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.
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
size
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'
size = 'tiny'?
str. Given Python's duck typing you can set any variable to anything, so there is no formulation that would prevent such an assignment.How can I explicitly define what values can [a] given variable have?. While enums are relevant, it doesn't seem like you can restrict in Python a general variable to have a limited set of values. Class fields are another matter.enum library. Even if you make size an instance of a class that restricts instance variable values (which is actually what you are doing by using enum, just doing it Pythonically), you can again just do size = 'tiny' and shed size's enumityYou 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")
sizevariable would be type string, unlesssmall,medium, andlargeare variables that are assigned to integers. Please clarify your question though, because as of now, it is very ambiguous .