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It's cumbersome and burdensome because you need for every nonstandard default value:
1) define a class with __repr__();
2) instantiate a sentinel;
3) check for the sentinel in the function and replace it but an actual value.
 class _ExternalAttrDefault:
 def __repr__():
 return '(stat.S_IRUSR|stat.S_IRUSR)<<16'
 _external_attr_default = _ExternalAttrDefault()
 def open(self, name, mode='r', external_attr=_external_attr_default):
 if external_attr is _external_attr_default:
 external_attr = (stat.S_IRUSR|stat.S_IRUSR)<<16
 ...
Instead of just:
 def open(self, name, mode='r', external_attr=(stat.S_IRUSR|stat.S_IRUSR)<<16):
 """
 Foo.open(name, mode='r', external_attr=(stat.S_IRUSR|stat.S_IRUSR)<<16)
 """
 ...