Python: Swapping the keys and values of a dictionary

A function to swap the keys and immutable values of a dictionary.

def swap_dict(a_dict):
    return {val:key for key, val in a_dict.items()}

Example of usage:

>>> print(swap_dict({1: 'a', 2: 'b', 3: 'c'}))
{'a': 1, 'c': 3, 'b': 2}

How to store functions in a dictionary in Python

This example uses a dictionary to store functions.

To call the function, use the dictionary key.

>>> f = {'plus_all':sum, 'find_max':max}
>>> f['find_max'](1,4,5,3,2,6)
6
>>> f['plus_all'](range(10))
45

How to sum the values of a Python dictionary

Suppose that we have a Python dictionary,

x = {'a': 3, 'b': 2, 'c':7}

and, we want to add the values i.e. 3 + 2 + 7 = 12.

Here are the three ways to add the values.

Using list comprehension

>>> x = {'a': 3, 'b': 2, 'c':7}
>>> sum([i for i in x.values()])
12

Using for loop I

>>> x = {'a': 34, 'b': 2, 'c':7}
>>> total = 0
>>> for i in x.values():
total += i

Using for loop II

>>> x = {'a': 34, 'b': 2, 'c':7}
>>> total = 0
>>> for key in x.keys():
total += x[key]

How to convert two Python lists into a dictionary

To convert two Python lists into a dictionary. First, pair the two list with the zip() function, as key-value pairs. The last step is to use the dict() constructor to build a dictionary directly from the key-value pairs.

>>> x = ['a', 'b', 'c']
>>> y = [1 ,2 ,3]
>>> dict(zip(x,y))
{'a': 1, 'c': 3, 'b': 2}

We can also zip the two lists together, as list of tuples, and use the dict() constructor for the iterator of that list.

>>> x = ['a', 'b', 'c']
>>> y = [1, 2, 3]
>>> dict(iter(list(zip(x, y))))
{'a': 1, 'c': 3, 'b': 2}

Find max or min in Python dictionary

Suppose that mydict is a dictionary defined by

mydict = {'a': 2, 'c': 5, 'b': 1, 'd': 4}

How to find the key for the max or min value in the items?

Applying

max(mydict)

would produce 'd'. This is wrong. The answer should be 'c'.

Doing it this way, the Python try to find the max in the key part. In this case, 'd' is the max, in alphabetical order.

The correct way is,

max(mydict, key = lambda x: mydict.get(x) )

Similarly, to find the min,

min(mydict, key = lambda x: mydict.get(x) )

Sort Python dictionary

Suppose that mydict is a dictionary defined by

mydict = {'a': 2, 'c': 5, 'b': 1, 'd': 4}

Remember that dictionary has no function sort, since it is unordered, not like a list, or tuple.

However, the key function, introduced in version 2.4, is helpful in sorting a dictionary.

To sort by items, and return the keys and items,

sorted( mydict.items(), key=lambda x: x[1] )

output:

[('b', 1), ('a', 2), ('d', 4), ('c', 5)]

To return only the keys, sorted by the items,

sorted( mydict.keys(), key=lambda x: mydict[x] )

output:

['b', 'a', 'd', 'c']

Note that, adding reverse=True at the end of the sorted function would produce a decending order list.

sorted( mydict.items(), key=lambda x: x[1] )

output:

[('c', 5), ('d', 4), ('a', 2), ('b', 1)]

Happy sorting.