如何合并元组或将列表转换为元组？

Question

I have a tuple, INSTALLED_APPS, and I want to merge extra items into it to get my local apps in the tuple too without altering the main settings file. I came this far:

DEFAULT_APPS = list(INSTALLED_APPS)
MY_APPS_LIST = DEFAULT_APPS.append('south')

however if I try to convert this into a tuple again by running:

INSTALLED_APPS = tuple(MY_APPS_LIST)

I get:

TypeError: 'NoneType' object is not iterable

The question is rather fundamental I'd say, but I can't really find "the" method for this, or even any method that works for me at all... I did find that both list() and tuple() in the Django shell return an empty object of that type, so I don't understand where the TypeError might be coming from...

Help appreciated!

Answer 1

append doesn't return a new list -- it modifies the original list, returning None . You want MY_APPS_LIST = DEFAULT_APPS + ['south']

Answer 2

list methods operate in-place, and hence return None . If you want to add another element to an existing list and only return the result then just add them.

MY_APPS_LIST = DEFAULT_APPS + ['south']

Answer 3

note that one can use + to concatenate tuples, so if INSTALLED_APPS is a tuple:

INSTALLED_APPS = DEFAULT_APPS + ('south',)

INSTALLED_APPS will be a tuple with 'south' as its last element

Answer 4

The list.append method does NOT return the list you appended to. It is an in-place method. You should be doing

DEFAULT_APPS = list(INSTALLED_APPS)
DEFAULT_APPS.append('south')
MY_APPS_LIST = DEFAULT_APPS