Python 3.5.2错误UndefinedValueError

Question

我的应用程序配置为使用mysql，但是当我尝试运行时./manage.py迁移此错误ocours

root@renato-VirtualBox:/home/renato/unb-alerta# ./manage.py migrate
Traceback (most recent call last):
File "./manage.py", line 10, in <module>
execute_from_command_line(sys.argv)
 File "/home/renato/unb-alerta/venv/lib/python3.5/site-   packages/django/core/management/__init__.py", line 367, in    execute_from_command_line
 utility.execute()
  File "/home/renato/unb-alerta/venv/lib/python3.5/site- packages/django/core/management/__init__.py", line 316, in execute
  settings.INSTALLED_APPS
  File "/home/renato/unb-alerta/venv/lib/python3.5/site-   packages/django/conf/__init__.py", line 53, in __getattr__
  self._setup(name)
  File "/home/renato/unb-alerta/venv/lib/python3.5/site-packages/django/conf/__init__.py", line 41, in _setup
  self._wrapped = Settings(settings_module)
  File "/home/renato/unb-alerta/venv/lib/python3.5/site- packages/django/conf/__init__.py", line 97, in __init__
  mod = importlib.import_module(self.SETTINGS_MODULE)
  File "/home/renato/unb- alerta/venv/lib/python3.5/importlib/__init__.py", line 126, in  import_module
  return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in    _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
 File "/home/renato/unb-alerta/unb_alerta/settings.py", line 139, in  <module>
  cast=db_url,
  File "/home/renato/unb-alerta/venv/lib/python3.5/site- packages/decouple.py", line 197, in __call__
  return self.config(*args, **kwargs)
  File "/home/renato/unb-alerta/venv/lib/python3.5/site-  packages/decouple.py", line 77, in __call__
  return self.get(*args, **kwargs)
  File "/home/renato/unb-alerta/venv/lib/python3.5/site- packages/decouple.py", line 64, in get
 raise UndefinedValueError('%s option not found and default value was  not defined.' % option)
decouple.UndefinedValueError: DATABASE_URL option not found and  default value was not defined.

我的.env有DATABASE_URL结束我的settings.py有这个配置

DATABASES = {
   'default': config(
     'DATABASE_URL',
      cast=db_url,
   )
}