奇怪的python 2.7 dict行为：关键不在里面，而明显在那里

Question

我编写了一个解析器，该解析器采用JSON配置并从中创建对象。 我首先创建一个众所周知的对象，并尝试动态导入一个模块（可能来自用户），同时通过该模块的定义的creator方法加载其类。

这是一些测试代码：

import json
import imp
import os.path as path
from lib.config.members import Member
from lib.tasks.task import Task


class Parser(object):

    def __init__(self):
        self._loadedMods = {"tasks": {}}

    def _load_module(self, clazz, modPart):
        """
        imports and caches a module.

        :param clazz: the filename of the module (i.e email, ping...)
        :param modPart: the folder of the module. (i.e services, parsers...)
        :return: the imported/cached module, or throws an error if it couldn't find it
        """
        mods = self._loadedMods[modPart]
        if clazz in mods:
            return mods["class"]
        else:
            #mod = __import__(clazz)

            p = path.join("lib", modPart, clazz + ".py")
            mod = imp.load_source(clazz, p)
            mods[clazz] = mod
            return mod

    def replace_with_import(self, objList, modPart, items_func, class_check):
        """
        replaces configuration dicts with their objects by importing and creating it in the first step.
        In the second step the original list of json config dicts gets replaced by the loaded objects

        :param objList: the list of objects which is iterated on
        :param modPart: the folder from the module (i.e tasks, parsers)
        :param items_func: function to get a pointer on the list of json-config-objects to replace. Takes one argument and
        should return a list of
        :param class_check: currently unsupported
        """
        for obj in objList:
            repl = []
            items = items_func(obj)
            for clazzItem in items:
                try:

                    clazz = clazzItem["class"]
                    mod = self._load_module(clazz, modPart)
                    item = mod.create(clazzItem)
                    if class_check(item):
                        repl.append(item)
                    else:
                        print " ignoring class " + clazzItem["class"] + "! It does not pass the class check!"

                except ImportError, err:
                    print "could not import " + clazz + ": " + str(clazzItem) + "! reason:"
                    print str(err)
                except KeyError, k:
                    print "Key " + str(k) + " not in classItem " + str(clazzItem)
                except Exception, e:
                    print "Error while replacing class ( " + clazz + " :" + str(e) + ")"

            del items[:]
            items.extend(repl)

    def _create_raw_Object(self, jsonDict, msgName, creator):
        """
        creates an Main object from the configuration, but just parses raw data and hands it to the object

        :param jsonDict: the configuration file part as dict
        :param msgName: name of object for error message
        :param creator: function pointer which is taking two arguments: identifier of the object and arguments.
        :should return an object
        :return: a list of objects returned by creator
        """
        items = []
        for key, val in jsonDict.items():
            try:
                item = creator(key, val)
                items.append(item)
            except Exception, e:
                print "ignoring " + msgName + ": " + key + "! reason:"
                print str(e)
        return items

jsonFile = '''
{
    "members":{
        "homer":{
            "name": "Homer Simpson",
            "comment": "Security Inspector",
            "tasks": [{"class":"email", "type": "donut", "args": {"rcpt": "homer_j_simpson@burnscorp.sp"}},
            {"class":"email", "type": "do", "args": {"rcpt": "my_other_mail@burnscorp.sp"}}]
        }
    }
}
'''

jsonDict = json.loads(jsonFile)

parser = Parser()

creator = lambda name, values: Member(name, **values)
members = parser._create_raw_Object(jsonDict["members"], "Members", creator)

items_func = lambda member: member.get_tasks()
class_check = lambda task: isinstance(task, Task)
parser.replace_with_import(members, "tasks", items_func, class_check)

for d in members:
    print d.__dict__

如您所见，一个Member可以有一个列表，这些列表中的任务是任意的，并且应该在其class属性中定义要导入的任务，但是一旦其中两个具有相同的类值（不应中断json我们定义它的方式）我得到一个奇怪的KeyError ：

Key 'class' not in classItem {u'args': {u'rcpt': u'my_other_mail@burnscorp.sp'}, u'type': u'do', u'class': u'email'}

为什么会出现这个奇怪的错误？ 非常高兴能给我任何提示的任何提示，因为我感到绝望，将其调试了几个小时。

我认为Member和Email / Task类是不相关的，但为了完整起见，我会发布它们：

lib / config / members.py

class Member:
    def __init__(self, id, name="",  comment="", tasks=None):
        self.id = id
        self.name = name
        self.tasks = []
        self.add_task(tasks)
        self.comment = comment

    def get_id(self):
        return self.id

    def add_task(self, task):
        if task is None:
            return
        if isinstance(task, list):
            self.tasks.extend(task)
        else:
            self.tasks.append(task)

    def get_tasks(self):
        return self.tasks

lib / tasks / [task | email] .py

class Task:
    """
    Base class for all built-in Tasks.
    """

    def set_task_type(self, taskType):
        """
        sets the type of this task.

        Be aware! this method can only get called once!

        :param taskType: the type of this task
        """
        if hasattr(self, "_taskType"):
            raise Exception("taskType is only allowed to set once!")
        self.taskType = taskType  

    def get_task_type(self):
        """
        :return: the type set by set_type_task
        """
        return self._taskType

"""
The email task.
"""

from lib.tasks.task import Task


class EmailTask(Task):
    def __init__(self, **kwargs):
        self.set_task_type(kwargs["type"])
        self.recipient = kwargs["args"]["rcpt"]

    def execute_task(self, msg):
        pass

def create(taskDict):
    return EmailTask(**taskDict)

Answer 1

看来您正在通过在replace_with_import中用自己的自定义print替换它来吃掉实际的异常。 正如我在评论部分所指出的。

通常，您希望使try块变小并且非常可预测，确切地知道可以引发什么以及应该在代码中处理什么。 try块中的复杂度越低越好。