Python中处理由其他库导入的库引发异常的最佳实践是什么？

Question

在Python中处理从其他库导入的库引发异常的适当方法是什么？

例如，我有一个名为“pycontrol”的库，它被导入到我的主程序中。 “pycontrol”导入了“suds”库。而“suds”库则导入了“urllib2”库。我注意到当“suds”库在访问通过“urllib2”访问的远程资源时遇到问题时，这些异常会向上传递到我的主程序中。

我目前最好的猜测是将urllib2和suds导入到全局命名空间中，并捕获它们抛出的典型异常，以及在“pycontrol”中未处理的异常。

还有其他最佳实践吗，可以用来解决这个问题？

以下是代码片段的基本想法（不导入suds或urllib2到全局命名空间）：

    import pycontrol.pycontrol as pc

    print "Connecting to iControl API on LTM %s..." % ltm
    try:
        b = pc.BIGIP(hostname=ltm, username=user, password=pw, 
            wsdls=wsdl_list, fromurl=True,
            debug=soap_debug)
    except (<whattocatch>), detail:
        print "Error: could not connect to iControl API on LTM %s... aborting!" % ltm
        print "Details: %s" % detail
        exitcode = 1
    else:
        print "Connection successfully established."

以下是一个示例回溯：

Connecting to iControl API on LTM s0-bigip1-lb2.lab.zynga.com...
Traceback (most recent call last):
  File "./register.py", line 507, in <module>
    main()
  File "./register.py", line 415, in main
    b = build_bigip_object(ltm, user, pw, WSDLS, soap_debug = False)
  File "./register.py", line 85, in build_bigip_object
    debug=soap_debug)
  File "build/bdist.macosx-10.6-universal/egg/pycontrol/pycontrol.py", line 81, in __init__
  File "build/bdist.macosx-10.6-universal/egg/pycontrol/pycontrol.py", line 103, in _get_clients
  File "build/bdist.macosx-10.6-universal/egg/pycontrol/pycontrol.py", line 149, in _get_suds_client
  File "/Library/Python/2.6/site-packages/suds/client.py", line 111, in __init__
    self.wsdl = reader.open(url)
  File "/Library/Python/2.6/site-packages/suds/reader.py", line 136, in open
    d = self.fn(url, self.options)
  File "/Library/Python/2.6/site-packages/suds/wsdl.py", line 136, in __init__
    d = reader.open(url)
  File "/Library/Python/2.6/site-packages/suds/reader.py", line 73, in open
    d = self.download(url)
  File "/Library/Python/2.6/site-packages/suds/reader.py", line 88, in download
    fp = self.options.transport.open(Request(url))
  File "/Library/Python/2.6/site-packages/suds/transport/https.py", line 60, in open
    return  HttpTransport.open(self, request)
  File "/Library/Python/2.6/site-packages/suds/transport/http.py", line 62, in open
    return self.u2open(u2request)
  File "/Library/Python/2.6/site-packages/suds/transport/http.py", line 118, in u2open
    return url.open(u2request, timeout=tm)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 383, in open
    response = self._open(req, data)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 401, in _open
    '_open', req)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 361, in _call_chain
    result = func(*args)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 1138, in https_open
    return self.do_open(httplib.HTTPSConnection, req)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 1105, in do_open
    raise URLError(err)
urllib2.URLError: <urlopen error [Errno 8] nodename nor servname provided, or not known>

- perfectfromnowon

Answer 1

我认为你已经回答了自己的问题。在你的模块中导入urllib2并捕获异常。

from urllib2 import URLError

try:
    # something
except URLError, e:
    # Do something in case of error.

- badzil

---

这就是我想到的（并一直在做的）。我猜我认为有一种更“Pythonic”的处理方式。看起来我也不应该担心是什么引发了异常，只需捕获我尝试使用的库（pycontrol）调用中抛出的任何异常即可。 - perfectfromnowon

不要试图捕获你无法处理的异常。如果你对 ValueError 这类异常无法做出任何有用的处理，那么捕获它也没有意义。 - Ethan Furman

Answer 2

为什么需要捕获特定的异常呢？毕竟，从b = pc.BIGIP(...)引发的任何异常（不仅限于URLError）都意味着您无法继续执行。

我建议：

import traceback

try:
    b = pc.BIGIP(...)
except:
    traceback.print_exc()
    exitcode = 1
else:
    do_something_with_connection(b)

另一个想法：为什么要费心去捕获异常呢？当Python解释器遇到未处理的异常时，它会将堆栈跟踪转储到stderr并退出程序：

b = bc.BIGIP(...)
do_something_with_connection(b)

或者如果你需要写入错误日志：

import logging
import sys

def main():
   b = bc.BIGIP(...)
   do_something_with_connection(b)

if __name__ == "__main__":
    try:
        main()
    except:
        logging.exception("An unexpected error occured")
        sys.exit(1)

- codeape

---

感谢您提供的好建议。异常本身实际上可以传达有关远程API发生了什么的有用信息。有时，这些异常实际上并非致命的，我只需要在远程API操作失败时执行一些逻辑即可。 - perfectfromnowon