A function that takes 4 or 7 as input and returns 7 or 4 [f(4) = 7 OR f(7) = 4] WITHOUT USING if-statement

swap_without_if.py

"""Implement a function which takes an integer which does the following:

    - if input is 4: it returns 7
    - if input is 7: it returns 4
    Otherwise (unspecified).

However:
    CANNOT USE `if-statement`.

To Run: 
   - Make sure you `pip install pytest`.
Then:
    ```$ pytest -v swap_without_if.py```
"""
import pytest
from typing import Dict


def f_ternary(x: int) -> int:
    """Uses an `if-expression` not if statement.

    Args:
        x (int) - Given either 4 or 7.

    Return:
        int -- Flips the input.
        f(4) = 7
        f(7) = 4
    """
    assert x == 4 or x == 7, "input must be 4 or 7"

    return 7 if x == 4 else 4


def f_old_ternary(x: int) -> int:
    """Uses ternary operator.


    Args:
        x (int) - Given either 4 or 7.

    Return:
        int -- Flips the input.
        f(4) = 7
        f(7) = 4
    """
    assert x == 4 or x == 7, "input must be 4 or 7"

    return x == 4 and 7 or 4


def f_dict(x: int) -> int:
    """Uses dictionary indexing.

    Args:
        x (int) - Given either 4 or 7.

    Return:
        int -- Flips the input.
        f(4) = 7
        f(7) = 4
    """
    assert x == 4 or x == 7, "input must be 4 or 7"

    return {4: 7, 7: 4}[x]


# Uses one-liner dictionary indexing.
f_dict2: Dict[int, int] = {4: 7, 7: 4}.__getitem__


def f_list(x: int) -> int:
    """Uses a list indexing.

    Args:
        x (int) - Given either 4 or 7.

    Return:
        int -- Flips the input.
        f(4) = 7
        f(7) = 4
    """
    assert x == 4 or x == 7, "input must be 4 or 7"

    return [0, 0, 0, 0, 7, 0, 0, 4][x]


def f_addition(x: int) -> int:
    """Uses integer addtion/subtraction.

    Args:
        x (int) - Given either 4 or 7.

    Return:
        int -- Flips the input.
        f(4) = 7
        f(7) = 4
    """
    assert x == 4 or x == 7, "input must be 4 or 7"

    return 11 - x


def f_bitflip(x: int) -> int:
    """Using a clever bit flip operation.

    Note:
        ```python
        >>> bin(4)
        '0b100'
        >>> bin(7)
        '0b111'
        >>> bin(4 ^ 0b11)  # == 7
        '0b111'
        >>> bin(7 ^ 0b11)  # == 4
        '0b100'

        ```
    Args:
        x (int) - Given either 4 or 7.

    Return:
        int -- Flips the input.
        f(4) = 7
        f(7) = 4
    """
    assert x == 4 or x == 7, "input must be 4 or 7"

    return x ^ 0b11


# `pytest` mark decorator for creating parameters.
params = pytest.mark.parametrize(
    'f',
    (
        f_ternary,
        f_old_ternary,
        f_dict,
        f_dict2,
        f_list,
        f_addition,
        f_bitflip,
    ),
)


@params
def test_4_returns_7(f):
    assert f(4) == 7

    with pytest.raises((AssertionError, KeyError)):
        f(2)


@params
def test_7_returns_4(f):
    assert f(7) == 4

    with pytest.raises((AssertionError, KeyError)):
        f(2)