using try-else block in actual case

fields.py

# South support; see http://south.aeracode.org/docs/tutorial/part4.html#simple-inheritance
try:
    from south.modelsinspector import add_introspection_rules
except ImportError:
    pass
else:
    add_introspection_rules([], [r"^picklefield\.fields\.PickledObjectField"])

test_database.py

try:
    import sqlalchemy  # noqa
except ImportError:
    DatabaseBackend = Task = TaskSet = None
else:
    from celery.backends.database import DatabaseBackend
    from celery.db.models import Task, TaskSet