Question
Answer and Explanation
In Python, declaring variables that are strictly mutually exclusive, meaning only one can hold a value at a time, isn't a built-in language feature. However, you can achieve this behavior programmatically using several methods. Here are a few effective approaches:
1. Using a Class with Instance Control:
- Create a class to encapsulate the mutually exclusive variables. Implement logic within the class to ensure that only one variable can hold a non-None value at any time. This is a flexible and object-oriented way to enforce this behavior.
Example:
class MutexVars:
def __init__(self):
self._var1 = None
self._var2 = None
def set_var1(self, value):
if self._var2 is not None:
raise ValueError("Cannot set var1 when var2 is set.")
self._var1 = value
def set_var2(self, value):
if self._var1 is not None:
raise ValueError("Cannot set var2 when var1 is set.")
self._var2 = value
def get_var1(self):
return self._var1
def get_var2(self):
return self._var2
def clear_vars(self):
self._var1 = None
self._var2 = None
You can then use this like:
mutex = MutexVars()
mutex.set_var1(10)
print(mutex.get_var1()) # Output: 10
try:
mutex.set_var2(20)
except ValueError as e:
print(e) # Output: Cannot set var2 when var1 is set.
mutex.clear_vars()
mutex.set_var2(20)
print(mutex.get_var2()) #Output: 20
2. Using a Dictionary or Similar Structure:
- You can use a dictionary to represent the variables, where only one key can have a non-None value, and check before assigning any other values.
Example :
def MutexVarsDict():
state = { 'var1':None, 'var2':None}
def set_var(name, value):
other = 'var2' if name == 'var1' else 'var1'
if state[other] is not None:
raise ValueError(f"Cannot set {name} when {other} is set.")
state[name] = value
def clear_vars():
state['var1'] = None
state['var2'] = None
def get_var(name):
return state[name]
return set_var, clear_vars, get_var
set_var,clear_vars, get_var = MutexVarsDict()
set_var('var1',10)
print(get_var('var1')) # Output: 10
try:
set_var('var2', 20)
except ValueError as e:
print(e) # Output: Cannot set var2 when var1 is set.
clear_vars()
set_var('var2',20)
print(get_var('var2')) #Output: 20
3. Using Flags or Boolean Variables:
- For a simpler case, particularly with just two mutually exclusive states, you can use a boolean flag. When one variable is set, the other flag is implicitly turned off.
Example:
var1_set = False
var2_set = False
value_var1 = None
value_var2 = None
def set_var1(value):
global var1_set, var2_set, value_var1, value_var2
if var2_set:
raise ValueError("Cannot set var1 when var2 is set")
var1_set = True
value_var1 = value
def set_var2(value):
global var1_set, var2_set, value_var1, value_var2
if var1_set:
raise ValueError("Cannot set var2 when var1 is set")
var2_set = True
value_var2 = value
def clear_vars():
global var1_set, var2_set, value_var1, value_var2
var1_set = False
var2_set = False
value_var1 = None
value_var2 = None
def get_var1():
global value_var1
return value_var1
def get_var2():
global value_var2
return value_var2
set_var1(10)
print(get_var1()) # Output: 10
try:
set_var2(20)
except ValueError as e:
print(e) # Output: Cannot set var2 when var1 is set.
clear_vars()
set_var2(20)
print(get_var2())# Output: 20
Choosing the right approach depends on your specific needs. The class-based solution is robust for complex cases, while flags or boolean values can simplify things when you only have two variables and simple state control is needed. Using a dictionary is somewhere in the middle.