Python 默认行为
Python defaultdict behaviour
我 运行 今天有些行为让我有点吃惊。
from collections import defaultdict
d = defaultdict(lambda: "test")
现在 d[0]returns "test" 符合预期，但 d.get(0) 实际上 returns None。这是预期的行为吗？
是的，这是预期的，documented 行为（强调原文）：
Note that __missing__() is not called for any operations besides __getitem__(). This means that get() will, like normal dictionaries, return None as a default rather than using default_factory.
是的，这是预期的。除了 mydict[x]、 所有其他方法与常规 dict 完全相同。来自 documentation:
Return a new dictionary-like object. defaultdict is a subclass of the
built-in dict class. It overrides one method and adds one writable
instance variable. The remaining functionality is the same as for the
dict class and is not documented here.
它覆盖的方法是__missing__，进一步阐述：
This method is called by the __getitem__() method of the dict class
when the requested key is not found; whatever it returns or raises is
then returned or raised by __getitem__().
Note that __missing__() is not called for any operations besides
__getitem__(). This means that get() will, like normal dictionaries, return None as a default rather than using default_factory.
添加了重点。
我 运行 今天有些行为让我有点吃惊。
from collections import defaultdict
d = defaultdict(lambda: "test")
现在 d[0]returns "test" 符合预期，但 d.get(0) 实际上 returns None。这是预期的行为吗？
是的，这是预期的，documented 行为（强调原文）：
Note that
__missing__()is not called for any operations besides__getitem__(). This means thatget()will, like normal dictionaries, returnNoneas a default rather than usingdefault_factory.
是的，这是预期的。除了 mydict[x]、 所有其他方法与常规 dict 完全相同。来自 documentation:
Return a new dictionary-like object. defaultdict is a subclass of the built-in dict class. It overrides one method and adds one writable instance variable. The remaining functionality is the same as for the dict class and is not documented here.
它覆盖的方法是__missing__，进一步阐述：
This method is called by the
__getitem__()method of the dict class when the requested key is not found; whatever it returns or raises is then returned or raised by__getitem__().Note that
__missing__()is not called for any operations besides__getitem__(). This means thatget()will, like normal dictionaries, returnNoneas a default rather than using default_factory.
添加了重点。