How to check if a list of keys exists in a Python dictionary ?

Introduction

To check if a list of keys exists in a Python dictionary, you can use a few different approaches. Here are some methods:

Using python built-in function all()

Let's consider a real-world scenario where we retrieve data from a URL using a GET request, resulting in a dictionary like this:

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d = {
    'token': 'j23feh4g5h2c',
    'first_name': 'John',
    'last_name': 'Doe',
    'username': 'JohnDoe',
    'email': 'John.Doe@gmail.com'
}

In this dictionary, each key represents a parameter that was expected in the response. For example, the dictionary includes keys such as 'token', 'first_name', 'last_name', 'username', and 'email'.

To ensure that all the necessary parameters are indeed present in the dictionary, we can define a list of expected parameters:

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parameters = ['token', 'first_name', 'last_name', 'username', 'email']

Now, we need a way to verify that every key in this parameters list exists in the dictionary d. This is where the all() function in Python comes in handy. The all() function will evaluate whether all elements in an iterable are True.

By using all() in combination with a generator expression, we can efficiently check for the existence of each key in the dictionary:

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all(param in d for param in parameters)

Explanation:

(1) Generator Expression: param in d for param in parameters:
- This expression iterates through each item in the parameters list.
- For each param, it checks whether param exists as a key in the dictionary d.
- The expression returns True if the key exists and False otherwise.

(2) all() Function: all(param in d for param in parameters)

• The all() function will return True only if every check (param in d) within the generator expression evaluates to True.
• If even one parameter is missing (i.e., if the expression returns False for any param), the all() function will return False.

Example Result:

If all the expected parameters are present in the dictionary d, the expression will return True. Otherwise, it will return False.

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# Example usage:
if all(param in d for param in parameters):
    print("All required parameters are present.")
else:
    print("Some required parameters are missing.")

In this scenario, since all parameters ('token', 'first_name', 'last_name', 'username', and 'email') are present in the dictionary d, the output would be:

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All required parameters are present.

This method provides a concise and efficient way to ensure that all required data is present in a dictionary retrieved from an external source.

Using a Loop

You can iterate through the list of keys and check if each key is in the dictionary.

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def check_keys_exist(dictionary, keys):
    for key in keys:
        if key not in dictionary:
            return False
    return True

# Example usage
my_dict = {'a': 1, 'b': 2, 'c': 3}
keys_to_check = ['a', 'b']

print(check_keys_exist(my_dict, keys_to_check))

Output:

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True

Using Set Subset

If you have a list of keys and want to check if they all exist in the dictionary, you can use set operations.

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def check_keys_exist(dictionary, keys):
   return set(keys).issubset(dictionary.keys())

Example usage

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my_dict = {'a': 1, 'b': 2, 'c': 3}
keys_to_check = ['a', 'b']

print(check_keys_exist(my_dict, keys_to_check))

Output:

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True

Check Existence for Each Key (With Return of Missing Keys)

If you want to find out which keys are missing, you can modify the loop to return the missing keys.

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def find_missing_keys(dictionary, keys):
    return [key for key in keys if key not in dictionary]

Example usage

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my_dict = {'a': 1, 'b': 2, 'c': 3}
keys_to_check = ['a', 'd']

missing_keys = find_missing_keys(my_dict, keys_to_check)

print(missing_keys)

Output:

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['d']

These methods will allow you to check if all the keys from a list exist in a dictionary, with different approaches depending on your specific needs.

References