Django source: manage.py

#!/usr/bin/env python
import os
import sys

if __name__ == "__main__":
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mypro.settings")
    try:
        from django.core.management import execute_from_command_line
    except ImportError:
        # The above import may fail for some other reason. Ensure that the
        # issue is really that Django is missing to avoid masking other
        # exceptions on Python 2.
        try:
            import django
        except ImportError:
            raise ImportError(
                "Couldn't import Django. Are you sure it's installed and "
                "available on your PYTHONPATH environment variable? Did you "
                "forget to activate a virtual environment?"
            )
        raise
    execute_from_command_line(sys.argv)

• os.environ
  A mapping object representing the string environment.

For example, environ['HOME'] is the pathname of your home directory.

>>> import os
>>> os.environ['HOME']
'/home/alphacocoa'

• dict.setdefault(key[, default])
  If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.

>>> os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mypro.settings")
'mypro.settings'
>>> os.environ['DJANGO_SETTINGS_MODULE']
'mypro.settings'
>>> os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mypro")
'mypro.settings'              ; If key is in the dictionary, return its value.
>>> os.environ['DJANGO_SETTINGS_MODULE']
'mypro.settings'