Python: check if key exists in dictionary (6 Ways)

Suppose we have a dictionary of string and int i.e.

Now we want to check if key ‘test’ exist in this dictionary or not.

There are different ways to do this. Let’s discuss them one by one.

Python: check if key in dictionary using if-in statement

We can directly use the ‘in operator’ with the dictionary to check if a key exist in dictionary or nor. The expression,

Will evaluate to a boolean value and if key exist in dictionary then it will evaluate to True, otherwise False. Let’s use this to check if key is in dictionary or not. For example,

Output:

Here it confirms that the key ‘test’ exist in the dictionary.

Now let’s test a negative example i.e. check if key ‘sample’ exist in the dictionary or not i.e.

Output:

Here it confirms that the key ‘sample’ does not exist in the dictionary.

Python: check if dict has key using get() function

In python, the dict class provides a method get() that accepts a key and a default value i.e.

Behavior of this function,

• If given key exists in the dictionary, then it returns the value associated with this key,
• If given key does not exists in dictionary, then it returns the passed default value argument.
• If given key does not exists in dictionary and Default value is also not provided, then it returns None.

Let’s use get() function to check if given key exists in dictionary or not,

Output:

Here it confirmed that the key ‘sample’ does not exist in the dictionary.

We passed ‘sample’ argument in the get() function, without any default value. As our dictionary does not contain ant key ‘sample’ and no default value is provided, therefore it returned None.

If we pass the default value along with the key and if key does not exist in the dictionary, then it returns the default value. For example,

Output:

Here it confirmed that the key ‘sample’ does not exist in the dictionary.

We passed ‘sample’ argument in the get() function, along with the default value -1. As our dictionary does not contain ant key ‘sample’, so get() function returned the the default value.

We cannot always be sure with the result of dict.get(), that key exists in dictionary or not . Therefore, we should use dict.get() to check existence of key in dictionary only if we are sure that there cannot be an entry of key with given default value.

Python: check if key in dict using keys()

keys() function of the dictionary returns a sequence of all keys in the dictionary. So, we can use ‘in’ keyword with the returned sequence of keys to check if key exist in the dictionary or not. For example,

Output:

Here it confirms that the key ‘test’ exist in the dictionary.

python check if key in dictionary using try/except

If we try to access the value of key that does not exist in the dictionary, then it will raise KeyError. This can also be a way to check if exist in dict or not i.e.

Output:

Here it confirms that the key ‘test’ exist in the dictionary.

In function check_key_exist(), it access the value of given key. If key does not exist then KeyError occurs, in that case it returns False, otherwise it returns True

Check if key not in dictionary in python using ‘if not in’ statement

In all the above example, we checked if key exist in dictionary or not. But if want to check if key doesn’t exist in dictionary then we can directly use ‘not in’ with dictionary i.e.

Output:

Here it confirms that the key ‘test’ exist in the dictionary.

Check if key exist in dictionary using has_key() function

dict provides a function has_key() to check if key exist in dictionary or not. But this function is discontinued in python 3. So, below example will run in python 2.7 only i.e.

Output:

Here it confirms that the key ‘test’ exist in the dictionary.

The complete example is as follows.

Output

Python: check if key exists in dictionary (6 Ways)

原文：https://www.cnblogs.com/a00ium/p/13693459.html