Using Dictionaries

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We'll learn to:

• Use a key to get a value from a dictionary
• Check for existence of keys
• Iterate throgh keys and values in dictionaries

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Get a key

Once you have a dictionary, you can access the values in it by providing the key.

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Get an Invalid Key

If a key does not exist, python will throw a KeyError

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To avoid KeyError first check

One way to avoid the KeyError is to first check if the key exist in the dictionary.

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This will not throw an error, because key_to_check in building_heights will return False, and so we never try to access the key.

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Try/Except to get a key

When we try to access a key that doesn't exist, the program will go into the except block and print "That key doesn't exist!".

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Safely Get a Key

Dictionaries have a .get() method to search for a value instead of the my_dict[key]. If the key you are trying to .get() does not exist, it will return None by default:

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Specify a value if the key doesn't exist

You can also specify a value to return if the key doesn't exist.

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Delete a Key

Sometimes we want to get a key removed from the dictionary. We can use .pop() to do this. Just like with .get(), we can provide a default value to return if the key does not exist.

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.pop() works to delete items from a dictionary, when you know the key value.

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Get All Keys

Sometimes we want to operate on all keys in a dictionary. We can get a roster of the keys without including the values using the built-in list function:

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Dictionaries also have a .keys() method that returns a dict_keys object (view) which provides a look at the current state of the dictionary, without the user being able to modify anything. The dict_keys object returned by .keys() is a set of the keys in the dictionary. You cannot add or remove elements from a dict_keys object, but it can be used in the place of a list for iteration:

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Get All Values

Dictionaries have a .values() method that returns a dict_values object with all the values in the dictionary. Similarly, it can be used in the place of a list for iteration.

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There is no built-in function to get all of the values as a list, for most purposes, the dict_values object will act that way, but if you really want to have a list, you can use:

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Get All Items

You can get both the keys and the values with the .items() method. Like .keys() and .values(), it returns a dict_list object. Each element of the dict_list returned by .items() is a tuple consisting of (key, value).