Python globals and keyword arguments

Question

Why does test2() below print "True False"? I would expect "False False".

I expect test2() to change the global value EC to be False, so ec should also be False.

Why not?

Is there a straightforward way to get the "False False" behavior?

EC = True

def test1(ec=EC):
    print(ec, EC)

def test2():

    global EC
    EC=False

    test1()

question from:https://stackoverflow.com/questions/65837698/python-globals-and-keyword-arguments

Answer 1

First, that's a default value, not a keyword argument. If you want to pass keyword arguments, that would look like this:

def test1(ec):
    print(ec)

test1(ec=True)

Second, unlike most languages with default argument values, Python evaluates default values at function definition time, not function call time. This is an extremely unusual design decision that causes a lot of problems. The typical workaround is to use a sentinel value like None as the default, and compute the "real" default inside the function if the sentinel is detected:

EC = True

def test1(ec=None):
    if ec is None:
        ec = EC
    print(ec, EC)

def test2():

    global EC
    EC=False

    test1()