Question

我只是乞求学习python（来自C）；我发现自己对于定义属于一个类的变量的方式感到非常困惑（尽管不在乎“类类型”还是“实例类型”，我的问题更基本）。到目前为止，我已经尝试了以下代码行：

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'accounts.apps.AccountsConfig',
    'offense',
    'background_task',


]


Exception in thread django-main-thread:
Traceback (most recent call last):
  File "c:\users\kiran.tanweer\appdata\local\programs\python\python37-32\Lib\threading.py", line 917, in _bootstrap_inner
    self.run()
  File "c:\users\kiran.tanweer\appdata\local\programs\python\python37-32\Lib\threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\utils\autoreload.py", line 54, in wrapper
    fn(*args, **kwargs)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\core\management\commands\runserver.py", line 117, in inner_run
    self.check(display_num_errors=True)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\core\management\base.py", line 390, in check
    include_deployment_checks=include_deployment_checks,
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\core\management\base.py", line 377, in _run_checks
    return checks.run_checks(**kwargs)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\core\checks\registry.py", line 72, in run_checks
    new_errors = check(app_configs=app_configs)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\core\checks\urls.py", line 13, in check_url_config
    return check_resolver(resolver)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\core\checks\urls.py", line 23, in check_resolver
    return check_method()
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\urls\resolvers.py", line 398, in check
    for pattern in self.url_patterns:
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\utils\functional.py", line 80, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\urls\resolvers.py", line 579, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\utils\functional.py", line 80, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\site-packages\django\urls\resolvers.py", line 572, in urlconf_module
    return import_module(self.urlconf_name)
  File "C:\Users\kiran.tanweer\Envs\dash\lib\importlib\__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "C:\Users\kiran.tanweer\Documents\dashboard-latest\telenor\telenor\urls.py", line 26, in <module>
    from . import views
  File "C:\Users\kiran.tanweer\Documents\dashboard-latest\telenor\telenor\views.py", line 425, in <module>
    from background_task import background
ImportError: cannot import name 'background' from 'background_task' (unknown location)

此代码令人惊讶地返回“ 3”（而我预料到错误），这将我带到了这里。我不明白为什么解释器能够推断出属于“ A”类的变量（我从未定义过）。

Answer 1

从C的角度来看，想象一下Python属性存储在哈希表中，因此可以在运行时动态地对其进行修改和查询。

考虑一下：

class A:
    pass

print(A.__dict__)
{'__module__': '__main__', '__dict__': <attribute '__dict__' of 'A' objects>, '__weakref__': <attribute '__weakref__' of 'A' objects>, '__doc__': None}

像您一样修改A时，您正在修改其__dict__ member：

A.a = 1

print(A.__dict__)
{'__module__': '__main__', '__dict__': <attribute '__dict__' of 'A' objects>, '__weakref__': <attribute '__weakref__' of 'A' objects>, '__doc__': None, 'a': 1}

请注意，它现在如何包含a

print(A.__dict__['a'])
1

属于类的变量定义

1 个答案: