Python function global variables?

Question

I know I should avoid using global variables in the first place due to confusion like this, but if I were to use them, is the following a valid way to go about using them? (

Answer 1

Here is one case that caught me out, using a global as a default value of a parameter.

globVar = None    # initialize value of global variable

def func(param = globVar):   # use globVar as default value for param
    print 'param =', param, 'globVar =', globVar  # display values

def test():
    global globVar
    globVar = 42  # change value of global
    func()

test()
=========
output: param = None, globVar = 42

I had expected param to have a value of 42. Surprise. Python 2.7 evaluated the value of globVar when it first parsed the function func. Changing the value of globVar did not affect the default value assigned to param. Delaying the evaluation, as in the following, worked as I needed it to.

def func(param = eval('globVar')):       # this seems to work
    print 'param =', param, 'globVar =', globVar  # display values

Or, if you want to be safe,

def func(param = None)):
    if param == None:
        param = globVar
    print 'param =', param, 'globVar =', globVar  # display values