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Dictionaries are collections of data that are unordered, changeable and indexed. They are found within a set of curly brackets ‘{}’. Each key within a dictionary is separated by a colon and each each element in the dictionary is separated by a comma. An empty dictionary without any items is {}.

The purpose of defining a dictionary is to access the contents of the dictionary through the key. The values of a dictionary can be of any type, but the keys must be of an immutable data type such as strings, numbers, or tuples. (tuples are immutable lists).

Example: In this example, we are defining a dictionary to link the three letter code of an amino acid to the single letter code. These act as key-value pairs.

dictionary = {
    'Arg': 'R', # a key-value pair
    'Ala': 'A',
    'Lys': 'L',
    'His': 'H',

  • Using an if condition, we check for the presence of Lys in the dictionary. If it is present, the pair of Lys is printed.