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The Python Dict

A Dictionary (a.k.a "dict") in python is a built-in datatype that can be used to store key-value pairs. This allows you to treat a dict like it's a database to store and organize data.

The special thing about dictionaries is the way they are implemented. It's similar to how a filing cabinet works. By just specifying the key which is like a book title, the python interpreter can just go to the location of the key and check for the content (value) of that key.

Dictionaries can use almost any datatypes, like strings, integers etc, as the key of dictionaries. However, values that are not hashable, such as, lists, dictionaries or other mutable types (that are compared by value rather than by object identity) may not be used as keys.

Numeric types used for keys obey the normal rules for numeric comparison. For example if two numbers compare equal (such as 1 and 1.0) then they can be used interchangeably to index the same dictionary entry. (Note, however, that since computers store floating-point numbers as approximations it is usually unwise to use them as dictionary keys.)

An important requirement of a dictionary is that the keys must be unique, which means - you can never have a duplicate key in your dictionary.

Creating a Empty Dictionary

To create an empty dictionary just use a pair of braces:

    >>> teams = {} # Creates a empty dictionary
    >>> type(teams)
    >>> <class 'dict'>

Creating a Non-Empty Dictionary

To create a non-empty dictionary with some initial values, place a comma-seperated list of key-value pairs:

    >>> teams = {'barcelona': 1875, 'chelsea': 1910}
    >>> teams
    {'barcelona': 1875, 'chelsea': 1910}

Alternative Ways To Create a Dictionary

You could create a dictionary by using the dict() method:

    >>> players = dict( [('messi','argentina'), ('ronaldo','portugal'), ('kaka','brazil')] ) # sequence of key-value pair is passed  
    >>> players
    {'ronaldo': 'portugal', 'kaka': 'brazil', 'messi': 'argentina'}
    >>> 
    >>> # If keys are simple strings, it's quite easier to specify pairs using keyword arguments
    ...
    >>> dict( totti = 38, zidane = 43 )
    {'zidane': 43, 'totti': 38}

Dict comprehensions can be used as well to create dictionaries from arbitrary key and value expressions:

    >>> {x: x**2 for x in (2, 4, 6)}
    {2: 4, 4: 16, 6: 36}

Adding New Key-Vaule Pair to Existing Dictionary

    >>> teams['santos'] = 1787
    >>> teams
    {'chelsea': 1910, 'barcelona': 1875, 'santos': 1787} # Notice the order - Dictionaries are unordered !
    >>> # extracting value - Just provide the key
    ...
    >>> teams['barcelona']
    1875

Deleting a Key-Value Pair From a Dictionary

del operator is used to delete a key-value pair from the dict. In scenarios where a key that's already in use is again used to store values, the old value associated with that key is completely lost. Also, keep in mind that it's an error to extract the value using an non-existent key.

    >>> del teams['santos']
    >>> teams
    {'chelsea': 1910, 'barcelona': 1875}

Replacing Value of a Key in a Dictionary

    >>> teams['chelsea'] = 2017 # overwriting    
    >>> teams
    {'chelsea': 2017, 'barcelona': 1875}

Searching for Existing Keys in a Dictionary

The in keyword can be used to check whether a key exist in the dict or not:

    >>> 'sanots' in teams
    False    
    >>> 'barcelona' in teams
    True
    >>> 'chelsea' not in teams
    False

Getting All of the Keys or Values In the Dictionary

keys is a built-in method that can be used to get the keys of a given dictionary. To extract the keys present in a dict as lists:

    >>> club_names = list(teams.keys())    
    >>> club_names
    ['chelsea', 'barcelona']

values is a built-in method that can be used to get the values of a given dictionary. To extract the values present in a dict as lists:

    >>> club_names = list(teams.values())    
    >>> club_names
    [1910, 1875]

get is a built-in method that can be used to get values from a dictionary by providing the key. This method is very useful if you need to check existance of values for a particular key. You can also use it to return default values in case a key-value pair is missing in the dictionary.

    >>> players = {'ronaldo': 'portugal', 'kaka': 'brazil', 'messi': 'argentina'}
    >>> players.get('ronaldo')
    portugal
    >>> players.get('joe')
    None
    >>> players.get('joe', 'Missing Key')
    Missing Key

has_key is a built-in method that can be used to check if a key exists in the dictionary.

    >>> items = {'a': 'apple', 'b': 'banana', 'c': 'cat'}
    >>> items.has_key('a')
    True
    >>> items.has_key('d')
    False

Yet another way of creating dictionary is using the dict() method:

    >>> players = dict( [('messi','argentina'), ('ronaldo','portugal'), ('kaka','brazil')] ) # sequence of key-value pair is passed  
    >>> players
    {'ronaldo': 'portugal', 'kaka': 'brazil', 'messi': 'argentina'}
    >>> 
    >>> # If keys are simple strings, it's quite easier to specify pairs using keyword arguments
    ...
    >>> dict( totti = 38, zidane = 43 )
    {'zidane': 43, 'totti': 38}

Dict comprehensions can be used as well to create dictionaries from arbitrary key and value expressions:

    >>> {x: x**2 for x in (2, 4, 6)}
    {2: 4, 4: 16, 6: 36}

Looping in Dictionary

To simply loop over the keys in the dictionary, rather than the keys and values:

    >>> d = {'x': 1, 'y': 2, 'z': 3} 
    >>> for key in d:
    ...     print(key) # do something
    ...
    x
    y
    z

Looping in Dictionary for Python Version 2

To loop over both keys and values you can use the following:

    >>> for key, item in d.iteritems():
    ...     print items
    ...
    1
    2
    3

Looping in Dictionary for Python Version 3

Use items() instead of iteeritems():

    >>> for key, item in d.items():
    ...     print(key, items)
    ...
    x 1
    y 2
    z 3

To loop over just the values in the dictionary, do this in Python 3.x:

    >>> for value in d.values():
    ...     print(value)
    ...
    1
    2
    3

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