创建列表或将列表追加到列表(如果已经存在)的有效方法? [英] Efficient way to either create a list, or append to it if one already exists?

问题描述

我正在遍历一堆具有多对多相关性的元组，并且我想制作一个字典，其中(a，b)的每个b都有一个对应于b的所有a的列表. .在元组摘要循环中，每次都在字典的键b处测试列表，然后寻找a，然后附加a(如果还不存在)似乎很尴尬；但我还没有找到更好的方法.是否存在?还有其他更漂亮的方法吗?

I'm going through a whole bunch of tuples with a many-to-many correlation, and I want to make a dictionary where each b of (a,b) has a list of all the a's that correspond to a b. It seems awkward to test for a list at key b in the dictionary, then look for an a, then append a if it's not already there, every single time through the tuple digesting loop; but I haven't found a better way yet. Does one exist? Is there some other way to do this that's a lot prettier?

推荐答案

请参见 setdefault()方法的文档:

See the docs for the setdefault() method:

setdefault(key [，默认])
如果键是 在字典中，返回其值. 如果不是，请插入值为 默认值并返回默认值.默认 默认为无.

setdefault(key[, default])
If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.

您可以将其用作单个调用，如果存在b，则将得到b；如果尚不存在，则将b设置为空列表-无论哪种方式，都返回b:

You can use this as a single call that will get b if it exists, or set b to an empty list if it doesn't already exist - and either way, return b:

>>> key = 'b'
>>> val = 'a'
>>> print d
{}
>>> d.setdefault(key, []).append(val)
>>> print d
{'b': ['a']}
>>> d.setdefault(key, []).append('zee')
>>> print d
{'b': ['a', 'zee']}

通过一个简单的"not in"检查将其合并，您已经完成了三行的工作:

Combine this with a simple "not in" check and you've done what you're after in three lines:

>>> b = d.setdefault('b', [])
>>> if val not in b:
...   b.append(val)
... 
>>> print d
{'b': ['a', 'zee', 'c']}

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