python - Python 生成器可以非延迟调用吗？

Question

我知道在Python中，生成器是延迟调用的。例如:

>>> def G():
...     print('this was evaluated now 1')
...     yield 1
...     print('this was evaluated now 2')
...     yield 2
...
>>> g = G()
>>> next(g)
this was evaluated now 1
1
>>> next(g)
this was evaluated now 2
2

仅在调用第一个 next(g) 后才对 print('this wasvalued now 1') 行进行求值。

我想知道是否有一种简单的方法来非延迟地调用生成器。这意味着当调用 g = G() 时，该函数将计算直到并包括第一个 yield 结果的所有内容，而不实际产生。然后，在第一次调用 next(g) 时，将产生已经计算的结果，并且还将计算直到并包括第二个 yield 结果的所有内容。等等。

如何实现这一目标？

<小时/>

以下是此非惰性方案下的预期行为:

>>> g = G()
this was evaluated now 1
>>> next(g)
1
this was evaluated now 2
>>> next(g)
2

<小时/>

这是一个解决方案尝试，但不起作用:

>>> class NonLazyGenerator():
...     def __init__(self,G):
...         self.g = G()
...         self.next_value = next(self.g)
...
...     def __next__(self):
...         current_value = self.next_value
...         try:
...             self.next_value = next(self.g)
...         except StopIteration:
...             pass
...         return current_value
...
>>> g = NonLazyGenerator(G)
this was evaluated now 1
>>> next(g)
this was evaluated now 2
1
>>> next(g)
2

这会失败，因为该值仅在 return 语句之后生成，而直到下一个 yield 的所有计算都发生在 return 之前> 声明。这个例子让我意识到，可能无法执行我正在寻找的东西，因为它需要在函数返回后执行步骤(可能需要多线程)。

Answer 1

您可能可以为其编写某种装饰器，例如:

def eagergenerator(mygen):
    class GeneratorWrapper:
        def __init__(self, *args, **kwargs):
            self.g = mygen(*args, **kwargs)
            self.last = next(self.g)
        def __iter__(self):
            return self
        def __next__(self):
            if self.last is self:
                raise StopIteration
            fake_yield = self.last
            try:
                self.last = next(self.g)
                return fake_yield
            except StopIteration:
                self.last = self
                return fake_yield
    return GeneratorWrapper

然后你可以简单地装饰你的普通发电机:

@eagergenerator
def G():
    print("one")
    yield 1
    print("two")
    yield 2

其工作原理如下:

>>> g = G()                               
one                                       
>>> next(g)                               
two                                       
1                                         
>>> next(g)                               
2                                         
>>> next(g)                               
Traceback (most recent call last):        
  File "<stdin>", line 1, in <module>     
  File "eagergen.py", line 10, in __next__
    raise StopIteration                   
StopIteration                             
>>>