[英]Using a loop to .setdefault on Dict Creates Nested Dict
I'm trying to understand why 我想知道为什么
tree = {}
def add_to_tree(root, value_string):
"""Given a string of characters `value_string`, create or update a
series of dictionaries where the value at each level is a dictionary of
the characters that have been seen following the current character.
"""
for character in value_string:
root = root.setdefault(character, {})
add_to_tree(tree, 'abc')
creates {'a': {'b': {'c': {}}}}
创建
{'a': {'b': {'c': {}}}}
while 而
root = {}
root.setdefault('a', {})
root.setdefault('b', {})
root.setdefault('c', {})
creates {'a': {}, 'b': {}, 'c': {}}
创建
{'a': {}, 'b': {}, 'c': {}}
What is putting us into the assigned dict value on each iteration of the loop? 是什么让我们在循环的每次迭代中进入指定的dict值?
root.setdefault(character, {})
returns root[character]
if character
is a key in root
or it returns the empty dict {}
. root.setdefault(character, {})
返回root[character]
如果character
是root
的键,或者返回空dict {}
。 It is the same as root.get(character, {})
except that it also assigns root[character] = {}
if character
is not already a key in root
. 它与
root.get(character, {})
相同,只是如果character
不是root
的键,它也会指定root[character] = {}
。
root = root.setdefault(character, {})
reassigns root
to a new dict if character
is not already a key in the original root
. 重新分配
root
到一个新的字典,如果character
是不是已经在原来的一个重要root
。
In [4]: root = dict()
In [5]: newroot = root.setdefault('a', {})
In [6]: root
Out[6]: {'a': {}}
In [7]: newroot
Out[7]: {}
In contrast, using root.setdefault('a', {})
without reassigning its return value to root
works: 相反,使用
root.setdefault('a', {})
而不将其返回值重新分配给root
工作:
tree = {}
def add_to_tree(root, value_string):
"""Given a string of characters `value_string`, create or update a
series of dictionaries where the value at each level is a dictionary of
the characters that have been seen following the current character.
"""
for character in value_string:
root.setdefault(character, {})
add_to_tree(tree, 'abc')
print(tree)
# {'a': {}, 'c': {}, 'b': {}}
For anyone else who is as slow as me. 对于任何和我一样慢的人。 The answer to, "Why does the (above) function produce
{'a': {'b': {'c': {}, 'd': {}}}}
and not {'a': {}, 'b': {}, 'c': {}}
?" 答案为“为什么(上面)函数产生
{'a': {'b': {'c': {}, 'd': {}}}}
而不是{'a': {}, 'b': {}, 'c': {}}
?“ is: 是:
Because we're looping within a function and reassigning the the result to root each time, it's kind of like in the TV infomercials where they keep saying, “but wait! 因为我们在一个函数中循环并且每次都将结果重新分配给root,这有点像在他们一直说的电视广告中,“但是等等! there's more!”.
还有更多!”。 So when .setdefault gets called on 'a', before that gets returned, it's result,
{'a': {}}
is held {inside the loop} while it's run on 'b', which yields {'b': {}}
, within {'a': {}}
and that is held to the side and {'a': {}}
is run, then the whole thing is returned from the loop and applied to tree. 所以当.setdefault在'a'上被调用时,在返回之前,结果是
{'a': {}}
被保持在{循环内},而它在'b'上运行,产生{'b': {}}
,在{'a': {}}
,并且保持在侧面并运行{'a': {}}
,然后整个事物从循环返回并应用于树。 Note that each time, what is actually returned by .setdefault IS the default, which in this case is {}
. 请注意,每次,.setdefault实际返回的内容都是默认值,在本例中为
{}
。 Here is a Python Visualizer illustration of the process. 这是一个Python Visualizer插图的过程。
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