60 lines
1.7 KiB
Markdown
60 lines
1.7 KiB
Markdown
|
---
|
|||
|
|
title: Generators
|
|||
|
|
localeTitle: 发电机
|
|||
|
|
---
|
|||
|
|
## 发电机
|
|||
|
|
|
|||
|
|
生成器是一种特殊类型的函数，它允许您在不结束函数的情况下返回值。它通过使用`yield`关键字来实现。与`return`类似， `yield`表达式将向调用者返回一个值。两者之间的关键区别在于`yield`将暂停该函数，允许将来返回更多值。
|
|||
|
|
|
|||
|
|
生成器是可迭代的，因此它们可以与for循环或任何其他迭代一起使用。
|
|||
|
|
|
|||
|
|
```python
|
|||
|
|
def my_generator():
|
|||
|
|
yield 'hello'
|
|||
|
|
yield 'world'
|
|||
|
|
yield '!'
|
|||
|
|
|
|||
|
|
for item in my_generator():
|
|||
|
|
print(item)
|
|||
|
|
|
|||
|
|
# output:
|
|||
|
|
# hello
|
|||
|
|
# world
|
|||
|
|
# !
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
与其他迭代器一样，生成器可以传递给`next`函数以检索下一个项目。当生成器没有更多值可以生成时，会引发`StopIteration`错误。
|
|||
|
|
|
|||
|
|
```python
|
|||
|
|
g = my_generator()
|
|||
|
|
print(next(g))
|
|||
|
|
# 'hello'
|
|||
|
|
print(next(g))
|
|||
|
|
# 'world'
|
|||
|
|
print(next(g))
|
|||
|
|
# '!'
|
|||
|
|
print(next(g))
|
|||
|
|
# Traceback (most recent call last):
|
|||
|
|
# File "<stdin>", line 1, in <module>
|
|||
|
|
# StopIteration
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
当您需要创建大量值但不需要将它们全部保存在内存中时，生成器特别有用。例如，如果您需要打印第一百万个斐波那契数字，通常会返回一百万个值的列表并迭代列表以打印每个值。但是使用生成器，您可以一次返回一个值：
|
|||
|
|
|
|||
|
|
```python
|
|||
|
|
def fib(n):
|
|||
|
|
a = 1
|
|||
|
|
b = 1
|
|||
|
|
for i in range(n):
|
|||
|
|
yield a
|
|||
|
|
a, b = b, a + b
|
|||
|
|
|
|||
|
|
for x in fib(1000000):
|
|||
|
|
print(x)
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
### 更多信息
|
|||
|
|
|
|||
|
|
* [PEP 255](https://www.python.org/dev/peps/pep-0255/)
|
|||
|
|
* [Python Wiki](https://wiki.python.org/moin/Generators)
|
|||
|
|
* [收益表文件](https://docs.python.org/2/reference/simple_stmts.html#yield)
|