Working with dictionaries

There are a large number of methods that can be used with dictionaries. We won't talk about all of them, but some of the more common ones are shown in the following screenshot:

Line 36 checks to see whether a specified key exists within the dictionary.

Line 37 returns all the items that exist within the dictionary—both keys and their associated values. For more flexibility, you can look for just a dictionary's keys (line 38) or just the values (line 39).

To remove entries within a dictionary, you can delete single items, as demonstrated in line 40. To remove all entries in dictionary d1, you would use the d1.clear() method .

Since dictionaries are changeable, you can add and delete values to them without creating a new dictionary object, as shown in lines 23 and 40. Adding a new object to a dictionary only requires making a new key and value, whereas lists will return an index out-of-bounds error if the offset is past the end of the list. Therefore, you must use append() to add values to lists but simply make new key value entries for dictionaries.

The following screenshot is a more realistic dictionary example. The following example creates a table that maps programming language names (the keys) to their creators (the values). You fetch a creator name by indexing on the language name:

From this example, you might notice that the last command is similar to string and list iteration using the for command. However, you'll also notice that, since dictionaries aren't sequences (that is, the stored items are indexed by keyword and not position), you can't use the standard for statement. You must use the keys() method to return a list of all the keys which you can then iterate through like a normal list.

You may have also noticed that dictionaries can act like light weight databases. The preceding example creates a table, where the programming language column is matched by the creator's row. If you have a need for a database, you might want to consider using a dictionary instead. If the data will fit, you will save yourself a lot of unnecessary coding and reduce the headaches you would get from dealing with a full-blown database. Granted, you don't have the flexibility and power of a true database, but for quick-and-dirty solutions, dictionaries will suffice. 

There are a few key points about dictionaries that you should be aware of:

Sequence operations don't work. As previously stated, dictionaries are mappings, not sequences. Because there's no order to dictionary items, functions such as concatenation and slicing don't work.

Assigning new indexes adds entries. Keys can be created when making a dictionary (that is, when you initially create the dictionary) or by adding new values to an existing dictionary. The process is similar and the end result is the same.

Keys can be anything immutable. The previous examples showed keys as string objects, but any non-mutable object (such as numbers) can be used for a key. Numbers can be used to create a list-like object but without the ordering. Tuples are sometimes used to make compound keys; class instances that are designed not to change can also be used if needed.