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。只要一个人工作，代码就需要继续前进（即它不应该再尝试其他方式）。如果它们都失败了，那么剧本就会爆炸。

所以 - 我认为是什么，但很可能不是 - 天才中风我试过这个：

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？

是吗？是这样吗？如果没有：还有什么我做错了吗？如果是这样的话：你怎么做这样的事情呢？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中指定连接参数，但那不是我的问题，确定:)）

修改

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中每行的最大字符数）：

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

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，如果您不想在必要时拨打getpass()，则需要检查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。

将所有这些结合在一起：

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}

Answer 2

你很亲密。在这种情况下，最好的做法是在except块中嵌套第二次和后续的尝试。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())

“尝试直到没有异常被引发”的Python习惯用法

2 个答案: