How can I access global variable inside class in Python

By declaring it global inside the function that accesses it:

g_c = 0

class TestClass():
    def run(self):
        global g_c
        for i in range(10):
            g_c = 1
            print(g_c)

The Python documentation says this, about the global statement:

The global statement is a declaration which holds for the entire current code block.