Python 的 collections.defaultdict returns 默认值是正确的，除非没有首先引用该键

Question

基本上，如果我设置 defaultdict 并按键引用它，它将正常工作，return 我设置为默认值。但是，如果我在其上使用 .get()，则默认值不是 returned。这是我能给出的最简单的例子。

basedict = defaultdict(dict)
assert(basedict['junk'] == {}) # Pass
assert(basedict.get('junk') == {}) # Pass
assert(basedict.get('popcorn') == {}) # Fail

为了完成，我正在处理一个个的字典，我最初遇到问题的代码看起来更像这样

from collections import defaultdict

basedict = defaultdict(dict)
assert(basedict['junk'] == {})

basedict[69] = defaultdict(lambda: 1000)
assert(basedict[69]['junk'] == 1000)
assert(basedict[69].get('junk') == 1000)
assert(basedict[69].get('junk', 420) == 1000)

# The above works fine, but if I call .get on a default dict using a key I've
# never referenced before it returns None or the .get supplied default value
assert(basedict[69].get('garbage') == 1000) # Returns None
assert(basedict[69].get('garbage', 420) == 1000) # Returns 420
assert(basedict[69].get('dumpster', 420) == 1000) # Returns 420

# But if I place a an assert before the calling .get(garbage) that 
# checks basedict[69]['garbage'] then the asserts after work until I get 
# to 'dumpster' at which point it fails again
# It also fails if I set the defaultdict to something other than a lambda
basedict[7] = defaultdict(dict)
assert(basedict[7]['taco'] == {}) # Pass
assert(basedict[7].get('taco') == {}) # Pass
assert(basedict[7].get('burrito') == {}) # Fail

Answer 1

defaultdict.get() 不填充密钥，不。这是设计，否则会破坏该方法的目的。这同样适用于会员测试：

>>> from collections import defaultdict
>>> d = defaultdict(dict)
>>> 'foo' in d
False
>>> d['foo']
{}
>>> 'foo' in d
True

如果您需要为丢失的密钥调用默认工厂，请使用 defaultdict.__getitem__（例如 defaultdict_instance[key]）。

如果您需要设置默认值其他而不是工厂提供的值，请使用dict.setdefault():

>>> d = defaultdict(dict)
>>> d.setdefault('bar', 42)
42
>>> d.setdefault('bar', 30)
42
>>> d
defaultdict(<type 'dict'>, {'bar': 42})

如果您只需要获取默认值，请使用 dict.get():

>>> d = defaultdict(dict)
>>> d.get('bar', {}).get('baz', 1000)
1000

请注意，我链接了 .get() 调用；如果缺少第一个键，第一个 .get() returns 一个空字典。

Answer 2

我最近运行遇到了这个问题，基本上不得不做这样的事情：

from collections import defaultdict

class BetterDict(defaultdict):
    def get(self, item, default=None):
        # default is not really used, but leaving here for compatibility
        # This will either return the actual value, or the defaultdict's
        # default, as generated by default_factory
        return self.__getitem__(item)

现在：

basedict = BetterDict(dict)
assert(basedict['junk'] == {}) # Pass
assert(basedict.get('junk') == {}) # Pass
assert(basedict.get('popcorn') == {}) # Pass

Python 的 collections.defaultdict returns 默认值是正确的，除非没有首先引用该键

Python's collections.defaultdict returns default value correctly unless the key isn't referenced first

python

collections

dictionary

python-3.x