Python dictionaries

Table of contents:

• Get the len of dictionary
• Get the value from the dictionary
• What is dictionary comprehension?
• How to invert a dictionary (keys and values)?
• How to remove elements with specific keys, values?
• Get dictionary keys and values
• Convert two sequences (lists) into dictionary
• Sorting a dictionary

Dictionaries are created using {} like this or using dict().

Example:

d = {'a':5, 'b':9, 'c':-1}

or you can convert them from tuples:

Example:

t = ('a',5), ('b',9), ('c',-1), ('d',4)
print(type(t))
d2 = dict(t)
d2

Output:

<class 'tuple'>
{'a': 5, 'b': 9, 'c': -1, 'd': 4}

Get the len of dictionary

Example:

d = {'a':5, 'b':9, 'c':-1}
len(d)

Output:

3

Get the value from the dictionary

Example:

d = {'a':5, 'b':9, 'c':-1}
d['a']

Output:

5

What is dictionary comprehension?

In dict comprehension we traverse all the elements and create a new dic nd.

Example:

d = {'a':5, 'b':9, 'c':-1}
nd = {k:v*v for k,v in d.items()}
nd

Output:

{'a': 25, 'b': 81, 'c': 1}

How to invert a dictionary (keys and values)?

As we know how to use dictionary comprehension we can use the same for key-values inversion.

Example:

d = {'a':5, 'b':9, 'c':-1}
d_inv = {v: k for k, v in d.items()}
d_inv

Output:

{5: 'a', 9: 'b', -1: 'c'}

How to remove elements with specific keys, values?

There is a way to remove the key from the dictionary if we use del, pop, popitem and clear:

Example:

d = {'a':5, 'b':9, 'c':-1, 'd':4}
del d['d']
print(d)
d.pop('c')
print(d)
d.popitem()
print(d)
d.clear()
print(d)

Output:

{'a': 5, 'b': 9, 'c': -1}
{'a': 5, 'b': 9}
{'a': 5}
{}

The other way is to use dictionary comprehension like before with the if condition, but then we deal with the new dictionary:

Example:

d = {'a':5, 'b':9, 'c':-1}
nd = {k:v*v for k,v in d.items() if k!='c'}
nd

Output:

{'a': 25, 'b': 81}

The same output would be for:

Example:

d = {'a':5, 'b':9, 'c':-1}
nd ={k:v*v for k,v in d.items() if v>0}
nd

Get dictionary keys and values

This example dissolves dict to keys and values and creates new exactly the same dictionary from those keys and values:

Example:

d ={'a':5, 'b':9, 'c':-1}
k = d.keys()
v = d.values()
k,v

Output:

(dict_keys(['a', 'b', 'c']), dict_values([5, 9, -1]))

About the same output you can get with:

Example:

d ={'a':5, 'b':9, 'c':-1}
k,v = zip(*d.items())
k,v

Output:

(('a', 'b', 'c'), (5, 9, -1))

Convert two sequences (lists) into dictionary

Example:

k,v = ['a', 'b', 'c'], [5, 9, -1]
d = dict(zip(k, v))
d

Output:

{'a': 5, 'b': 9, 'c': -1}

Sorting a dictionary

While it is not possible to sort a dictionary, since dictionaries are orderless, it is possible to use list comprehension to create new dictionary sorted the way we like. Plain simple sorted can be used for that.

Example:

d ={'a':5, 'b':9, 'c':-1}
nd = {k:v for k, v in sorted(d.items(), key=lambda _: _[1])}
print(nd)

Output:

{'c': -1, 'a': 5, 'b': 9}

The trick is, the way we inserted items to the dictionary is how they will be presented with print.