"尝试直到没有异常被引发"的Python习惯用法

Question

我希望我的代码能够自动尝试多种方式来创建数据库连接.一旦工作,代码需要继续前进(即它不应该再尝试其他方式).如果它们都失败了,那么脚本就会爆炸.

所以在 - 我的想法,但很可能不是 - 天才的一击,我试过这个:

import psycopg2
from getpass import getpass

# ouch, global variable, ooh well, it's just a simple script eh
CURSOR = None

def get_cursor():
    """Create database connection and return standard cursor."""

    global CURSOR

    if not CURSOR:
        # try to connect and get a cursor
        try:
            # first try the bog standard way: db postgres, user postgres and local socket
            conn = psycopg2.connect(database='postgres', user='postgres')
        except psycopg2.OperationalError:
            # maybe user pgsql?
            conn = psycopg2.connect(database='postgres', user='pgsql')
        except psycopg2.OperationalError:
            # maybe it was postgres, but on localhost? prolly need password then
            conn = psycopg2.connect(database='postgres', user='postgres', host='localhost', password=getpass())
        except psycopg2.OperationalError:
            # or maybe it was pgsql and on localhost
            conn = psycopg2.connect(database='postgres', user='pgsql', host='localhost', password=getpass())

        # allright, nothing blew up, so we have a connection
        # now make a cursor
        CURSOR = conn.cursor()

    # return existing or new cursor
    return CURSOR

但似乎第二个和后续的except语句不再捕获OperationalErrors了.可能是因为Python只在try ... except语句中捕获一次异常？

是这样吗？如果没有:还有什么我做错了吗？如果是这样的话:你怎么做这样的事呢？有标准的习语吗？

(我知道有很多方法可以解决这个问题,比如让用户在命令行上指定连接参数,但这不是我的问题,确定:))

编辑:

我接受了retracile的优秀答案,我接受了gnibbler对使用for..else结构的评论.最后的代码变成了(抱歉,我并没有真正遵循pep8的每行最大字符数限制):

编辑2:正如您从Cursor类的注释中看到的那样:我真的不知道如何调用这种类.它不是真正的单例(我可以有多个不同的Cursor实例)但是在调用get_cursor时我每次都会得到相同的游标对象.那就像一个单身工厂？:)

import psycopg2
from getpass import getpass
import sys

class UnableToConnectError(Exception):
    pass

class Cursor:
    """Cursor singleton factory?"""

    def __init__(self):
        self.CURSOR = None

    def __call__(self):
        if self.CURSOR is None:
            # try to connect and get a cursor
            attempts = [
                    {'database': 'postgres', 'user': 'postgres'},
                    {'database': 'postgres', 'user': 'pgsql'},
                    {'database': 'postgres', 'user': 'postgres', 'host': 'localhost', 'password': None},
                    {'database': 'postgres', 'user': 'pgsql', 'host': 'localhost', 'password': None},
                    ]

            for attempt in attempts:
                if 'password' in attempt:
                    attempt['password'] = getpass(stream=sys.stderr) # tty and stderr are default in 2.6, but 2.5 uses sys.stdout, which I don't want
                try:
                    conn = psycopg2.connect(**attempt)

                    attempt.pop('password', None)
                    sys.stderr.write("Succesfully connected using: %s\n\n" % attempt)

                    break # no exception raised, we have a connection, break out of for loop
                except psycopg2.OperationalError:
                    pass
            else:
                raise UnableToConnectError("Unable to connect: exhausted standard permutations of connection dsn.")

            # allright, nothing blew up, so we have a connection
            # now make a cursor
            self.CURSOR = conn.cursor()

        # return existing or new cursor
        return self.CURSOR
get_cursor = Cursor()

Answer 1

大约:

attempts = [
    { 'database'='postgres', 'user'='pgsql', ...},
    { 'database'='postgres', 'user'='postgres', 'host'='localhost', 'password'=getpass()},
    ...
]
conn = None
for attempt in attempts:
    try:
        conn = psycopg2.connect(**attempt)
        break
    except psycopg2.OperationalError:
        pass
if conn is None:
    raise a ruckus
CURSOR = conn.cursor()

现在,如果你不想打电话,getpass()除非有必要,你想要检查if 'password' in attempt: attempt['password'] = getpass()左右.

现在关于全球......

class MyCursor:
    def __init__(self):
        self.CURSOR = None
    def __call__(self):
        if self.CURSOR is None:
            <insert logic here>
        return self.CURSOR

get_cursor = MyCursor()

......虽然我认为还有其他几种方法可以完成同样的事情.

将它们整合在一起:

class MyCursor:
    def __init__(self):
        self.CURSOR = None
    def __call__(self):
        if self.CURSOR is None:
            attempts = [
                {'database'='postgres', 'user'='postgres'},
                {'database'='postgres', 'user'='pgsql'},
                {'database'='postgres', 'user'='postgres', 'host'='localhost', 'password'=True},
                {'database'='postgres', 'user'='pgsql', 'host'='localhost', 'password'=True},
            ]
            conn = None
            for attempt in attempts:
                if 'password' in attempt:
                    attempt['password'] = getpass()
                try:
                    conn = psycopg2.connect(**attempt)
                    break # that didn't throw an exception, we're done
                except psycopg2.OperationalError:
                    pass
            if conn is None:
                raise a ruckus # nothin' worked
            self.CURSOR = conn.cursor()
        return self.CURSOR
get_cursor = MyCursor()

_{注意:完全未经测试}