Python的习惯用法'；尝试直到没有引发异常'；

我希望我的代码自动尝试多种方法来创建数据库连接。一旦一种方法起作用，代码就需要继续前进（即，它不应该再尝试其他方法）。如果它们都失败了，那么脚本可能会崩溃

所以，在-我认为是，但很可能不是-一个天才的笔触中，我尝试了这个：

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

但第二个和后续的except语句似乎不再捕捉操作错误。可能是因为Python在try…except语句中只捕获一次异常

是这样吗？如果不是：还有什么我做错了吗？如果是这样的话：那你是怎么做的？有标准的成语吗

（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，但这不是我的问题好：）

编辑：

我接受了retracile的优秀回答，并接受了gnibbler使用for..else结构的评论。最后的代码是（对不起，我没有真正遵循pep8中每行最多字符数的限制）：

编辑2:正如您在Cursor类的注释中所看到的：我真的不知道如何调用这种类。它不是一个真正的单例（我可以有多个不同的游标实例），但是当调用get_Cursor时，每次都会得到相同的游标对象。所以它就像一个单身工厂？：）

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，如果您不想调用

getpass（）

，除非有必要，那么您应该检查

在尝试中的'password'是否为：trunt['password']=getpass（）

等等

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()

---

注意：完全未经测试

您已接近成功。在这种情况下，最好的做法可能是在except块中嵌套第二次和后续尝试。因此，代码的关键部分如下所示：

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?
        try:
            conn = psycopg2.connect(database='postgres', user='pgsql')
        except psycopg2.OperationalError:
            # maybe it was postgres, but on localhost? prolly need password then
            try:
                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())

---

不必将

conn

初始化为

None

，您只需使用

上的
else
子句，用于
+1@David:中断可确保返回第一个工作的连接。随着数据库数量的增加，此解决方案可以很好地扩展，并且连接字符串已经很好地从代码中抽象出来。@gnibbler-Woah，我忘了Python有那个很棒的功能（）@retracile，你是对的，出于某种原因，我认为中断将一事无成。@David：没问题；如果你不愿意，我不会告诉任何人；）-呃，完全不是蟒蛇。使您看起来像C++ COM程序员。
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()

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?
        try:
            conn = psycopg2.connect(database='postgres', user='pgsql')
        except psycopg2.OperationalError:
            # maybe it was postgres, but on localhost? prolly need password then
            try:
                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())