Question

基本上，如果我设置defaultdict并按键引用它，它将正常工作并返回我设置为默认值。但是，如果我在其上使用.get()，则不会返回默认值。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，我正在使用dicts和我的代码，我最初遇到的问题看起来有点像这样

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，没有。这是按设计，否则会破坏该方法的目的。这同样适用于会员资格测试：

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

如果您需要为缺失密钥调用默认工厂，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，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，我锁定了.get()次来电;如果缺少第一个键，则第一个.get()返回一个空字典。

Answer 2

我最近遇到了这个问题，基本上必须做这样的事情：

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正确返回默认值，除非首先引用该键

2 个答案: