如何检查 Python 是否已分配变量

Question

I would like to check, whether a variable has been assigned or not. Based on the answers in this question Referring to the null object in Python , I tried it with if outputVariable==None but this does not work and throws the error "UnboundLocalError: local variable 'outputVariable' referenced before assignment".

I have the following code:

def testMethod(assignVariable):
    if assignVariable == True:
        outputVariable = 5
        
    if outputVariable==None:
        outputVariable = 0
 
    return outputVariable


returnValue = testMethod(False)

Any idea how I can check before the return statement, if outputVariable has a value. If it does not, it should be given a value of 0

Update: Based on the accepted answer here How do I check if a variable exists? , I tried the following but it was also not successfull:

def testMethod(assignVariable):
    if assignVariable == True:
        outputVariable = 5
        
    if outputVariable in locals():
        pass
    else:
        outputVariable = 0       

    return outputVariable


returnValue = testMethod(False)

Answer 1

This question has already been anwered here .

If the variable has no value, it is non-existent because you cannot declare a variable without any sort of initialization.

Answer 2

Here is a simple way to help you:

def tryVar(var):
    try:
        val = var
    except NameError:
        return None
    return val

In your code, just use tryVar(--) wherever you want to get a variable, you can check if said variable exists by simply:

if tryVar(X) is None:
   >>>DO WHATEVER

Solution for python 3.8+ if you want to assign variable, and if it doesnt exist assign 0 to it in one line:

Z = x if tryVar(x) is None else x:=0

this assigns the variable if its not assigned, and returns its value, additionally if you don't want to do anything with the values you can also:

x if tryVar(x) is None else x:=0

Answer 3

You can't check for a variable that is not defined . Even None is a well-defined value for an object; it's a good default value in many scenarios.

This is the idiomatic way to achieve what you want:

def testMethod(assignVariable):

    return assignVariable or 0

returnValue = testMethod(False)

You can avoid that intermediate variable as it is not adding any value except being a placeholder; function is optional if it's for one-time use. Python takes care of all these checks internally. There is a ternary operator also if you need one.