Project: Reverse a dict in Python 2.7 & 3

This project's goal is to compute average run times of different methods of reversing a dict's keys and values in Python 2 & 3

• method 1: makes use of dict comprehension, and the dictionary must contain unique values
• method 2: the dictionary doesn't contain unique values and saves all the keys with the same values in a list
• method 3: makes use of map(reversed, iter), useful when the type and order of the original dictionary must be preserved (e.g. OrderedDict)

I ran some tests, and method 1 is the one that provides the best average run time for reversing the keys and values of a dictionary. See Table 1 (Python 2) and Table 2 (Python 3) for the average run times of the different methods based on the number of items.

Check the project's README for details on how to install and use the program.

You can also check my blog post Python tips: reverse a dictionary where I discuss the implementation of the different methods and the results of the comparaisons of the methods based on their average run times.

Note: I tested the code with Python 2.7.15 and 3.6.5

Contents

1. Results
   1. Major updates
   2. Table 1: Avg run times of different methods (Python 2)
   3. Table 2: Avg run times of different methods (Python 3)
2. Method 1: unique values, solution based on dict comprehension
   1. Python 2.7 with dict.iteritems()
   2. Python 2.7 with dict.items()
   3. Python 3
3. Method 2: non-unique values
   1. Python 2.7 with dict.get()
   2. Python 3 with dict.get()
   3. Python 2.7 with dict.setdefault()
   4. Python 3 with dict.setdefault()
4. Method 3: type and order preserved
   1. Python 2.7
   2. Python 3
5. References

Results

Major updates

• I updated the code base and re-ran the python commands to re-populate the following two tables with new results. The changes consisted in implementing the important missing option --use_setdefault which I thought was already implemented when I generated the results the first time. Big mistake on my part! Thus the old results for the rows Method 2: setdefault were generated actually with dict.get() instead of dict.setdefault() :(
• Also, I factorized methods.py where the different dict-reversing methods are defined by putting all the common code from Python 2 & 3 methods into base classes.

Method 1: unique-values, solution based on dict

Python 2.7 with dict.iteritems()

my_dict = { 'a': 1, 'b':2, 'c': 3, 'd':4, 'e':5}
inv_dict = {v: k for k, v in my_dict.iteritems()}

Python 2.7 with dict.items()

my_dict = { 'a': 1, 'b':2, 'c': 3, 'd':4, 'e':5}
inv_dict = {v: k for k, v in my_dict.items()}

Python 3

my_dict = { 'a': 1, 'b':2, 'c': 3, 'd':4, 'e':5}
inv_dict = {v: k for k, v in my_dict.items()}

Method 2: non-unique values

Python 2.7 with dict.get()

my_dict = {1: 'a', 2:'b', 3: 'c', 4: 'a', 5: 'c'}
inv_dict = {}
for k, v in my_dict.iteritems():
    inv_dict[v] = inv_dict.get(v, [])
    inv_dict[v].append(k)

Python 3 with dict.get()

from collections import OrderedDict

def reverse_dict(orig_dict):
    inv_dict = {}
    for k, v in orig_dict.items():
        inv_dict[v] = inv_dict.get(v, [])
        inv_dict[v].append(k)

my_dict = OrderedDict({1: 'a', 2:'b', 3: 'c', 4: 'a', 5: 'c'})
reverse_dict(my_dict)

Python 2.7 with dict.setdefault()

my_dict = {1: 'a', 2:'b', 3: 'c', 4: 'a', 5: 'c'}
inv_dict = {}
for key, value in my_dict.iteritems():
    inv_dict.setdefault(value, []).append(key)

Python 3 with dict.setdefault()

my_dict = {1: 'a', 2:'b', 3: 'c', 4: 'a', 5: 'c'}
inv_dict = {}
for key, value in my_dict.items():
    inv_dict.setdefault(value, []).append(key)

Method 3: type and order preserved

Python 2.7

def reverse_mapping(f):
    return f.__class__(map(reversed, f.iteritems()))

my_dict = {1: 'a', 2:'b', 3: 'c', 4: 'd', 5: 'e'}
inv_dict = reverse_mapping(my_dict)

Python 3

def reverse_mapping(f):
    return f.__class__(map(reversed, f.items()))

my_dict = {1: 'a', 2:'b', 3: 'c', 4: 'd', 5: 'e'}
inv_dict = reverse_mapping(my_dict)

References

• Method 1 code
• Method 2 code with dict.get()
• Method 2 code with dict.setdefault()
• Method 3 code