Append multiple items to a list on a for loop in python-CodePudding

I have a nested python for loop and need to append 2 times a value, is the code below PEP8 valid? Or there is a better pythonic way to to write the function?

def function():
    empty_list = []
    my_list = ['a', 'b', 'c']
    for letter_1 in my_list: 
        for letter_2 in my_list:
            empty_list.append(letter_1)
            empty_list.append(letter_2)
    return empty_list

CodePudding user response：

Assuming that your desired output is:

Desired output:

['a','a','a','b','a','c',  # letter_1, with loop of letter_2
 'b','a','b','b','b','c',  # letter_2, with loop of letter_2
 'c','a','c','b','c','c']  # letter_3, with loop of letter_2

An alternative (more "pythonic"?) way to write your function is to use the itertools library and list comprehensions:

def alt_function(my_list = ['a', 'b', 'c']):
    iterable = chain.from_iterable([a b for a, b in product(my_list, repeat=2)])
    return list(iterable)
    
alt_function()

Output

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

CodePudding user response：

Your code is right and PEP8 compliant. I would remove the my_list from the function block and make it a function's parameter. I would suggest using list.extend() to perform the operation you need in one line. In order to make it a bit more Pythonic I would add typing hints and the function's docstring. The code would look like this:

from typing import List

def function(my_list: List) -> List:
    """Function's docstring.

    Args:
        my_list (List): List of characters.

    Returns:
        List: Processed list of characters.
    """
    empty_list = []
    for a in my_list:
        for b in my_list:
            empty_list.extend((a, b))
    return empty_list

I don't know which IDE you use, but on Visual Studio Code you can download some extensions to generate docstrings automatically from your function's/classes' signature and typing hints. And also, there's extensions to automatically lint Python code to be PEP8 compliant.

I would also add a small test to make sure my function works as expected. Something like this:

assert function(['a', 'b', 'c']) == ['a', 'a', 'a', 'b', 'a', 'c',
                                     'b', 'a', 'b', 'b', 'b', 'c', 'c', 'a', 'c', 'b', 'c', 'c']