freeCodeCamp/guide/english/python/data-structures/dictionaries/index.md

title
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. Hash-table-like structure makes it easy to check for existence - which means that we can easily determine if a specific key is present in the dictionary without needing to examine every element. The Python interpreter can just go to the location key and check if the key is there.

Dictionaries can use almost any arbitrary datatypes, like strings, integers etc, for keys. However, values that are not hashable, that is, values containing 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: 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.)

One most important requirement of a dictionary is that the keys must be unique.
To create an empty dictionary just use a pair of braces:

    >>> teams = {}
    >>> type(teams)
    >>> <class 'dict'>

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}

It's easy to add key-value pairs to an 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

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

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

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]

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

To loop over both key and value you can use the following:
For Python 2.x:

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

Use items() for Python 3.x:

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