yield的功能類似于return，但是不同之處在于它返回的是生成器。
生成器
生成器是通過一個或多個yield表達式構成的函數，每一個生成器都是一個迭代器（但是迭代器不一定是生成器）。
如果一個函數包含yield關鍵字，這個函數就會變為一個生成器。
生成器并不會一次返回所有結果，而是每次遇到yield關鍵字后返回相應結果，并保留函數當前的運行狀態，等待下一次的調用。
由于生成器也是一個迭代器，那么它就應該支持next方法來獲取下一個值。
基本操作
# 通過`yield`來創建生成器
def func():
for i in xrange(10);
yield i
# 通過列表來創建生成器
[i for i in xrange(10)]
# 通過`yield`來創建生成器
def func():
for i in xrange(10);
yield i
# 通過列表來創建生成器
[i for i in xrange(10)]
Python
# 調用如下
>>> f = func()
>>> f # 此時生成器還沒有運行
>>> f.next() # 當i=0時，遇到yield關鍵字，直接返回
>>> f.next() # 繼續上一次執行的位置，進入下一層循環
...
>>> f.next()
>>> f.next() # 當執行完最后一次循環后，結束yield語句，生成StopIteration異常
Traceback (most recent call last):
File "
", line 1, in
StopIteration
>>>
# 調用如下
>>> f = func()
>>> f # 此時生成器還沒有運行
>>> f.next() # 當i=0時，遇到yield關鍵字，直接返回
>>> f.next() # 繼續上一次執行的位置，進入下一層循環
...
>>> f.next()
>>> f.next() # 當執行完最后一次循環后，結束yield語句，生成StopIteration異常
Traceback (most recent call last):
File "
", line 1, in
StopIteration
>>>
除了next函數，生成器還支持send函數。該函數可以向生成器傳遞參數。
>>> def func():
... n = 0
... while 1:
... n = yield n #可以通過send函數向n賦值
...
>>> f = func()
>>> f.next() # 默認情況下n為0
>>> f.send(1) #n賦值1
>>> f.send(2)
>>>
>>> def func():
... n = 0
... while 1:
... n = yield n #可以通過send函數向n賦值
...
>>> f = func()
>>> f.next() # 默認情況下n為0
>>> f.send(1) #n賦值1
>>> f.send(2)
>>>
應用
最經典的例子，生成無限序列。
常規的解決方法是，生成一個滿足要求的很大的列表，這個列表需要保存在內存中，很明顯內存限制了這個問題。
def get_primes(start):
for element in magical_infinite_range(start):
if is_prime(element):
return element
def get_primes(start):
for element in magical_infinite_range(start):
if is_prime(element):
return element
如果使用生成器就不需要返回整個列表，每次都只是返回一個數據，避免了內存的限制問題。
def get_primes(number):
while True:
if is_prime(number):
yield number
number += 1
def get_primes(number):
while True:
if is_prime(number):
yield number
number += 1
生成器源碼分析
生成器的源碼在Objects/genobject.c。
調用棧
在解釋生成器之前，需要講解一下Python虛擬機的調用原理。
Python虛擬機有一個棧幀的調用棧，其中棧幀的是PyFrameObject，位于Include/frameobject.h。
typedef struct _frame {
PyObject_VAR_HEAD
struct _frame *f_back; /* previous frame, or NULL */
PyCodeObject *f_code; /* code segment */
PyObject *f_builtins; /* builtin symbol table (PyDictObject) */
PyObject *f_globals; /* global symbol table (PyDictObject) */
PyObject *f_locals; /* local symbol table (any mapping) */
PyObject **f_valuestack; /* points after the last local */
/* Next free slot in f_valuestack. Frame creation sets to f_valuestack.
Frame evaluation usually NULLs it, but a frame that yields sets it
to the current stack top. */
PyObject **f_stacktop;
PyObject *f_trace; /* Trace function */
/* If an exception is raised in this frame, the next three are used to
* record the exception info (if any) originally in the thread state. See
* comments before set_exc_info() -- it's not obvious.
* Invariant: if _type is NULL, then so are _value and _traceback.
* Desired invariant: all three are NULL, or all three are non-NULL. That
* one isn't currently true, but "should be".
*/
PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
PyThreadState *f_tstate;
int f_lasti; /* Last instruction if called */
/* Call PyFrame_GetLineNumber() instead of reading this field
directly. As of 2.3 f_lineno is only valid when tracing is
active (i.e. when f_trace is set). At other times we use
PyCode_Addr2Line to calculate the line from the current
bytecode index. */
int f_lineno; /* Current line number */
int f_iblock; /* index in f_blockstack */
PyTryBlock f_blockstack[CO_MAXBLOCKS]; /* for try and loop blocks */
PyObject *f_localsplus[1]; /* locals+stack, dynamically sized */
} PyFrameObject;
typedef struct _frame {
PyObject_VAR_HEAD
struct _frame *f_back; /* previous frame, or NULL */
PyCodeObject *f_code; /* code segment */
PyObject *f_builtins; /* builtin symbol table (PyDictObject) */
PyObject *f_globals; /* global symbol table (PyDictObject) */
PyObject *f_locals; /* local symbol table (any mapping) */
PyObject **f_valuestack; /* points after the last local */
/* Next free slot in f_valuestack. Frame creation sets to f_valuestack.
Frame evaluation usually NULLs it, but a frame that yields sets it
to the current stack top. */
PyObject **f_stacktop;
PyObject *f_trace; /* Trace function */
/* If an exception is raised in this frame, the next three are used to
* record the exception info (if any) originally in the thread state. See
* comments before set_exc_info() -- it's not obvious.
* Invariant: if _type is NULL, then so are _value and _traceback.
* Desired invariant: all three are NULL, or all three are non-NULL. That
* one isn't currently true, but "should be".
*/
PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
PyThreadState *f_tstate;
int f_lasti; /* Last instruction if called */
/* Call PyFrame_GetLineNumber() instead of reading this field
directly. As of 2.3 f_lineno is only valid when tracing is
active (i.e. when f_trace is set). At other times we use
PyCode_Addr2Line to calculate the line from the current
bytecode index. */
int f_lineno; /* Current line number */
int f_iblock; /* index in f_blockstack */
PyTryBlock f_blockstack[CO_MAXBLOCKS]; /* for try and loop blocks */
PyObject *f_localsplus[1]; /* locals+stack, dynamically sized */
} PyFrameObject;
棧幀保存了給出代碼的的信息和上下文，其中包含最后執行的指令，全局和局部命名空間，異常狀態等信息。f_valueblock保存了數據，b_blockstack保存了異常和循環控制方法。
舉一個例子來說明，
def foo():
x = 1
def bar(y):
z = y + 2 #
def foo():
x = 1
def bar(y):
z = y + 2 #
那么，相應的調用棧如下，一個py文件，一個類，一個函數都是一個代碼塊，對應者一個Frame，保存著上下文環境以及字節碼指令。
c ---------------------------
a | bar Frame | -> block stack: []
l | (newest) | -> data stack: [1, 2]
l ---------------------------
| foo Frame | -> block stack: []
s | | -> data stack: [.bar at 0x10d389680>, 1]
t ---------------------------
a | main (module) Frame | -> block stack: []
c | (oldest) | -> data stack: []
k ---------------------------
c ---------------------------
a | bar Frame | -> block stack: []
l | (newest) | -> data stack: [1, 2]
l ---------------------------
| foo Frame | -> block stack: []
s | | -> data stack: [.bar at 0x10d389680>, 1]
t ---------------------------
a | main (module) Frame | -> block stack: []
c | (oldest) | -> data stack: []
k ---------------------------
每一個棧幀都擁有自己的數據棧和block棧，獨立的數據棧和block棧使得解釋器可以中斷和恢復棧幀（生成器正式利用這點）。
Python代碼首先被編譯為字節碼，再由Python虛擬機來執行。一般來說，一條Python語句對應著多條字節碼（由于每條字節碼對應著一條C語句，而不是一個機器指令，所以不能按照字節碼的數量來判斷代碼性能）。
調用dis模塊可以分析字節碼，
from dis import dis
dis(foo)
0 LOAD_CONST 1 (1) # 加載常量1
3 STORE_FAST 0 (x) # x賦值為1
6 LOAD_CONST 2 (
) # 加載常量2
9 MAKE_FUNCTION 0 # 創建函數
12 STORE_FAST 1 (bar)
15 LOAD_FAST 1 (bar)
18 LOAD_FAST 0 (x)
21 CALL_FUNCTION 1 # 調用函數
24 RETURN_VALUE
from dis import dis
dis(foo)
0 LOAD_CONST 1 (1) # 加載常量1
3 STORE_FAST 0 (x) # x賦值為1
6 LOAD_CONST 2 (
) # 加載常量2
9 MAKE_FUNCTION 0 # 創建函數
12 STORE_FAST 1 (bar)
15 LOAD_FAST 1 (bar)
18 LOAD_FAST 0 (x)
21 CALL_FUNCTION 1 # 調用函數
24 RETURN_VALUE
其中，
第一行為代碼行號；
第二行為偏移地址；
第三行為字節碼指令；
第四行為指令參數；
第五行為參數解釋。
第一行為代碼行號；
第二行為偏移地址；
第三行為字節碼指令；
第四行為指令參數；
第五行為參數解釋。
生成器源碼分析
由了上面對于調用棧的理解，就可以很容易的明白生成器的具體實現。
生成器的源碼位于object/genobject.c。
生成器的創建
PyObject *
PyGen_New(PyFrameObject *f)
{
PyGenObject *gen = PyObject_GC_New(PyGenObject, &PyGen_Type); # 創建生成器對象
if (gen == NULL) {
Py_DECREF(f);
return NULL;
}
gen->gi_frame = f; # 賦予代碼塊
Py_INCREF(f->f_code); # 引用計數+1
gen->gi_code = (PyObject *)(f->f_code);
gen->gi_running = 0; # 0表示為執行，也就是生成器的初始狀態
gen->gi_weakreflist = NULL;
_PyObject_GC_TRACK(gen); # GC跟蹤
return (PyObject *)gen;
}
PyObject *
PyGen_New(PyFrameObject *f)
{
PyGenObject *gen = PyObject_GC_New(PyGenObject, &PyGen_Type); # 創建生成器對象
if (gen == NULL) {
Py_DECREF(f);
return NULL;
}
gen->gi_frame = f; # 賦予代碼塊
Py_INCREF(f->f_code); # 引用計數+1
gen->gi_code = (PyObject *)(f->f_code);
gen->gi_running = 0; # 0表示為執行，也就是生成器的初始狀態
gen->gi_weakreflist = NULL;
_PyObject_GC_TRACK(gen); # GC跟蹤
return (PyObject *)gen;
}
send與next
next與send函數，如下
static PyObject *
gen_iternext(PyGenObject *gen)
{
return gen_send_ex(gen, NULL, 0);
}
static PyObject *
gen_send(PyGenObject *gen, PyObject *arg)
{
return gen_send_ex(gen, arg, 0);
}
static PyObject *
gen_iternext(PyGenObject *gen)
{
return gen_send_ex(gen, NULL, 0);
}
static PyObject *
gen_send(PyGenObject *gen, PyObject *arg)
{
return gen_send_ex(gen, arg, 0);
}
從上面的代碼中可以看到，send和next都是調用的同一函數gen_send_ex，區別在于是否帶有參數。
static PyObject *
gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
{
PyThreadState *tstate = PyThreadState_GET();
PyFrameObject *f = gen->gi_frame;
PyObject *result;
if (gen->gi_running) { # 判斷生成器是否已經運行
PyErr_SetString(PyExc_ValueError,
"generator already executing");
return NULL;
}
if (f==NULL || f->f_stacktop == NULL) { # 如果代碼塊為空或調用棧為空，則拋出StopIteration異常
/* Only set exception if called from send() */
if (arg && !exc)
PyErr_SetNone(PyExc_StopIteration);
return NULL;
}
if (f->f_lasti == -1) { # f_lasti=1 代表首次執行
if (arg && arg != Py_None) { # 首次執行不允許帶有參數
PyErr_SetString(PyExc_TypeError,
"can't send non-None value to a "
"just-started generator");
return NULL;
}
} else {
/* Push arg onto the frame's value stack */
result = arg ? arg : Py_None;
Py_INCREF(result); # 該參數引用計數+1
*(f->f_stacktop++) = result; # 參數壓棧
}
/* Generators always return to their most recent caller, not
* necessarily their creator. */
f->f_tstate = tstate;
Py_XINCREF(tstate->frame);
assert(f->f_back == NULL);
f->f_back = tstate->frame;
gen->gi_running = 1; # 修改生成器執行狀態
result = PyEval_EvalFrameEx(f, exc); # 執行字節碼
gen->gi_running = 0; # 恢復為未執行狀態
/* Don't keep the reference to f_back any longer than necessary. It
* may keep a chain of frames alive or it could create a reference
* cycle. */
assert(f->f_back == tstate->frame);
Py_CLEAR(f->f_back);
/* Clear the borrowed reference to the thread state */
f->f_tstate = NULL;
/* If the generator just returned (as opposed to yielding), signal
* that the generator is exhausted. */
if (result == Py_None && f->f_stacktop == NULL) {
Py_DECREF(result);
result = NULL;
/* Set exception if not called by gen_iternext() */
if (arg)
PyErr_SetNone(PyExc_StopIteration);
}
if (!result || f->f_stacktop == NULL) {
/* generator can't be rerun, so release the frame */
Py_DECREF(f);
gen->gi_frame = NULL;
}
return result;
}
static PyObject *
gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
{
PyThreadState *tstate = PyThreadState_GET();
PyFrameObject *f = gen->gi_frame;
PyObject *result;
if (gen->gi_running) { # 判斷生成器是否已經運行
PyErr_SetString(PyExc_ValueError,
"generator already executing");
return NULL;
}
if (f==NULL || f->f_stacktop == NULL) { # 如果代碼塊為空或調用棧為空，則拋出StopIteration異常
/* Only set exception if called from send() */
if (arg && !exc)
PyErr_SetNone(PyExc_StopIteration);
return NULL;
}
if (f->f_lasti == -1) { # f_lasti=1 代表首次執行
if (arg && arg != Py_None) { # 首次執行不允許帶有參數
PyErr_SetString(PyExc_TypeError,
"can't send non-None value to a "
"just-started generator");
return NULL;
}
} else {
/* Push arg onto the frame's value stack */
result = arg ? arg : Py_None;
Py_INCREF(result); # 該參數引用計數+1
*(f->f_stacktop++) = result; # 參數壓棧
}
/* Generators always return to their most recent caller, not
* necessarily their creator. */
f->f_tstate = tstate;
Py_XINCREF(tstate->frame);
assert(f->f_back == NULL);
f->f_back = tstate->frame;
gen->gi_running = 1; # 修改生成器執行狀態
result = PyEval_EvalFrameEx(f, exc); # 執行字節碼
gen->gi_running = 0; # 恢復為未執行狀態
/* Don't keep the reference to f_back any longer than necessary. It
* may keep a chain of frames alive or it could create a reference
* cycle. */
assert(f->f_back == tstate->frame);
Py_CLEAR(f->f_back);
/* Clear the borrowed reference to the thread state */
f->f_tstate = NULL;
/* If the generator just returned (as opposed to yielding), signal
* that the generator is exhausted. */
if (result == Py_None && f->f_stacktop == NULL) {
Py_DECREF(result);
result = NULL;
/* Set exception if not called by gen_iternext() */
if (arg)
PyErr_SetNone(PyExc_StopIteration);
}
if (!result || f->f_stacktop == NULL) {
/* generator can't be rerun, so release the frame */
Py_DECREF(f);
gen->gi_frame = NULL;
}
return result;
}
字節碼的執行
PyEval_EvalFrameEx函數的功能為執行字節碼并返回結果。
# 主要流程如下，
for (;;) {
switch(opcode) { # opcode為操作碼，對應著各種操作
case NOP:
goto fast_next_opcode;
...
...
case YIELD_VALUE: # 如果操作碼是yield
retval = POP();
f->f_stacktop = stack_pointer;
why = WHY_YIELD;
goto fast_yield; # 利用goto跳出循環
}
}
fast_yield:
...
return vetval; # 返回結果
# 主要流程如下，
for (;;) {
switch(opcode) { # opcode為操作碼，對應著各種操作
case NOP:
goto fast_next_opcode;
...
...
case YIELD_VALUE: # 如果操作碼是yield
retval = POP();
f->f_stacktop = stack_pointer;
why = WHY_YIELD;
goto fast_yield; # 利用goto跳出循環
}
}
fast_yield:
...
return vetval; # 返回結果
舉一個例子，f_back上一個Frame，f_lasti上一次執行的指令的偏移量，
import sys
from dis import dis
def func():
f = sys._getframe(0)
print f.f_lasti
print f.f_back
yield 1
print f.f_lasti
print f.f_back
yield 2
a = func()
dis(func)
a.next()
a.next()
import sys
from dis import dis
def func():
f = sys._getframe(0)
print f.f_lasti
print f.f_back
yield 1
print f.f_lasti
print f.f_back
yield 2
a = func()
dis(func)
a.next()
a.next()
結果如下，其中第三行的英文為操作碼，對應著上面的opcode，每次switch都是在不同的opcode之間進行選擇。
Python
0 LOAD_GLOBAL 0 (sys)
3 LOAD_ATTR 1 (_getframe)
6 LOAD_CONST 1 (0)
9 CALL_FUNCTION 1
12 STORE_FAST 0 (f)
15 LOAD_FAST 0 (f)
18 LOAD_ATTR 2 (f_lasti)
21 PRINT_ITEM
22 PRINT_NEWLINE
23 LOAD_FAST 0 (f)
26 LOAD_ATTR 3 (f_back)
29 PRINT_ITEM
30 PRINT_NEWLINE
31 LOAD_CONST 2 (1)
34 YIELD_VALUE # 此時操作碼為YIELD_VALUE，直接跳轉上述goto語句，此時f_lasti為當前指令，f_back為當前frame
35 POP_TOP
36 LOAD_FAST 0 (f)
39 LOAD_ATTR 2 (f_lasti)
42 PRINT_ITEM
43 PRINT_NEWLINE
44 LOAD_FAST 0 (f)
47 LOAD_ATTR 3 (f_back)
50 PRINT_ITEM
51 PRINT_NEWLINE
52 LOAD_CONST 3 (2)
55 YIELD_VALUE
56 POP_TOP
57 LOAD_CONST 0 (None)
60 RETURN_VALUE
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