import re
import pandas as pd


def list_comprehension(l1: list):
    """For input list l1 of integers, return list of elements
    whose square is less than sum of all elements"""

    return None


def nested_list_comprehension(l1: list, l2: list):
    """For input lists l1, l2, return a list of elements common
    to both l1 and l2."""

    return None


def dict_comprehension(l1: list):
    """For input list l1 of integers, return a dictionary with
    keys from l1, and values are squares of keys"""

    return None


def generator(l1: list, l2: list):
    """For input lists l1 and l2 of integers, return a generator
    that gives all pairings (e1, e2) of element e1 from l1 and
    element e2 of l2."""

    raise NotImplementedError("Not implemented")


if __name__ == "__main__":
    l1 = [1, 2, 3, 4]
    l2 = [2, 4, 6, 8]

    print("\n-------------------\n")
    print("Inputs:\n")
    print(f"l1: {l1}")
    print(f"l2: {l2}")
    print("\n-------------------\n")

    print("list_comprehension:\n")
    print(f"Desired output: [2, 4]")
    print(f"Output        : {list_comprehension(l2)}")
    print("\n-------------------\n")

    print("nested_list_comprehension:\n")
    print(f"Desired output: [2, 4]")
    print(f"Output        : {nested_list_comprehension(l1, l2)}")
    print("\n-------------------\n")

    print("dict_comprehension:\n")
    print("Desired output: {1: 1, 2: 4, 3: 9, 4: 16}")
    print(f"Output:       : {dict_comprehension(l1)}")
    print("\n-------------------\n")

    print("generator:\n")
    print("Desired output:")
    print("  (1, 2)")
    print("  (1, 4)")
    print("  (1, 6)")
    print("  (1, 8)")
    print("  (2, 2)")
    print("  (2, 4)")
    print("  (2, 6)")
    print("  (2, 8)")
    print("  (3, 2)")
    print("  (3, 4)")
    print("  (3, 6)")
    print("  (3, 8)")
    print("  (4, 2)")
    print("  (4, 4)")
    print("  (4, 6)")
    print("  (4, 8)")
    print("Output:       :")
    try:
        for pair in generator(l1, l2):
            print(f"  {pair}")
    except NotImplementedError as e:
        print(e)