您不能真正将其定义为默认值，因为默认值是在定义方法（在任何实例存在之前）时评估的。通常的模式是改为执行以下操作：
class C:
def __init__(self, format):
self.format = format
def process(self, formatting=None):
if formatting is None:
formatting = self.format
print(formatting)
self.format仅当formatting是时使用None。
为了说明默认值的工作原理，请参见以下示例：
def mk_default():
print("mk_default has been called!")
def myfun(foo=mk_default()):
print("myfun has been called.")
print("about to test functions")
myfun("testing")
myfun("testing again")
mk_default has been called!
about to test functions
myfun has been called.
myfun has been called.
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