一文搞懂 Python 中的 yield

def read(self, sql, params=()):
    stmt = ibm_db.prepare(self.connection, sql)    for index, param in enumerate(params):        ibm_db.bind_param(stmt, index + 1, param)    ibm_db.execute(stmt)    row = ibm_db.fetch_tuple(stmt)    while row:        yield row        row = ibm_db.fetch_tuple(stmt)

>>> def iter_fun():...     print("a")...     yield 1...     print("b")...     yield 2...     print("c")...     yield 3...>>> iter_fun()<generator object iter_fun at 0x107e372a0>>>> for i in iter_fun():...     print(i)...a1b2c3>>> x = iter_fun()>>> x.__next__()a1>>> x.__next__()b2>>> x.__next__()c3>>> x.__next__()Traceback (most recent call last):  File "<stdin>", line 1, in <module>StopIteration

In [14]: def normal_fun(num):    ...:     result = []    ...:     for i in range(0,num):    ...:         if i%2 == 0:    ...:             result.append(i)    ...:     return result    ...:
In [14]: def iter_fun(num):    ...:     for i in range(0,num):    ...:         if i %2 == 0:    ...:             yield i    ...:
In [15]: %time for i in iter_fun(1000000): a = iCPU times: user 97 ms, sys: 2.55 ms, total: 99.6 msWall time: 97.2 ms
In [16]: %time for i in normal_fun(1000000): a = iCPU times: user 115 ms, sys: 13.6 ms, total: 129 msWall time: 128 ms
In [17]: %time for i in normal_fun(100000000): a = iCPU times: user 10.8 s, sys: 668 ms, total: 11.4 sWall time: 11.4 s
In [18]: %time for i in iter_fun(100000000): a = iCPU times: user 9.1 s, sys: 6.49 ms, total: 9.11 sWall time: 9.12 s

with open("file.txt","r") as reader:    for line in reader.readlines():        #do something        pass

with open("file.txt","r") as reader:    for line in reader:        #do something        pass

>>> with open("/etc/passwd","r") as reader:...     dir(reader)...['_CHUNK_SIZE', '__class__', '__del__', '__delattr__', '__dict__', '__dir__', '__doc__', '__enter__', '__eq__', '__exit__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__lt__', '__ne__', '__new__', '__next__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '_checkClosed', '_checkReadable', '_checkSeekable', '_checkWritable', '_finalizing', 'buffer', 'close', 'closed', 'detach', 'encoding', 'errors', 'fileno', 'flush', 'isatty', 'line_buffering', 'mode', 'name', 'newlines', 'read', 'readable', 'readline', 'readlines', 'reconfigure', 'seek', 'seekable', 'tell', 'truncate', 'writable', 'write', 'write_through', 'writelines']>>>

In [20]: def fun():    ...:     print("a")    ...:     a = yield 1    ...:     print("b")    ...:     print("a = ",a)    ...:     b = yield a    ...:     print("c")    ...:     print("b = ",b)    ...:
In [21]: x = fun()
In [22]: x.__next__()aOut[22]: 1
In [23]: x.send(4)ba =  4Out[23]: 4
In [24]: x.send(5)cb =  5---------------------------------------------------------------------------StopIteration                             Traceback (most recent call last)<ipython-input-24-9183f5e81876> in <module>----> 1 x.send(5)

import time
def consume():    r = ''    while True:        n = yield r        if not n:            return        print('[consumer] consuming %s...' % n)        time.sleep(1)        r = 'well received'
def produce(c):    next(c)    n = 0    while n < 5:        n = n + 1        print('[producer] producing %s...' % n)        r = c.send(n)        print('[producer] consumer return: %s' % r)    c.close()
if __name__=='__main__':    c = consume()    produce(c)

[producer] producing 1...[consumer] consuming 1...[producer] consumer return: well received[producer] producing 2...[consumer] consuming 2...[producer] consumer return: well received[producer] producing 3...[consumer] consuming 3...[producer] consumer return: well received[producer] producing 4...[consumer] consuming 4...[producer] consumer return: well received[producer] producing 5...[consumer] consuming 5...[producer] consumer return: well received