Python dict keys as statements

Question

Was wondering if something like the following was possible:

rarity = {>= 75: 'Common', <= 20 : 'Rare', >= 5: 'Legendary'}

I can *guess* what you're trying to do here, but please spell it out a bit more. — deceze, Jun 01 '18 at 14:32
Possible duplicate of [Replacements for switch statement in Python?](https://stackoverflow.com/questions/60208/replacements-for-switch-statement-in-python) — Ivan Vinogradov, Jun 01 '18 at 14:41

score 1 · Answer 1 · answered Jun 01 '18 at 14:35

1

In Python 2.7 this will raise a syntax error. It feels like abuse of dictionary (key-value storage) concept. Maybe you should rework your code and you could use 'Common', 'Rare' as keys and values as ranges, i.e. range(5,20), range(20), etc.

answered Jun 01 '18 at 14:35

rook

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Ivan Vinogradov · Answer 2 · 2018-06-01T14:44:43.357

1

This cannot be done with dict in python. You probably need an ordinary function for your task:

def check(x):
    if x >= 75:
        return 'Common'
    if x <= 20:
        ...

Remember that order of checks and return statements matters.

edited Jun 01 '18 at 14:44

answered Jun 01 '18 at 14:39

Ivan Vinogradov

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the question is very unclear but there are ways to use a dict like this e.g. `x = 6; {x >= 75: 'Common', x<= 20 : 'Rare', x>= 5: 'Legendary'}[True]` – Chris_Rands Jun 01 '18 at 14:45
1

@Chris_Rands except OP has made a mistake, for Rare it should be `x>=20` and if you use that in your code you'll see only the last true condition will be returned. So your code will fail for `x=25` – anishtain4 Jun 01 '18 at 14:54
@anishtain4 We can only guess what the question is about but assuming a single output for a single integer input (also assumed in Ivan's code), it could be done in a dict like I showed above – Chris_Rands Jun 01 '18 at 15:02
1

@Chris_Rands yes, this is a very poorly written question. However it's clear that OP is looking for a short form of switch. I don't see any reason for a switch command generally, `if ... elif` does the same job. – anishtain4 Jun 01 '18 at 15:19

Robᵩ · Answer 3 · 2018-06-01T14:54:16.867

I can't see a way to do this with better than O(k) performance, where k is the number of keys in your sort-of dict.

If you are not seeking dict's O(1) performance, and just want a dict-like syntax, you can implement a mapping object yourself, like so:

from collections.abc import Mapping

class CallDict(Mapping):
    def __init__(self, *pairs):
        self._pairs = pairs
    def __iter__(self):
        return iter(())
    def __len__(self):
        return len(self._pairs)
    def __getitem__(self, x):
        for func, value in self._pairs:
            if func(x):
                return value
        raise KeyError("{} satisfies no condition".format(x))

# Conditions copied directly from OP, but probably wrong.
cd = CallDict(
    ((lambda x: x >= 75), "Common"),
    ((lambda x: x <= 20), "Rare"),
    ((lambda x: x >= 5), "Legendary"),
)


assert cd[1] == 'Rare'
assert cd[10] == 'Rare'
assert cd[50] == 'Legendary'
assert cd[100] == 'Common'

Note that I don't advocate creating a `Mapping` object for this particular case. For such a simple case, I'd prefer to see `rarity = HowRareIsIt(10)`. But knowing that one *can* create a mapping object may be useful for other, similar, cases. — Robᵩ, Jun 01 '18 at 14:59

Python dict keys as statements

3 Answers3