As I'm using Python for every day tasks, I find myself repeating some tasks over and over again. Here I present a collection of code snippets for the usually performed tasks, so you can use them straight ahead in your project whenever you need them and speed up your implementation. All snippets are related to general-purpose snippets that you can use for any application.

Sort dictionary according to a desired order list

sample_dict = {'1': 'Item 1', '2': 'Item 2', '3': 'Item 3', '4': 'Item 4', '5': 'Item 5'}
desired_order_list = ['3', '2', '1', '5', '4']

reordered_dict = { k: sample_dict[k] for k in desired_order_list }

The reordered_dict is: {'3': 'Item 3', '2': 'Item 2', '1': 'Item 1', '5': 'Item 5', '4': 'Item 4'}

Find duplicates in a list

Given a list with potentially repeated items, we want to find which elements appear more than once. This snippet uses a list comprehension and set to extract all duplicates without repetition.

items = [1, 2, 2, 3, 4, 4, 5]
duplicates = list(set([x for x in items if items.count(x) > 1]))

The duplicates is: [2, 4]

Flatten a nested list

When you have a list of lists (a nested list), this script flattens it into a single list containing all the elements in order. It’s useful for simplifying nested structures into one continuous list.

nested = [[1, 2], [3, 4], [5]]
flat = [item for sublist in nested for item in sublist]

The flat is: [1, 2, 3, 4, 5]

Count frequency of elements in a list

This snippet counts how many times each element appears in a list, returning a dictionary-like object (Counter) where keys are the items and values are their counts. Perfect for quick frequency analysis.

from collections import Counter

lst = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
freq = Counter(lst)

The freq is: Counter({'apple': 3, 'banana': 2, 'orange': 1})

Merge two lists into a dictionary

We have two lists in Python and we want to merge them in a dictionary, where one list's (keys_list) items will act as the dictionary's keys and the other's (values_list) as the values.

keys_list = ['a', 'b', 'c']
values_list = ['red', 'green', 'blue']

new_dict = dict(zip(keys_list, values_list))

The new_dict is: {'a': 'red', 'b': 'green', 'c': 'blue'}

Cartesian product of lists

Suppose there are two lists: ['a', 'b', 'c'], ['1', '2', '3'] and we want this: ['a1','a2','a3','b1','b2','b3','c1','c2','c3']. This concept is called a Cartesian product, and the stdlib itertools.product will build one for you.

from itertools import product

list_1 = ['a','b','c']
list_2 = ['1','2','3']

product = list(map(''.join, product(list_1, list_2)))

The product is: ['a1', 'a2', 'a3', 'b1', 'b2', 'b3', 'c1', 'c2', 'c3'].

Sort a list of dictionaries

Another frequent task is when we have two or more lists, and we want to collect them all in one big list of lists, where all the first items of the smaller list form the first list in the bigger list.

from operator import itemgetter

list_of_dicts = [
    { "student": "A", "grade": 15, },
    { "student": "B", "grade": 20, },
    { "student": "C", "grade": 18, }
]

sorted_list_of_dicts = sorted(list_of_dicts, key=itemgetter('grade'))

The sorted_list_of_dicts is: [{ "student": "B", "grade": 20, }, { "student": "C", "grade": 18, },{ "student": "A", "grade": 15}].

By default the order of sorting is ascending. If you want to sort the list in descending order you should use the additional argument reverse=True in the 'sorted' function.