python - 运行函数时导入的模块变为 None

Question

更新：在这篇文章的底部有更多的调试信息，它揭示了 python 状态中的一些非常奇怪的东西。

我有一个模块，它可以导入 django User 对象等。

导入工作正常，代码加载。但是，当您在该模块中调用使用 User 对象的函数时，它会错误地指出 User 是 NoneType。

还有一些其他的导入，以及一些模块级别的全局变量，在调用函数时它们也是 None 。

奇怪的是，这只是我们的暂存环境（Ubuntu 12.04）中的一个问题。它在本地运行良好，这可能最类似于使用额外的 python 包进行开发工作的登台。生产上也很好。

有没有人遇到过这种情况，并且有任何想法可能导致它？

这是代码：

import urllib
import time
import urlparse

# Django imports
from django.db.models.signals import post_delete
from django.db import models
from django.contrib.auth.models import User

from backends.cache.dualcache import cache

# Piston imports
from managers import TokenManager, ConsumerManager
from signals import consumer_post_delete

KEY_SIZE = 18
SECRET_SIZE = 32
VERIFIER_SIZE = 10

CONSUMER_STATES = (
    ('pending', 'Pending'),
    ('accepted', 'Accepted'),
    ('canceled', 'Canceled'),
    ('rejected', 'Rejected')
)


def generate_random(length=SECRET_SIZE):
    return User.objects.make_random_password(length=length)


class Consumer(models.Model):
    name = models.CharField(max_length=255)
    description = models.TextField()

    key = models.CharField(max_length=KEY_SIZE)
    secret = models.CharField(max_length=SECRET_SIZE)

    status = models.CharField(max_length=16, choices=CONSUMER_STATES, default='pending')

    objects = ConsumerManager()

    def __unicode__(self):
        return u"Consumer %s with key %s" % (self.name, self.key)

    def generate_random_codes(self):
        key = User.objects.make_random_password(length=KEY_SIZE)
        secret = generate_random(SECRET_SIZE)

        while Consumer.objects.filter(key__exact=key, secret__exact=secret).count():
            secret = generate_random(SECRET_SIZE)

        self.key = key
        self.secret = secret
        self.save()

这是解决方法，这基本上意味着在函数中再次导入您需要的内容：

import urllib
import time
import urlparse

# Django imports
from django.db.models.signals import post_delete
from django.db import models
from django.contrib.auth.models import User

from backends.cache.dualcache import cache

# Piston imports
from managers import TokenManager, ConsumerManager
from signals import consumer_post_delete

KEY_SIZE = 18
SECRET_SIZE = 32
VERIFIER_SIZE = 10

CONSUMER_STATES = (
    ('pending', 'Pending'),
    ('accepted', 'Accepted'),
    ('canceled', 'Canceled'),
    ('rejected', 'Rejected')
)


def generate_random(length=SECRET_SIZE):
    return User.objects.make_random_password(length=length)


class Consumer(models.Model):
    name = models.CharField(max_length=255)
    description = models.TextField()

    key = models.CharField(max_length=KEY_SIZE)
    secret = models.CharField(max_length=SECRET_SIZE)

    status = models.CharField(max_length=16, choices=CONSUMER_STATES, default='pending')

    objects = ConsumerManager()

    def __unicode__(self):
        return u"Consumer %s with key %s" % (self.name, self.key)

    def generate_random_codes(self):
        from piston.models import KEY_SIZE, SECRET_SIZE, Consumer
        from django.contrib.auth.models import User
        from piston.models import generate_random

        key = User.objects.make_random_password(length=KEY_SIZE)
        secret = generate_random(SECRET_SIZE)

        while Consumer.objects.filter(key__exact=key, secret__exact=secret).count():
            secret = generate_random(SECRET_SIZE)

        self.key = key
        self.secret = secret
        self.save()

这是堆栈跟踪。该错误是由以下行引起的：

key = User.objects.make_random_password(length=KEY_SIZE)

在 generate_random_codes 函数中。

Traceback:
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in wrapper
  366.                 return self.admin_site.admin_view(view)(*args, **kwargs)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
  89.         response = view_func(request, *args, **kwargs)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/contrib/admin/sites.py" in inner
  196.             return view(request, *args, **kwargs)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapper
  25.             return bound_func(*args, **kwargs)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/utils/decorators.py" in bound_func
  21.                 return func(self, *args2, **kwargs2)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/db/transaction.py" in inner
  224.                 return func(*args, **kwargs)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in add_view
  970.             form = ModelForm(initial=initial)
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/forms/models.py" in __init__
  234.             self.instance = opts.model()
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/db/models/base.py" in __init__
  349.                 val = field.get_default()
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/db/models/fields/related.py" in get_default
  983.         field_default = super(ForeignKey, self).get_default()
File "/sites/tellybug/shared/webserver/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py" in get_default
  379.                 return self.default()
File "/sites/tellybug/releases/b92109dd526607b2af92ad6b7f494f3f06e31bb2/webserver/tellybug/tbapp/models/tellybugapp.py" in generate_new_consumer
  11.     consumer.generate_random_codes()
File "/sites/tellybug/releases/b92109dd526607b2af92ad6b7f494f3f06e31bb2/webserver/tellybug/piston/models.py" in generate_random_codes
  57.   key = User.objects.make_random_password(length=KEY_SIZE)

Exception Type: AttributeError at /admin/tbapp/tellybugapp/add/
Exception Value: 'NoneType' object has no attribute 'objects'

更新：这不仅仅是删除 User 对象 - 有些东西正在破坏函数中的整个上下文。

def generate_random_codes(self):
    """
    Used to generate random key/secret pairings. Use this after you've
    added the other data in place of save().

    c = Consumer()
    c.name = "My consumer"
    c.description = "An app that makes ponies from the API."
    c.user = some_user_object
    c.generate_random_codes()
    """
    import sys
    print "Globals", globals()
    print "Name ", __name__
    print "Package ", __package__
    print "Sys modules", sys.modules['piston.models'].__dict__
    key = User.objects.make_random_password(length=KEY_SIZE)

使用这些打印语句，输出为：

Globals {'ColumnFamilyMap': None, 'datetime': None, 'KEY_SIZE': None, 'TokenManager': None, 'ConsistencyLevel': None, 'Nonce': None, 'uuid': None, 'cache': None, 'urllib': None, '__package__': None, 'models': None, 'User': None,  .... }
Name  None
Package  None
Sys modules {'ColumnFamilyMap': <class 'pycassa.columnfamilymap.ColumnFamilyMap'>, 'datetime': <type 'datetime.datetime'>, 'KEY_SIZE': 18, 'NonceType': <class 'piston.models.NonceType'>, 'OAuthToken': <class 'piston.models.OAuthToken'>, 'TokenManager': <class 'piston.managers.TokenManager'>, 'ConsistencyLevel': <class 'pycassa.cassandra.ttypes.ConsistencyLevel'>, 'Nonce': <class 'piston.models.Nonce'>, 'uuid': <module 'uuid' from '/usr/lib/python2.7/uuid.pyc'>,  ...}

请注意，两者__package__和__name__都是未定义的，我认为这几乎是不可能的，并且虽然模块的 sys.modules 版本具有正确__dict__的，但 from 的返回值globals()是无意义的。

score 10 · Accepted Answer

这发生在导入模块中的函数在该模块被垃圾回收后仍在执行。

由于您的代码不足以重现该问题，因此这是一个显示行为的简化示例。创建一个包含以下内容的文件并从 Python 命令行或从另一个文件导入它。如果你只是在顶层运行它是行不通的。

import sys
import threading

x = "foo"

def run():
    while True:
        print "%s %s\n" % (sys, x)

threading.Thread(target = run).start()
sys.stdin.readline()

运行它：

$ python
>>> import evil_threading
<module 'sys' (built-in)> foo

<module 'sys' (built-in)> foo
... press Ctrl-C
None None

None None
... press Ctrl-\ to kill the Python interpreter

在 Python 关闭期间，模块设置为None. 这是在 3.4 中删除的晦涩 Python 行为。在此示例中，终止主线程会导致关闭，但另一个线程仍在运行，因此它将模块视为None.

这里有一个更简单的例子，它通过直接从 sys.modules 中删除模块引用来做同样的事情。

import sys
print sys
del sys.modules['__main__']
print sys

python - 运行函数时导入的模块变为 None

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