问题描述
我没有软件/计算机科学背景，但我喜欢用 Python 编写代码，并且通常可以理解为什么事情会更快.我真的很想知道为什么这个 for 循环比字典理解运行得更快.任何见解?
问题:给定一个带有这些键和值的字典a，返回一个以值作为键和键作为值的字典.(挑战:在一行中完成)
和代码
a = {'a':'hi','b':'hey','c':'yo'}b = {}对于 a.items() 中的 i,j:b[j]=i%% timeit 932 ns ± 37.2 ns 每个循环b = {v: k for k, v in a.items()}%% timeit 1.08 µs ± 16.4 ns 每个循环您正在使用太小的输入进行测试；虽然与列表推导相比，字典推导相对于 for 循环没有那么多的性能优势，但对于实际问题的大小，它可以并且确实击败了 for 循环，尤其是在定位到全局名称时.
您的输入仅包含 3 个键值对.改为使用 1000 个元素进行测试，我们看到时间非常接近:
>>>导入时间>>>来自随机导入选择，randint；从字符串导入 ascii_lowercase 作为字母>>>循环 = '''\... b = {}...对于 a.items() 中的 i,j:... b[j]=i...'''>>>dictcomp = '''b = {v: k for k, v in a.items()}'''>>>def rs(): return ''.join([choice(letters) for _ in range(randint(3, 15))])...>>>a = {rs(): rs() for _ in range(1000)}>>>连(一)1000>>>count, total = timeit.Timer(looped, 'from __main__ import a').autorange()>>>(total/count) * 1000000 # 每次运行微秒66.62004760000855>>>count, total = timeit.Timer(dictcomp, 'from __main__ import a').autorange()>>>(total/count) * 1000000 # 每次运行微秒64.5464928005822区别就在那里，dict comp 速度更快，但仅 只是 在这种规模下.使用 100 倍的键值对，差异会更大一些:
>>>a = {rs(): rs() for _ in range(100000)}>>>连(一)98476>>>count, total = timeit.Timer(looped, 'from __main__ import a').autorange()>>>total/count * 1000 #毫秒，不同的规模！15.48140200029593>>>count, total = timeit.Timer(dictcomp, 'from __main__ import a').autorange()>>>total/count * 1000 #毫秒，不同的规模！13.674790799996117当您考虑处理近 10 万个键值对时，这不是那 大区别.尽管如此，for 循环显然慢.
那么为什么 3 个元素的速度差异呢?因为推导式(字典、集合、列表推导式或生成器表达式)在底层实现为一个新的函数，并且调用该函数有一个基本成本，普通循环不必支付.
这是两种替代方案的字节码的反汇编；注意字典理解的顶级字节码中的 MAKE_FUNCTION 和 CALL_FUNCTION 操作码，有一个单独的部分说明该函数的作用，实际上几乎没有区别在这两种方法之间:
实质区别:循环代码使用LOAD_NAME for b 每次迭代，STORE_SUBSCR 将键值对存储在dict加载中.字典理解使用MAP_ADD 来实现与STORE_SUBSCR 相同的功能，但不必每次都加载那个b 名称.
但是只有3 次迭代，字典理解必须执行的MAKE_FUNCTION/CALL_FUNCTION 组合才是对性能的真正拖累:
创建一个带一个参数的函数对象并调用它需要超过 0.1 μs(对于我们传入的 None 值，使用额外的 LOAD_CONST )！这就是 3 个键值对的循环和理解时间之间的差异.
你可以将这比作惊讶的是，一个拿着铲子的人挖一个小洞的速度比反铲挖得快.反铲当然可以快速挖掘，但是如果您需要先启动反铲并移动到位，则有铲子的人可以更快地开始！
除了几个键值对(挖一个更大的洞)之外，函数 create 和 call 成本逐渐消失.在这一点上，字典理解和显式循环基本上做同样的事情:
- 取下一个键值对，将它们弹出堆栈
- 通过字节码操作调用
dict.__setitem__钩子，使用栈顶的两个项目(STORE_SUBSCR或MAP_ADD.这不会不能算作函数调用"，因为它都是在解释器循环中内部处理的.
这与列表推导不同，其中普通循环版本必须使用 list.append()，涉及属性查找和函数调用每次循环迭代.列表理解速度优势来自于这种差异；参见 Python 列表理解成本高
dict comprehension 增加的是，当将 b 绑定到最终的字典对象时，目标字典名称只需要查找一次.如果目标字典是一个全局而不是一个局部变量，那么理解就会获胜，毫无疑问:
所以只需使用字典理解.与 < 的区别要处理的 30 个元素太小而无法关心，而当您生成全局或拥有更多项目时，无论如何 dict 理解都会胜出.
I don't come from a software/computer science background but I love to code in Python and can generally understand why things are faster. I am really curious to know why this for loop runs faster than the dictionary comprehension. Any insights?
and the code
a = {'a':'hi','b':'hey','c':'yo'}
b = {}
for i,j in a.items():
b[j]=i
%% timeit 932 ns ± 37.2 ns per loop
b = {v: k for k, v in a.items()}
%% timeit 1.08 µs ± 16.4 ns per loop
You are testing with way too small an input; while a dictionary comprehension doesn't have as much of a performance advantage against a for loop when compared to a list comprehension, for realistic problem sizes it can and does beat for loops, especially when targeting a global name.
Your input consists of just 3 key-value pairs. Testing with 1000 elements instead, we see that the timings are very close instead:
>>> import timeit
>>> from random import choice, randint; from string import ascii_lowercase as letters
>>> looped = '''\
... b = {}
... for i,j in a.items():
... b[j]=i
... '''
>>> dictcomp = '''b = {v: k for k, v in a.items()}'''
>>> def rs(): return ''.join([choice(letters) for _ in range(randint(3, 15))])
...
>>> a = {rs(): rs() for _ in range(1000)}
>>> len(a)
1000
>>> count, total = timeit.Timer(looped, 'from __main__ import a').autorange()
>>> (total / count) * 1000000 # microseconds per run
66.62004760000855
>>> count, total = timeit.Timer(dictcomp, 'from __main__ import a').autorange()
>>> (total / count) * 1000000 # microseconds per run
64.5464928005822
The difference is there, the dict comp is faster but only just at this scale. With 100 times as many key-value pairs the difference is a bit bigger:
>>> a = {rs(): rs() for _ in range(100000)}
>>> len(a)
98476
>>> count, total = timeit.Timer(looped, 'from __main__ import a').autorange()
>>> total / count * 1000 # milliseconds, different scale!
15.48140200029593
>>> count, total = timeit.Timer(dictcomp, 'from __main__ import a').autorange()
>>> total / count * 1000 # milliseconds, different scale!
13.674790799996117
which is not that big a difference when you consider both processed nearly 100k key-value pairs. Still, the for loop is clearly slower.
So why the speed difference with 3 elements? Because a comprehension (dictionary, set, list comprehensions or a generator expression) is under the hood implemented as a new function, and calling that function has a base cost the plain loop doesn't have to pay.
Here's the disassembly for the bytecode for both alternatives; note the MAKE_FUNCTION and CALL_FUNCTION opcodes in the top-level bytecode for the dict comprehension, there is a separate section for what that function then does, and there are actually very few differences in between the two approaches here:
>>> import dis
>>> dis.dis(looped)
1 0 BUILD_MAP 0
2 STORE_NAME 0 (b)
2 4 SETUP_LOOP 28 (to 34)
6 LOAD_NAME 1 (a)
8 LOAD_METHOD 2 (items)
10 CALL_METHOD 0
12 GET_ITER
>> 14 FOR_ITER 16 (to 32)
16 UNPACK_SEQUENCE 2
18 STORE_NAME 3 (i)
20 STORE_NAME 4 (j)
3 22 LOAD_NAME 3 (i)
24 LOAD_NAME 0 (b)
26 LOAD_NAME 4 (j)
28 STORE_SUBSCR
30 JUMP_ABSOLUTE 14
>> 32 POP_BLOCK
>> 34 LOAD_CONST 0 (None)
36 RETURN_VALUE
>>> dis.dis(dictcomp)
1 0 LOAD_CONST 0 (<code object <dictcomp> at 0x11d6ade40, file "<dis>", line 1>)
2 LOAD_CONST 1 ('<dictcomp>')
4 MAKE_FUNCTION 0
6 LOAD_NAME 0 (a)
8 LOAD_METHOD 1 (items)
10 CALL_METHOD 0
12 GET_ITER
14 CALL_FUNCTION 1
16 STORE_NAME 2 (b)
18 LOAD_CONST 2 (None)
20 RETURN_VALUE
Disassembly of <code object <dictcomp> at 0x11d6ade40, file "<dis>", line 1>:
1 0 BUILD_MAP 0
2 LOAD_FAST 0 (.0)
>> 4 FOR_ITER 14 (to 20)
6 UNPACK_SEQUENCE 2
8 STORE_FAST 1 (k)
10 STORE_FAST 2 (v)
12 LOAD_FAST 1 (k)
14 LOAD_FAST 2 (v)
16 MAP_ADD 2
18 JUMP_ABSOLUTE 4
>> 20 RETURN_VALUE
The material differences: the looped code uses LOAD_NAME for b each iteration, and STORE_SUBSCR to store the key-value pair in dict loaded. The dictionary comprehension uses MAP_ADD to achieve the same thing as STORE_SUBSCR but doesn't have to load that b name each time.
But with 3 iterations only, the MAKE_FUNCTION / CALL_FUNCTION combo the dict comprehension has to execute is the real drag on the performance:
>>> make_and_call = '(lambda i: None)(None)'
>>> dis.dis(make_and_call)
1 0 LOAD_CONST 0 (<code object <lambda> at 0x11d6ab270, file "<dis>", line 1>)
2 LOAD_CONST 1 ('<lambda>')
4 MAKE_FUNCTION 0
6 LOAD_CONST 2 (None)
8 CALL_FUNCTION 1
10 RETURN_VALUE
Disassembly of <code object <lambda> at 0x11d6ab270, file "<dis>", line 1>:
1 0 LOAD_CONST 0 (None)
2 RETURN_VALUE
>>> count, total = timeit.Timer(make_and_call).autorange()
>>> total / count * 1000000
0.12945385499915574
More than 0.1 μs to create a function object with one argument, and call it (with an extra LOAD_CONST for the None value we pass in)! And that's just about the difference between the looped and comprehension timings for 3 key-value pairs.
You can liken this to being surprised that a man with a shovel can dig a small hole faster than a backhoe can. The backhoe can certainly dig fast, but a man with a shovel can get started faster if you need to get the backhoe started and moved into position first!
Beyond a few key-value pairs (digging a bigger hole), the function create and call cost fades away into nothingness. At this point the dict comprehension and the explicit loop basically do the same thing:
- take the next key-value pair, pop those on the stack
- call the
dict.__setitem__hook via a bytecode operation with the top two items on the stack (eitherSTORE_SUBSCRorMAP_ADD. This doesn't count as a 'function call' as it's all internally handled in the interpreter loop.
This is different from a list comprehension, where the plain loop version would have to use list.append(), involving an attribute lookup, and a function call each loop iteration. The list comprehension speed advantage comes from this difference; see Python list comprehension expensive
What a dict comprehension does add, is that the target dictionary name only needs to be looked up once, when binding b to the the final dictionary object. If the target dictionary is a global instead of a local variable, the comprehension wins, hands down:
>>> a = {rs(): rs() for _ in range(1000)}
>>> len(a)
1000
>>> namespace = {}
>>> count, total = timeit.Timer(looped, 'from __main__ import a; global b', globals=namespace).autorange()
>>> (total / count) * 1000000
76.72348440100905
>>> count, total = timeit.Timer(dictcomp, 'from __main__ import a; global b', globals=namespace).autorange()
>>> (total / count) * 1000000
64.72114819916897
>>> len(namespace['b'])
1000
So just use a dict comprehension. The difference with < 30 elements to process is too small to care about, and the moment you are generating a global or have more items, the dict comprehension wins out anyway.
这篇关于为什么这个循环比创建字典的字典理解更快?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！