Run code only for the first time a function is called

Question

firstCall = False

def buttonDown():
    if firstCall == True:
        print("Initial Call")
        firstCall = False
    else:
        print("Subsequent Call")


def buttonUp():
    firstCall = True
    
buttonDown()
buttonDown()
buttonDown()

I think this code translated would run in C#, but in Python reassigning firstCall to False to stop "Initial Call" from printing again results in an error UnboundLocalError: local variable 'firstCall' referenced before assignment

How should this logic be written?

I expected the output to be:

Initial Call Subsequent Call Subsequent Call

Instead I get an compile error. Commenting out the line that causes the error, firstCall = False, results in, of course:

Initial Call Initial Call Initial Call