user1877442 - 11 months ago 44

Python Question

I'm trying to make a function that will compare multiple variables to an integer and output a string of three letters. I was wondering if there was a way to translate this into Python. So say:

`x = 0`

y = 1

z = 3

Mylist = []

if x or y or z == 0 :

Mylist.append("c")

elif x or y or z == 1 :

Mylist.append("d")

elif x or y or z == 2 :

Mylist.append("e")

elif x or y or z == 3 :

Mylist.append("f")

which would return a list of

`["c", "d", "f"]`

Is something like this possible?

Answer

You misunderstand how boolean expressions work; they don't work like an English sentence and guess that you are talking about the same comparison for all names here. You are looking for:

```
if x == 1 or y == 1 or z == 1:
```

`x`

and `y`

are otherwise evaluated on their own (`False`

if `0`

, `True`

otherwise).

You can shorten that to:

```
if 1 in (x, y, z):
```

or better still:

```
if 1 in {x, y, z}:
```

using a `set`

to take advantage of the constant-cost membership test (`in`

takes a fixed amount of time whatever the left-hand operand is).

When you use `or`

, python sees each side of the operator as *separate* expressions. The expression `x or y == 1`

is treated as first a boolean test for `x`

, then if that is False, the expression `y == 1`

is tested.

This is due to operator precedence. The `or`

operator has a lower precedence than the `==`

test, so the latter is evaluated *first*.

However, even if this were *not* the case, and the expression `x or y or z == 1`

was actually interpreted as `(x or y or z) == 1`

instead, this would still not do what you expect it to do.

`x or y or z`

would evaluate to the first argument that is 'truthy', e.g. not `False`

, numeric 0 or empty (see boolean expressions for details on what Python considers false in a boolean context).

So for the values `x = 2; y = 1; z = 0`

, `x or y or z`

would resolve to `2`

, because that is the first true-like value in the arguments. Then `2 == 1`

would be `False`

, even though `y == 1`

would be `True`

.