User defined function for getting absolute value

Question

Using Wing ide 3.3.5

How would I write a function my_abs(value) that returns the absolute value of its x, without using the built-in abs function in Python.

For example:

Test                Result
print(my_abs(3.5))  3.5
print(my_abs(-7.0)) 7.0

Answer 1

A snarky answer would be to use the built-in __abs__() function instead:

>>> print((-7.0).__abs__())
7.0

---

Here is a naïve one-liner (basically equivalent to @Nabin's answer):

def simple_abs(num):
    return -num if num < 0 else num

---

However, it turns out that there is a bug, because

>>> -0.0 < 0
False
>>> simple_abs(-0.0)
-0.0                        ← Buggy
>>> abs(-0.0)
0.0                         ← Correct

It turns out that testing for negative zero in Python is hard to do. One possible test is str(-0.0) == '-0.0'. Here's what @JasonS helped me come up with:

import math

def my_abs(num):
    return -num if math.copysign(1, num) < 0 else num

Or, in longer, more readable form:

def my_abs(num):
    if math.copysign(1, num) < 0:
        return -num
    else:
        return num

Answer 2

A simple algorithm would be:

if number<0:
    return number*-1
else:
    return number

You have to check if the number is negative. Is so, then multiply it with -1 and return. Or you have to return the number without alteration.