据我所知，Python中的for x in a_generator: foo(x)循环大致相当于：
try:
while True:
foo(next(a_generator))
except StopIteration:
pass
这表明这样的事情：
for outer_item in a_generator:
if should_inner_loop(outer_item):
for inner_item in a_generator:
foo(inner_item)
if stop_inner_loop(inner_item): break
else:
bar(outer_item)
会做两件事：
y，直到x should_inner_loop(x)返回truthy，然后在内部for中循环，直到stop_inner_loop(thing)返回true。然后，外部循环恢复，其中内部停止。从我公认的不太好的测试中，它似乎表现如上。但是，我无法在规范中找到任何保证此行为在解释器中保持不变的内容。有没有说过或暗示我可以确定它总是这样的？它会导致错误，还是以其他方式执行？ （即做一些除上述内容之外的事情
N.B。上面的代码取自我自己的经验;我不知道它是否真的准确。这就是我要问的原因。
答案 0 :(得分：6)
TL; DR：CPython是安全的（但我找不到任何相关规范），虽然它可能不会做你想做的事。
首先，让我们谈谈你的第一个假设，即等价。
for循环实际调用对象上的第一个iter()，然后对其结果运行next()，直到获得StopIteration。
这是相关的字节码（Python的低级形式，由解释器本身使用）：
>>> import dis
>>> def f():
... for x in y:
... print(x)
...
>>> dis.dis(f)
2 0 SETUP_LOOP 24 (to 27)
3 LOAD_GLOBAL 0 (y)
6 GET_ITER
>> 7 FOR_ITER 16 (to 26)
10 STORE_FAST 0 (x)
3 13 LOAD_GLOBAL 1 (print)
16 LOAD_FAST 0 (x)
19 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
22 POP_TOP
23 JUMP_ABSOLUTE 7
>> 26 POP_BLOCK
>> 27 LOAD_CONST 0 (None)
30 RETURN_VALUE
GET_ITER调用iter(y)（它本身调用y.__iter__()）并将其结果推送到堆栈（将其视为一堆本地未命名变量），然后进入{{ {1}}，调用FOR_ITER（它本身调用next(<iterator>)），然后执行循环内的代码，<iterator>.__next__()使执行返回JUMP_ABSOLUTE。
现在，为安全起见：
以下是生成器的方法：https://hg.python.org/cpython/file/101404/Objects/genobject.c#l589
正如您在line 617所看到的那样，FOR_ITER的实施是__iter__()，您可以找到here的实现。 PyObject_SelfIter只返回对象（即生成器）本身。
因此，当您嵌套两个循环时，两者都迭代在同一个迭代器上。
而且，正如你所说，他们只是在上面打PyObject_SelfIter，所以它是安全的。
但要小心：内部循环将消耗外部循环不会消耗的项目。 即使这是你想做的事情，它也可能不太可读。
如果这不是您想要做的，请考虑itertools.tee()，它缓冲迭代器的输出，允许您迭代其输出两次（或更多）。只有当tee迭代器在输出流中保持彼此靠近时，这才有效。如果一个tee迭代器在使用另一个之前将完全耗尽，那么最好只需在迭代器上调用next()来实现其中的列表。
答案 1 :(得分：3)
不，它不安全（因为我们没有获得我们可能预期的结果）。
考虑一下：
{
"metadata": {
"version": 4.3,
"type": "Object",
"generator": "ObjectExporter"
},
"geometries": [
{
"uuid": "9848FD0D-E02D-4D2E-BB80-93CD4CA27AFF",
"type": "Geometry",
"data": {
"vertices": [-100,0,-100,100,0,-100,-100,0,-100,-100,0,100,-100,0,-90,100,0,-90,-90,0,-100,-90,0,100,-100,0,-80,100,0,-80,-80,0,-100,-80,0,100,-100,0,-70,100,0,-70,-70,0,-100,-70,0,100,-100,0,-60,100,0,-60,-60,0,-100,-60,0,100,-100,0,-50,100,0,-50,-50,0,-100,-50,0,100,-100,0,-40,100,0,-40,-40,0,-100,-40,0,100,-100,0,-30,100,0,-30,-30,0,-100,-30,0,100,-100,0,-20,100,0,-20,-20,0,-100,-20,0,100,-100,0,-10,100,0,-10,-10,0,-100,-10,0,100,-100,0,0,100,0,0,0,0,-100,0,0,100,-100,0,10,100,0,10,10,0,-100,10,0,100,-100,0,20,100,0,20,20,0,-100,20,0,100,-100,0,30,100,0,30,30,0,-100,30,0,100,-100,0,40,100,0,40,40,0,-100,40,0,100,-100,0,50,100,0,50,50,0,-100,50,0,100,-100,0,60,100,0,60,60,0,-100,60,0,100,-100,0,70,100,0,70,70,0,-100,70,0,100,-100,0,80,100,0,80,80,0,-100,80,0,100,-100,0,90,100,0,90,90,0,-100,90,0,100,-100,0,100,100,0,100,100,0,-100,100,0,100],
"normals": [],
"faces": []
}
},
{
"uuid": "DCC723E0-C1E0-4CB4-B047-56AF8FCDD3E2",
"type": "BoxGeometry",
"width": 4,
"height": 4,
"depth": 4
},
{
"uuid": "AE0EA8E4-47AC-4D29-AD68-CE00181F9E21",
"type": "BoxGeometry",
"width": 4,
"height": 4,
"depth": 4
},
{
"uuid": "0AE6C8FD-83BD-4946-BFCB-F767B01FE579",
"type": "BoxGeometry",
"width": 4,
"height": 4,
"depth": 4
},
{
"uuid": "6B40660B-CF8B-4CB8-AD55-E0F959C5979C",
"type": "BoxGeometry",
"width": 4,
"height": 4,
"depth": 4
}],
"materials": [
{
"uuid": "F174D367-B14B-47C9-9818-1D1A62540882",
"type": "LineBasicMaterial"
},
{
"uuid": "83848195-1452-40DA-827F-A498C1D870F9",
"type": "MeshPhongMaterial",
"color": 16777215,
"ambient": 16777215,
"emissive": 0,
"specular": 1118481,
"shininess": 30,
"opacity": 0.8,
"transparent": true
},
{
"uuid": "37C5C4FD-3402-4FE4-877D-E66BD18D797E",
"type": "MeshPhongMaterial",
"color": 16777215,
"ambient": 16777215,
"emissive": 0,
"specular": 1118481,
"shininess": 30,
"opacity": 0.8,
"transparent": true
},
{
"uuid": "FC6E4C1C-8191-4CE9-B687-A29C13EA9400",
"type": "MeshPhongMaterial",
"color": 16777215,
"ambient": 16777215,
"emissive": 0,
"specular": 1118481,
"shininess": 30,
"opacity": 0.8,
"transparent": true
},
{
"uuid": "CE06172B-3277-4B91-B113-3BC45963EE2E",
"type": "MeshPhongMaterial",
"color": 16777215,
"ambient": 16777215,
"emissive": 0,
"specular": 1118481,
"shininess": 30,
"opacity": 0.8,
"transparent": true
}],
"object": {
"uuid": "FDF89002-FA43-4B30-BEB1-8548BFF8592D",
"type": "Scene",
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,0,0,0,1],
"children": [
{
"uuid": "5E79AA2B-C280-4A2B-9312-440F701CBEE3",
"type": "PerspectiveCamera",
"fov": 50,
"aspect": 1,
"near": 0.1,
"far": 2000,
"matrix": [0.8944271802902222,-1.3716013880227251e-9,-0.4472135901451111,0,-0.15064871311187744,0.9415544867515564,-0.3012974262237549,0,0.4210759699344635,0.3368607759475708,0.842151939868927,0,50,50,100,1]
},
{
"uuid": "3984192A-DD7F-4689-A076-6770131B23AD",
"type": "Line",
"name": "Grid",
"geometry": "9848FD0D-E02D-4D2E-BB80-93CD4CA27AFF",
"material": "F174D367-B14B-47C9-9818-1D1A62540882",
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,0,0,0,1]
},
{
"uuid": "ACB0E066-532C-4444-9FA7-181C76BAAB95",
"type": "AmbientLight",
"color": 5592405,
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,0,0,0,1]
},
{
"uuid": "FFDE61E5-CA82-4BC9-B406-1B7A9006D1DD",
"type": "DirectionalLight",
"color": 16777215,
"intensity": 0.8,
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,0.5773502588272095,0.5773502588272095,0.5773502588272095,1]
},
{
"uuid": "A94B706F-7329-48A5-9CF8-040795E57314",
"type": "SpotLight",
"color": 16777215,
"intensity": 0.6,
"distance": 0,
"angle": 1.0471975511965976,
"exponent": 10,
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,-30,30,-100,1]
},
{
"uuid": "6608DD55-558B-4718-B953-F686D42B3744",
"type": "Mesh",
"geometry": "DCC723E0-C1E0-4CB4-B047-56AF8FCDD3E2",
"material": "83848195-1452-40DA-827F-A498C1D870F9",
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,-9.931706428527832,0,-9.857349395751953,1]
},
{
"uuid": "25A41D96-791D-4FBE-9CB9-5AEC0ECBE991",
"type": "Mesh",
"geometry": "AE0EA8E4-47AC-4D29-AD68-CE00181F9E21",
"material": "37C5C4FD-3402-4FE4-877D-E66BD18D797E",
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,2.9669816493988037,0,5.327826976776123,1]
},
{
"uuid": "B6FE254F-BCA8-41B1-A63C-4CB7CAA6F734",
"type": "Mesh",
"geometry": "0AE6C8FD-83BD-4946-BFCB-F767B01FE579",
"material": "FC6E4C1C-8191-4CE9-B687-A29C13EA9400",
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,1.5996896028518677,0,-7.717252254486084,1]
},
{
"uuid": "0EFA29CB-D365-4942-B54B-D43FFFB22D3E",
"type": "Mesh",
"geometry": "6B40660B-CF8B-4CB8-AD55-E0F959C5979C",
"material": "CE06172B-3277-4B91-B113-3BC45963EE2E",
"matrix": [1,0,0,0,0,1,0,0,0,0,1,0,8.166288375854492,0,14.097293853759766,1]
}]
}
}
当然，我们将打印0到19张。
现在让我们添加一些代码：
a = (_ for _ in range(20))
for num in a:
print(num)
唯一要打印的是a = (_ for _ in range(20))
for num in a:
for another_num in a:
pass
print(num)
。
到达外循环的第二次迭代时，内循环已经耗尽了生成器。
我们也可以这样做：
0如果它是安全的，我们预计会打印20到0到19，但我们实际上只打印了一次，原因与上面提到的相同。
答案 2 :(得分：2)
这不是你问题的答案，但我建议不要这样做，因为代码不可读。我花了一段时间才发现您使用y两次，即使这是您问题的全部内容。不要让未来的读者对此感到困惑。当我看到一个嵌套的循环时，我并不期待你做了什么，我的大脑也很难看到它。
我会这样做：
def generator_with_state(y):
state = 0
for x in y:
if isinstance(x, special_thing):
state = 1
continue
elif state == 1 and isinstance(x, signal):
state = 0
yield x, state
for x, state in generator_with_state(y):
if state == 1:
foo(x)
else:
bar(x)