# Filter

顾名思义，`filter` 过滤列表中的元素，并且返回一个由所有符合要求的元素所构成的列表，`符合要求`即函数映射到该元素时返回值为True。这里是一个简短的例子：

```python
number_list = range(-5, 5)
less_than_zero = filter(lambda x: x < 0, number_list)
print(list(less_than_zero))  
# 译者注：上面print时，加了list转换，是为了python2/3的兼容性
#        在python2中filter直接返回列表，但在python3中返回迭代器
#        因此为了兼容python3, 需要list转换一下

# Output: [-5, -4, -3, -2, -1]
```

这个 `filter` 类似于一个 `for` 循环，但它是一个内置函数，并且更快。

注意：如果 `map` 和 `filter` 对你来说看起来并不优雅的话，那么你可以看看另外一章：列表/字典/元组推导式。

> 译者注：大部分情况下推导式的可读性更好。


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