Checking if key exists in dictionary python

How to check if key exists in a python dictionary?

In this tutorial, we look at methods to check if a key exists in a dictionary in Python. We also break down the code to facilitate further understanding.

However, in case you are here only for the solution use this link.

Table of Contents — check if key exists in dictionary python

If you are here to learn how to check if a key exists in a dictionary in python it is most likely because of these two reasons. Either you are new to python or you tried using the has_key methods and received an error.

The latter is because python3 has removed the has_key methods. However, there are other equally efficient methods to check if a key exists in a dictionary.

Why do we check if a key exists in a python dictionary?

Dictionaries are common and extensively used data types in python. They are used to store key-value pairs and these values are accessed by their respective keys.

This is why it is a good practice to check if the key exists before you try to access its relevant value. Doing so would reduce the likelihood of facing errors.

Now let’s look at the different ways you can use to check if a key exists in a dictionary in Python.

Using the in operator to check if key exists in dictionary python:

In this method, we use the membership operator; in . This operator is used to check if one value is a member of another. It returns a boolean value.

In our case, we use the in operator to check if the key is a member of the dictionary.

Code to check if a key exists in dictionary in python:

dict_1 = if "a" in dict_1: print("Exists") else: print("Does not exist") #Output = "Exists" 

dict_1 = if "d" in dict_1: print("Exists") else: print("Does not exist") #Output = "Does not exist" 

Similarly, the not in operator can also be used to check if a key does not exist in a dictionary.

However, remember the in operator is case sensitive hence you could either ensure all your keys are in the same case or you could use the upper() or lower() methods respectively.

Checking if key exists using the get() method

The get() method is a dictionary method that returns the value of the associated key. If the key is not present it returns either a default value (if passed) or it returns None.

Using this method we can pass a key and check if a key exists in the python dictionary.

Syntax of get()

Here dict is the name of the dictionary you intent to work with

Parameters

Keyname — The keyname of the value to intent to return value — Optional, this value is returned in case the key does not exist

Code to check if the key exists in a dictionary using get()

dict_1 = if dict_1.get("a") is not None: print("Exists") else: print("Does not exist") #Output = "Exists" 

dict_1 = if dict_1.get("d") is not None: print("Exists") else: print("Does not exist") #Output = "Does not exist" 

While using this method keep in mind that this would not be accurate in case you have a key with the value None. If you don’t, this method will work fine.

This method returns the values, you could store them in a variable in case you intend to use it.

Closing Thoughts

Although both the aforementioned methods have their limitations, these are more efficient in comparison to the other methods.

Other methods include iterating over the dictionary and then comparing all the keys with a variable containing the key name.

Although all those methods work, they aren’t efficient and should only be used to facilitate understanding of the concepts. But in case you are a learner, do try them out.

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Python: Check if a Key (or Value) Exists in a Dictionary (5 Easy Ways)

In this tutorial, you’ll learn how to use Python to check if a key exists in a dictionary. You’ll also learn how to check if a value exists in a dictionary. You’ll learn how to do this using the in operator, the .get() method, the has_key() function, and the .keys() and .values() methods.

Knowing how to work with Python dictionaries is an important skill. This can be especially helpful when working with web APIs that return JSON data.

While we can easily index a dictionary, if a key doesn’t exist, a KeyError will be thrown. This will cause some significant problems in your program, unless these errors are handled.

A much more safe alternative to using dictionary indexing, is to first check if a given key exists in a dictionary. Let’s get started learning!

The Quick Answer: Use in to see if a key exists

What is a Python Dictionary?

Dictionaries in Python are one of the main, built-in data structures. They consist of key:value pairs that make finding an items value easy, if you know their corresponding key. One of the unique attributes of a dictionary is that keys must be unique, but that values can be duplicated.

Let’s take a look at how dictionaries look in Python. They’re created using <> curly brackets and the key:value pairs are separated by commas.

Let’s create a dictionary called ages , which, well, contains the ages of different people:

One way that you’re often taught to access a dictionary value is to use indexing via the [] square bracket accessing. In the next section, you’ll see how dictionary indexing works and why it’s not always the best option. Following that, you’ll learn different methods of ensuring that a key exists.

The Problem with Indexing a Python Dictionary

Indexing a dictionary is an easy way of getting a dictionary key’s value – if the given key exists in the dictionary. Let’s take a look at how dictionary indexing works. We’ll use dictionary indexing to get the value for the key Nik from our dictionary ages :

We can see here that this worked beautifully. That being said, let’s see if we try to get the value for the key Jill , which doesn’t exist in the dictionary:

>>> ages = >>> print(ages['Jill']) KeyError: 'Jill'

We can see here, that if we try to access a dictionary’s value for a key that doesn’t exist, that a KeyError is thrown. This has some huge implications for your code. Unless the error is explicitly handled, the program will fail. One way to avoid a KeyError is to ensure that a key actually exists in a Python dictionary.

That’s exactly what you’ll learn in the next few sections. Let’s get started!

Use Python to Check if a Key Exists: Python keys Method

Python dictionary come with a built-in method that allows us to generate a list-like object that contains all the keys in a dictionary. Conveniently, this is named the .keys() method.

Printing out dict.keys() looks like this:

print(ages.keys()) # Returns: dict_keys(['Matt', 'Katie', 'Nik', 'Jack', 'Alison', 'Kevin'])

We can see how that looks like a little bit like a list. We can now check if a key exists in that list-like object!

Let’s see how we can use the .keys() method to see if a key exists in a dictionary. Let’s use this method to see if a key exists:

# Check if a key exists in a Python dictionary by checking all keys ages = some_key = 'James' if some_key in ages.keys(): print('Key exists') else: print('Key doesn\'t exist') # Returns Key doesn't exist

We can see here that we check whether or not a provided key, some_key , exists in the keys of our dictionary. In this case, the key didn’t exist and the program printed out Key doesn’t exist .

In the next section, you’ll learn how to simplify this even further!

Use Python to Check if a Key Exists: Python in Operator

The method above works well, but we can simplify checking if a given key exists in a Python dictionary even further. We can actually omit the .keys() method entirely, and using the in operator will scan all keys in a dictionary.

Let’s see how this works in practise:

ages = some_key = 'Nik' if some_key in ages: print('Key exists') else: print('Key doesn\'t exist') # Returns: Key exists

We can see here that our method actually looks very similar to the above, but we’ve been able to strip out the .keys() method entirely! This actually helps the code read a bit more plain-language.

in the next section, you’ll learn how to actually retrieve a key’s value, even if a key doesn’t exist!

Use the .get Method to Check if a Key Exists in a Python Dictionary

Working with dictionaries in Python generally involves getting a key’s value – not just checking if it exists. You learned earlier that simply indexing the dictionary will throw a KeyError if a key doesn’t exist. How can we do this safely, without breaking out program?

The answer to this, is to use the Python .get() method. The .get() method will simply return None if a key doesn’t exist. Let’s try this out:

ages = print(ages.get('Jack')) print(ages.get('Jill')) # Returns: # 43 # None

We can see here that when the .get() method is applied to return a key that exists, that key’s value is correctly returned. When a key doesn’t exist, the program continues to run, but it returns None .

What is particularly helpful about the Python .get() method, is that it allows us to return a value, even if a key doesn’t exist.

Say we wanted our program to notify us that a key didn’t exist. We could ask the .get() method to return “Key doesn’t exist!”.

Let’s see how we can do this:

ages = print(ages.get('Jill', "The key doesn't exist")) # Returns: "The key doesn't exist!"

The .get() method is a great and safe way to see if a key exists in a Python dictionary. Now, let’s learn to see whether or not a given value exists in a Python dictionary.

Check if a Value Exists in a Python Dictionary Using .values()

Similar to the Python dictionary .keys() method, dictionaries have a corresponding .values() method, which returns a list-like object for all the values in a dictionary.

Let’s see how we can access all of a dictionary’s values:

ages = print(ages.values()) #Returns: dict_values([30, 29, 31, 43, 32, 38])

We can use this to see whether or not a value exists. Say we wanted to know if the age 27 existed in our dictionary, we could write the following:

ages = some_age = 27 if some_age in ages.values(): print('Age exists!') else: print("Age doesn't exist!") # Returns: Age doesn't exist!

Now, what if we wanted to return the key or keys for a given value. We can safely do this using a list comprehension, which will have three permutations:

1. Be an empty list, if the value doesn’t exist,
2. Have one item, if the value exists once
3. Have more than one item, if the value exists more than once

Let’s use a slightly modified dictionary to see this in action:

# Getting all keys of a certain value in a Python dictionary ages = value_43 = Checking if key exists in dictionary python print(value_43) # Returns: ['Jack', 'Jill']

We’ve created a list comprehension that adds each key, if the value of that key is equal to 43.

What are Python List Comprehensions? To learn more about list comprehensions, check out my comprehensive tutorial here and my in-depth video below!

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