defaults_demo.py

class ClassWithDefaults(object):
    def __init__(self, arg1, arg2, default1=False, default2=False):
        (self.arg1, self.arg2,
         self.default1, self.default2) = (arg1, arg2,
                                          default1, default2)

    def __repr__(self):
        return('{}(arg1={}, arg2={}, default1={}, default2={})'.format(
            self.__class__.__name__,
            self.arg1, self.arg2,
            self.default1, self.default2
        ))


def set_default_flag():
    defaults = list(ClassWithDefaults.__init__.__defaults__)
    print(defaults)
    defaults = (defaults[:1] + [True] + defaults[2:])
    print(defaults)
    try:
        ClassWithDefaults.__init__.__defaults__ = tuple(defaults)
    except:
        ClassWithDefaults.__init__.__func__.__defaults__ = tuple(defaults)

if __name__ == '__main__':
    x = ClassWithDefaults('arg1', 'arg2')
    print(x)
    set_default_flag()
    x = ClassWithDefaults('arg1', 'arg2')
    print(x)

script.out

class ClassWithDefaults(object):
    def __init__(self, arg1, arg2, default1=False, default2=False):
        (self.arg1, self.arg2,
         self.default1, self.default2) = (arg1, arg2,
                                          default1, default2)

    def __repr__(self):
        return('{}(arg1={}, arg2={}, default1={}, default2={})'.format(
            self.__class__.__name__,
            self.arg1, self.arg2,
            self.default1, self.default2
        ))


def set_default_flag():
    defaults = list(ClassWithDefaults.__init__.__defaults__)
    print(defaults)
    defaults = (defaults[:1] + [True] + defaults[2:])
    print(defaults)
    try:
        ClassWithDefaults.__init__.__defaults__ = tuple(defaults)
    except:
        ClassWithDefaults.__init__.__func__.__defaults__ = tuple(defaults)

if __name__ == '__main__':
    x = ClassWithDefaults('arg1', 'arg2')
    print(x)
    set_default_flag()
    x = ClassWithDefaults('arg1', 'arg2')
    print(x)
Interpreted:
====================
ClassWithDefaults(arg1=arg1, arg2=arg2, default1=False, default2=False)
[False, False]
[False, True]
ClassWithDefaults(arg1=arg1, arg2=arg2, default1=False, default2=True)
====================
Cythonized:
====================
ClassWithDefaults(arg1=arg1, arg2=arg2, default1=False, default2=False)
[False, False]
[False, True]
ClassWithDefaults(arg1=arg1, arg2=arg2, default1=False, default2=False)
====================

script.sh

cat ./defaults_demo.py

echo Interpreted:
echo '===================='
python defaults_demo.py
echo '===================='
cython --embed -o defaults_demo.c defaults_demo.py
gcc -Os -I /usr/include/python2.7 -o defaults_demo defaults_demo.c -lpython2.7 -lpthread -lm -lutil -ldl

echo Cythonized:
echo '===================='
./defaults_demo
echo '===================='