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Jakebut
Jakebut

Reputation: 77

Python - allowed values of a variable

How can I explicitly define what values can given variable have? Let's say I want value of variable size to be either 'small', 'medium', or 'big' and nothing else.

EDIT: I want to avoid a situation when variable is set to something from beyond the list (for example to 'tiny' in this case). Like enum in Java. This variable would be a class field.

Upvotes: 1

Views: 8606

Answers (3)

Galuoises
Galuoises

Reputation: 3283

You could define a Size class inside your class and then set the size attribute while checking whether the value is allowed. Below a minimal example:

from enum import Enum

class Trial:
    class Size(Enum):
        small = "small"
        medium = "medium"
        large = "large"

    def __init__(self, size):
        self._set_size(size)
 
    def _set_size(self, size):
        if size in set(item.value for item in self.Size):
            self._size = size
        else:
            raise ValueError("size value not valid")

Upvotes: 3

Craig Burgler
Craig Burgler

Reputation: 1779

You are describing an enumeration, which is supported in Python by the enum library:

from enum import Enum

class Size(Enum):
    small = 'small'
    medium = 'medium'
    big = 'big'

size = Size('big')
print(size)
try:
    size = Size('tiny')
except ValueError as e:
    print("invalid Size (", e.args[0].split()[0],
          "). Size must be one of 'small', 'medium' or 'big'", sep='')

Output:

Size.big
invalid Size ('tiny'). Size must be one of 'small', 'medium' or 'big'

Upvotes: 6

Yakuza
Yakuza

Reputation: 3882

Simplest way would be to always use dedicated method which would firstly validate input and if it's correct then set variable. Below you may find some example:

class Test:
   def __init__(self):
      self.__variable = None

   def set_variable(self, value):
      if value not in ('small', 'medium', 'big'):
         raise ValueError()
      self.__variable = value

Upvotes: 6

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