smonad 0.6

什么？

python中的monad，以及一些有用的函数。

这个包裹是Philip Xu的优质叉子。 Philip的包经过了修改，使之更适合Python开发人员。它 从优秀的lambda图书馆获得一些灵感 对于java和scala的Try monad

python开发人员可能会发现try monad特别有用，因为它允许您处理错误 作为价值观。

为什么？

python没有处理多个 批量操作。此外，这个图书馆的作者发现过度依赖 on类会导致代码的可组合性降低。

这个库借鉴了haskell和其他函数式编程的一些思想。 更好地处理上述用例的语言。而这个图书馆 以“monad”的名字命名，你不需要知道 单子的概念以便使用这个库。

将错误视为值

python没有将错误作为值处理的内置约定 除了尝试/除外。

smonad引入了包装failed或 容器类中的成功结果。

实用程序方法attempt执行可调用并包装引发的异常 在失败的课堂上。如果未引发异常，则返回成功

>>> from smonad.actions import attempt
>>> result = attempt(lambda: 1 / 0)
>>> print result
Failure(ZeroDivisionError('integer division or modulo by zero',))
>>> exc = result.value
>>> exc
ZeroDivisionError('integer division or modulo by zero',)
>>> # the following would fail as it does not catch the correct exception
>>> # result = attempt(lambda: 1 / 0, exception=ValueError)
>>> result = attempt(lambda: 1 / 1)
>>> print result
Success(1)
>>> result.value
1

您可以在自己的代码中实例化失败或成功，以指示 计算是否成功。

下面是创建使用失败/成功的aws实例的示例脚本 将故障传播到尽可能高的级别。

from collections import namedtuple
from smonad.types.ftry import Success, Failure
from smonad.utils import failed
import sys
import os


CloudInstance = namedtuple('CloudInstance', ['name', 'provider', 'instance_type'])

jenkins = CloudInstance(name='jenkins', provider='aws', instance_type='m3.medium')

git = CloudInstance(name='git', provider='aws', instance_type='m3.xlarge')

vault = CloudInstance(name='vault', provider='aws', instance_type='m3.xlarge')


class TryAgainLaterError(Exception):
    pass


def aws_create_instance(instance):
    if instance.name == 'jenkins':
        raise TryAgainLaterError("Can't create jenkins server right now")

    return instance


def create_cloud_instance(instance):
    try:
        created_instance = aws_create_instance(instance)
        return Success(created_instance)
    except TryAgainLaterError as e:
        return Failure(instance)


def make_my_servers():
    server_results = []
    for server in [jenkins, git, vault]:
        server_results.append(create_cloud_instance(server))

    # if there are any errors, let's retry once more
    failed_servers = filter(lambda r: failed(r), server_results)
    retry_results = []
    for exc, instance in failed_servers:
        retry_results.append(aws_create_instance(instance))

    if any([failed(i) for i in retry_results]):
        failed_servers = ",".join([i.value.name for i in retry_results if failed(i)])
        return Failure("Unable to create servers: %s" % failed_servers)

    return Success("Successfully created all servers")


if __name__ == "__main__":
    result = make_my_servers()
    if failed(result):
        sys.stderr.write("Error: %s\n" % result.value)
        os.sys.exit(1)
    else:
        print result.value

我们可以利用 在Try的recover方法中。recover应用恢复功能 失败的例子。它返回成功（v）不变。

def make_my_servers():
    server_results = []
    for server in [jenkins, git, vault]:
        server_results.append(create_cloud_instance(server))


    # The recover only applies ``create_cloud_instance`` to Failures, it returns the Success value otherwise
    server_results = map(lambda s: s.recover(create_cloud_instance), server_results)

    if any([failed(s) for s in server_results]):
        failed_servers = ",".join([i.value.name for i in server_results if failed(i)])
        return Failure("Unable to create servers: %s" % failed_servers)

    return Success("Successfully created all servers")

组成函数

>>> from smonad.decorators import maybe
>>> parse_int = maybe(int)
>>> parse_int(42)
Just(42)
>>> parse_int('42')
Just(42)
>>> parse_int('42.2')
Nothing

>>> parse_float = maybe(float)
>>> parse_float('42.2')
Just(42.2)

>>> from smonad.actions import tryout
>>> parse_number = tryout(parse_int, parse_float)
>>> tokens = [2, '0', '4', 'eight', '10.0']
>>> [parse_number(token) for token in tokens]
[Just(2), Just(0), Just(4), Nothing, Just(10.0)]

>>> @maybe
... def reciprocal(n):
...     return 1. / n
>>> reciprocal(2)
Just(0.5)
>>> reciprocal(0)
Nothing

>>> process = parse_number >> reciprocal
>>> process('4')
Just(0.25)
>>> process('0')
Nothing
>>> [process(token) for token in tokens]
[Just(0.5), Nothing, Just(0.25), Nothing, Just(0.1)]
>>> [parse_number(token) >> reciprocal for token in tokens]
[Just(0.5), Nothing, Just(0.25), Nothing, Just(0.1)]
>>> [parse_number(token) >> reciprocal >> reciprocal for token in tokens]
[Just(2.0), Nothing, Just(4.0), Nothing, Just(10.0)]