python之yield表達式

yield表達式用於generator function

調用generator function時，返回一個iterator（函數內語句不被會執行），調用iterator函數時，執行到yield表達式，

當前函數暫停執行，返回表達式的值到調用者，繼續調用iterator函數，從暫停處恢復執行。、

遇到yield表達式，與遇到其他表達式差不多，yield表達式也有值，一般為None。

與其他表達式的不同之處在於yield表達式會在yield處返回表達式的值

官方文檔描述如下：

    When a generator function is called, it returns an iterator known as a generator. That generator then controls the execution of the generator function. The execution starts when one of the generator’s methods is called. At that time, the execution proceeds to the first yield expression, where it is suspended again, returning the value of expression_list to the generator’s caller. By suspended, we mean that all local state is retained, including the current bindings of local variables, the instruction pointer, the internal evaluation stack, and the state of any exception handling. When the execution is resumed by calling one of the generator’s methods, the function can proceed exactly as if the yield expression were just another external call. The value of the yield expression after resuming depends on the method which resumed the execution. If __next__() is used (typically via either a for or the next() builtin) then the result is None. Otherwise, if send() is used, then the result will be the value passed in to that method.

    Yield expressions are allowed anywhere in a try construct. If the generator is not resumed before it is finalized (by reaching a zero reference count or by being garbage collected), the generator-iterator’s close() method will be called, allowing any pending finally clauses to execute.

    When the underlying iterator is complete, the value attribute of the raised StopIteration instance becomes the value of the yield expression. 

官方例子：

>>> def echo(value=None):
...     print("Execution starts when 'next()' is called for the first time.")
...     try:
...         while True:
...             try:
...                 value = (yield value)
...             except Exception as e:
...                 value = e
...     finally:
...         print("Don't forget to clean up when 'close()' is called.")
...
>>> generator = echo(1)
>>> print(next(generator))
Execution starts when 'next()' is called for the first time.
1
>>> print(next(generator))
None
>>> print(generator.send(2))
2
>>> generator.throw(TypeError, "spam")
TypeError('spam',)
>>> generator.close()
Don't forget to clean up when 'close()' is called.

模擬個iterator版range函數

def my_range(start, stop=None, step=1):
    if not stop:
        stop = start
        start = 0
    while start < stop:
        yield start
        start += step


if __name__ == '__main__':
    for i in my_range(10):
        print(i)
    for i in my_range(0, 10):
        print(i)
    for i in my_range(0, 10, 2):
        print(i)