Python 3 中的字典理解
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Dictionary Comprehension in Python 3
提问by Jon
I found the following stack overflow post about dict comprehensions in Python2.7and Python 3+: Create a dictionary with list comprehension in Pythonstating that I can apply dictionary comprehensions like this:
我发现了以下关于 dict 推导式 in Python2.7and 的堆栈溢出帖子Python 3+：在 Python 中创建一个带有列表推导式的字典，说明我可以像这样应用字典推导式：
d = {key: value for (key, value) in sequence}
I tried it in Python 3. However, it raises an exception.
我在 Python 3 中尝试过。但是，它引发了一个异常。
d = {'a':1, 'b':2, 'c':3, 'd':4}
{key : value for (key, value) in d}
{key : value for key, value in d}
Both versions raise a ValueErrorsaying that ValueError: need more than 1 value to unpack.
两个版本都提出了ValueError这样的说法ValueError: need more than 1 value to unpack。
What is the easiest / the most direct way to make a dictionary comprehension in Python3?
在 Python3 中进行字典理解的最简单/最直接的方法是什么？
采纳答案by Martijn Pieters
Looping over a dictionary only yields the keys. Use d.items()to loop over both keys and values:
循环字典只会产生键。使用d.items()循环遍历键和值：
{key: value for key, value in d.items()}
The ValueErrorexception you see is nota dict comprehension problem, nor is it limited to Python 3; you'd see the same problem in Python 2 or with a regular forloop:
ValueError您看到的异常不是字典理解问题，也不限于 Python 3；您会在 Python 2 或常规for循环中看到相同的问题：
>>> d = {'a':1, 'b':2, 'c':3, 'd':4}
>>> for key, value in d:
... print key, value
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: need more than 1 value to unpack
because each iteration there is only oneitem being yielded.
因为每次迭代只产生一个项目。
Without a transformation, {k: v for k, v in d.items()}is just a verbose and costly d.copy(); use a dict comprehension only when you do a little more with the keys or values, or use conditions or a more complex loop construct.
没有转换，{k: v for k, v in d.items()}只是冗长而昂贵d.copy()；仅当您对键或值执行更多操作，或者使用条件或更复杂的循环结构时才使用字典理解。
回答by RandallShanePhD
Well said above - you can drop items in Python3 if you do it this way:
上面说得好 - 如果你这样做，你可以在 Python3 中删除项目：
{key: d[key] for key in d}
{key: d[key] 用于输入 d}
d = {'a':1, 'b':2, 'c':3, 'd':4}
z = {x: d[x] for x in d}
z
>>>{'a': 1, 'b': 2, 'c': 3, 'd': 4}
and this provides for the ability to use conditions as well
这也提供了使用条件的能力
y = {x: d[x] for x in d if d[x] > 1}
y
>>>{'b': 2, 'c': 3, 'd': 4}
Enjoy!
享受！
回答by Soumyaranjan Das
Dictionary comprehension means generating items in the dictionary by some logic:
字典理解是指通过某种逻辑在字典中生成项目：
x = {p: p*p for p in range(10)}
print(x)
y = {q: q*3 for q in range(5,15) if q%2!=0}
print(y)
