python - Python如何区分显式传递的None作为内置参数

Question

我试验了下一段代码:

>>> f = object()

# It's obvious behavior:
>>> f.foo
Traceback (most recent call last):       
  File "<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute 'foo'

# However, the next one is surprising me!
>>> getattr(f, 'foo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute 'foo'

# And this one returns None as expected:
>>> getattr(f, 'foo', None)

然后我在 PyCharm IDE 中找到了 getattr() 的伪签名:

def getattr(object, name, default=None): # known special case of getattr
    """
    getattr(object, name[, default]) -> value

    Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y.
    When a default argument is given, it is returned when the attribute doesn't
    exist; without it, an exception is raised in that case.
    """
    pass

我的问题是 python 如何区分这两种在内部使用 getattr()(可能还有其他函数)的场景？是否有可能完全在客户端代码中执行类似的操作？

Answer 1

正如@scytale 所说，getattr 的伪签名与其实现并不完全对应。我已经看到尝试在纯 Python 中复制行为，看起来像这样:

class MyObject(object):
    __marker = object()

    def getvalue(key, default=__marker):
        ...
        if key is __marker:
             # no value supplied for default
             ....

换句话说，使用调用者无法轻易提供的标记值来检查是否没有给出默认值而不是 None。