python Certificate Python Training ... thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } if "model" in thisdict: print("Yes, 'model' is one of the keys in the thisdict dictionary") Try it Yourself »
Flexiple
flexiple.com › python › check-if-key-exists-in-dictionary-python
How to check if a key exists in a Python Dictionary? - Flexiple
To check if a key exists in a Python dictionary, you can use the has_key() method. However, it's important to note that this method is available in Python 2.x but has been removed in Python 3.x.
freeCodeCamp
freecodecamp.org › news › how-to-check-if-a-key-exists-in-a-dictionary-in-python
How to Check if a Key Exists in a Dictionary in Python – Python Dict Has Key
June 27, 2023 - Combining methods A good thing about working with Python dictionary methods is that you can combine them. For example, you can use the in operator to check if a key exists and the dict.get() to retrieve its value if it exists.
Interview Kickstart
interviewkickstart.com › home › blogs › learn › check whether a given key already exists in a dictionary in python
Check if Given Key Already Exists in Python Dictionary
December 18, 2025 - The has_key() method is a built-in ... we look at examples for has_key(), we’ll run the code in earlier versions of Python. ... The given key exists in the dictionary. The given key does not exist in the dictionary. We can also use the if-in statement to check if the key is ...
Reddit
reddit.com › r/learnpython › looping through a dictionary to check if key exists in a list of lists
r/learnpython on Reddit: Looping through a dictionary to check if key exists in a list of lists
January 16, 2024 -
Hi
I'm new to coding and python, and have been given a project to work on, as part of this project I need to do the following:
I have a dictionary:
base_ref_dict = {'0000000001': '',
'0000000002': '8.2',
'0000000003': '8.3',
'0000000004': '',
'0000000005': '8.2',
'0000000006': '8.3',
'0000000007': '',
'0000000008': '8.2',
'0000000009': '8.3',
'0000000010': '8.3'}
I then have a list of lists:
MyBaseList = [['0000000001', '1.2', '3.5', '8.1', '8.0'],
['0000000002', '1.2', '3.5', '8.1', 'Not Present'],
['0000000003', '1.2', '3.5', '8.1', '8.0'],
['0000000004', '1.2', '3.5', '8.1', '8.0'],
['0000000005', '1.2', '3.5', '8.1', 'Not Present'],
['0000000006', '1.2', '3.5', '8.1', '8.0'],
['0000000008', '1.2', '3.5', '8.1', 'Not Present'],
['0000000009', '1.2', '3.5', '8.1', '8.0']]
I want to take the key from `base_ref_dict` and check if it exists in `MyBaseList`. If it does not exist, I want to remove it from `base_ref_dict`.
So the final `base_ref_dict` should look like:
base_ref_dict = {'0000000001': '',
'0000000002': '8.2',
'0000000003': '8.3',
'0000000004': '',
'0000000005': '8.2',
'0000000006': '8.3',
'0000000008': '8.2',
'0000000009': '8.3''}
I've tried the following, but I think my iterations are all messed up:
for x in base_ref_dict: #loop through dictionary
for litems in MyBaseList: #loop through base list
if x != litems[0]: #if the key is not in the Inner list
del base_ref_dict[x] #delete from dictionary
break
In the final data both `base_ref_dict` and `MyBaseList` will hold approx. 80k records, so this needs to be as time efficient as possible
Top answer 1 of 5
4
More than one way to approach working with lists of lists, one being list comprehension which i used below. Also the way you're iterating through the dictionary and deleting on the fly will cause: "RuntimeError: dictionary changed size during iteration". You can use for key in list(dictionary) for key in list(base_ref_dict): if key not in (item[0] for item in MyBaseList): del base_ref_dict[key]
2 of 5
4
Don't loop through your dictionary keys. Loop through your list and then simply use litems[0] in base_ref_dict to see if it exists in the dictionary. This will be significantly faster due to how lookups in dictionaries work (look into hashmaps if you're interested in roughly what happens under the hood). EDIT: This does not answer the question
Scaler
scaler.com › home › topics › check if key exists in dictionary python
Check if Key Exists in Dictionary Python - Scaler Topics
November 16, 2023 - You can check if a key exists in the dictionary in Python using [] (index access operator) along with the try-except block. If you try to access a key that does not exist in Python dictionary using the [] operator, the program will raise a KeyError ...
Note.nkmk.me
note.nkmk.me › home › python
Check If a Key/Value Exists in a Dictionary in Python | note.nkmk.me
May 19, 2025 - To check whether a specific key-value pair exists in a dictionary, use the in operator with the items() method. Specify the pair as a tuple (key, value).
AskPython
askpython.com › home › 4 easy techniques to check if key exists in a python dictionary
4 Easy Techniques to Check if Key Exists in a Python Dictionary - AskPython
May 30, 2023 - Python in operator along with if statement can be used to check whether a particular key exists in the dictionary. Python in operator is basically used to check for memberships.
FavTutor
favtutor.com › blogs › check-key-exists-in-dictionary-python
Check if Key exists in Dictionary (or Value) with Python code
February 18, 2023 - student_dictionary = {1: "Rahul", 2: "Abhijeet", 3: "Roma", 4: "Arpita"} student_values = student_dictionary.values() if "Rahul" in student_values: print("Yes, the value 'Rahul' exists in the student dictionary") else: print("No, the value 'Rahul' ...
Codedamn
codedamn.com › news › python
Python Key in Dict: How to Check if a Key Exists
July 3, 2023 - If the key does not exist, it returns the default value. student = {"name": "John Doe", "age": 21, "courses": ["CS101", "MA101"]} if student.get("name") is not None: print("Key exists.") else: print("Key does not exist.") In this example, ...
Spark By {Examples}
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Python Check if Key Exists in Dictionary - Spark By {Examples}
May 31, 2024 - The first and most used method in the list is get() method which can be used to check if a key exists in a dictionary. The get() method in Python’s dictionary data structure allows you to determine if a specific key is present in the dictionary.
PythonHow
pythonhow.com › how › check-if-a-given-key-already-exists-in-a-dictionary
Here is how to check if a given key already exists in a dictionary in Python
To check if a given key exists in a dictionary in Python, you can use the in keyword. In this example, we create a dictionary called my_dict with three key-value pairs. Then, we use the in keyword to check if a given key exists in the dictionary. If the key exists, we print a message indicating ...
OneUptime
oneuptime.com › home › blog › how to check if key exists in dictionary in python
How to Check if Key Exists in Dictionary in Python
January 25, 2026 - The setdefault() method returns the value if the key exists, otherwise sets the default and returns it. user = {'name': 'Alice'} # Get existing value name = user.setdefault('name', 'Unknown') print(name) # 'Alice' # Key missing - sets default ...
Reddit
reddit.com › r/learnpython › checking dictionary's key and value
Checking dictionary's key and value : r/learnpython
August 19, 2021 - Yes. dict.get(x) checks the dict for a key equal to whatever x value is, and either 1) returns the associated value if the key exists, or 2) returns a default value if it doesn't.
Better Stack
betterstack.com › community › questions › python-check-if-key-exists-in-dictionary
How to check if a given key already exists in a dictionary in Python? | Better Stack Community
Alternatively, you can use the dict.get() method to check if a key exists in a dictionary. This method returns the value for the given key if it exists in the dictionary, and returns a default value (which you can specify) if the key does not exist. For example:
Stack Abuse
stackabuse.com › python-check-if-key-exists-in-dictionary
Python: Check if Key Exists in Dictionary
March 8, 2023 - Here it will either evaluate to True if the key exists or to False if it doesn't: key = 'orange' if key in fruits_dict: print('Key Found') else: print('Key not found') Now, since we don't have an orange in our dictionary, this is the result: ... This is the intended and preferred approach by ...
Pierian Training
pieriantraining.com › home › how to check if a key exists in a python dictionary
How to Check if a Key Exists in a Python Dictionary - Pierian Training
April 28, 2023 - In this example, we created a dictionary named `my_dict` that contains three key-value pairs. We then used the `in` keyword to check if the key `’name’` exists in the dictionary.