Python 中复制 dictionary 的快速方法

我有一个 Python 程序，它经常处理字典。我得把字典复印几千遍。我需要钥匙和相关内容的副本。该副本将会被编辑，并且不能链接到原件(例如，副本的更改不能影响原件)

键是字符串，值是整数(0/1)。

我目前使用一种简单的方法:

newDict = oldDict.copy()

对代码进行分析表明，复制操作占用了大部分时间。

有没有比 dict.copy()方法更快的替代方法? 什么是最快的？

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Can you provide a code sample so I can see how you are using copy() and in what context?

You could use 

new = dict(old)

But I dont think it will be faster.

小开

Depending on things you leave to speculation, you may want to wrap the original dictionary and do a sort of copy-on-write.

The "copy" is then a dictionary which looks up stuff in the "parent" dictionary, if it doesn't already contain the key --- but stuffs modifications in itself.

This assumes that you won't be modifying the original and that the extra lookups don't end up costing more.

小开

Appearantly dict.copy is faster, as you say.

[utdmr@utdmr-arch ~]$ python -m timeit -s "d={1:1, 2:2, 3:3}" "new = d.copy()"
1000000 loops, best of 3: 0.238 usec per loop
[utdmr@utdmr-arch ~]$ python -m timeit -s "d={1:1, 2:2, 3:3}" "new = dict(d)"
1000000 loops, best of 3: 0.621 usec per loop
[utdmr@utdmr-arch ~]$ python -m timeit -s "from copy import copy; d={1:1, 2:2, 3:3}" "new = copy(d)"
1000000 loops, best of 3: 1.58 usec per loop
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最佳答案

Looking at the C source for the Python dict operations, you can see that they do a pretty naive (but efficient) copy. It essentially boils down to a call to PyDict_Merge:

PyDict_Merge(PyObject *a, PyObject *b, int override)

This does the quick checks for things like if they're the same object and if they've got objects in them. After that it does a generous one-time resize/alloc to the target dict and then copies the elements one by one. I don't see you getting much faster than the built-in copy().

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The measurments are dependent on the dictionary size though. For 10000 entries copy(d) and d.copy() are almost the same.

a = {b: b for b in range(10000)}
In [5]: %timeit copy(a)
10000 loops, best of 3: 186 µs per loop
In [6]: %timeit deepcopy(a)
100 loops, best of 3: 14.1 ms per loop
In [7]: %timeit a.copy()
1000 loops, best of 3: 180 µs per loop
小开

I realise this is an old thread, but this is a high result in search engines for "dict copy python", and the top result for "dict copy performance", and I believe this is relevant.

From Python 3.7, newDict = oldDict.copy() is up to 5.5x faster than it was previously. Notably, right now, newDict = dict(oldDict) does not seem to have this performance increase.

There is a little more information here.


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