Python recursively appending list function

Question

Having issues with a recursive function that should be relatively simple to do, but can`t seem to get right.

I have a folder structure with folders that can contain other folders, images or files. Associated with each folder there are permissions. I want to have my function recursively build a list of the permissions that are associated with each folder.

I have a function has_read_permission(request) that returns True if the folder has permissions and False if not

I have built a function like so:

def get_child_perms(self, folder, request, perm_list):
        # Folder contains other folders
        if folder.get_children():
            # For every sub-folder
            for subfolder in folder.get_children():
                return perm_list.append(self.get_child_perms(subfolder, request, perm_list))
        else:
            # If folder doesn't have sub-folders containing folders
            return [folder.has_read_permission(request)]

I keep getting None

Given folders like so:

Folder (allowed) - Wont check this one
|_First Folder (allowed)
| |_First sub Folder (restricted)
| | |_File
| | |_File
| | |_Image
| |__Second Sub Folder (allowed)
|_Second Folder (allowed)

Then running get_child_perms() would return [True,False,True,True] or even [True, [False, True], True]

EDIT

Disregard edit -> asked other question Python Recursive function missing results

Changed abit,

def get_child_perms(self, folder, request, perm_list):
        if folder.get_children():
            for subfolder in folder.get_children():
                perm_list.append(self.get_child_perms(subfolder, request, perm_list))
            return perm_list
        else:
            return [folder.has_read_permission(request)]

getting:

[[True], [...], [True], [...], [...], [True], [True], [True], [True], [True], [True], [True], [True], [...], [True], [...]]

 Admin
 -Folder 1
   - Files
 -Folder 2
   - Files
 -Folder 3
   - Files 
 -Folder 4
   - SubFolder 1
      -SubSubFolder 1
         - Files
      - Files
   - SubFolder 2
      - SubSubFolder 2
           - Files
      - Files
 -Folder 5
   - SubFolder 3
       - Files
   - SubFolder 4
       - Files
   - SubFolder 5
       -Files
   - Files
 -Folder 6
   - Files
 -Folder 7
   - SubFoler 6
       - Files
   - Files
 -Folder 8
   - Files

Try to answer this question, will the for loop's body be executed if folder.get_children() returns and empty iterable? — thefourtheye
– thefourtheye, Commented Aug 19, 2015 at 17:58
Are you sure you want to return inside the loop? That will make it exit after at most one iteration. — Ry-
– Ry- ♦, Commented Aug 19, 2015 at 18:00
I don't understand your latest edit. If someone here solved your problem (i.e., you are no longer getting None), you should mark an answer accepted. Ask a new question if you have one, but you can't just keep editing this one until the function works just how you want it. — Two-Bit Alchemist
– Two-Bit Alchemist, Commented Aug 19, 2015 at 18:21

Anshul Goyal · Accepted Answer · 2015-08-20 04:51:24Z

4

for subfolder in folder.get_children():
    perm_list.append(self.get_child_perms(subfolder, request, perm_list))
return perm_list

.append is in place. It does not return anything.So you are getting None

edited Aug 20, 2015 at 4:51

Anshul Goyal

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answered Aug 19, 2015 at 17:58

vks

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Comments

Two-Bit Alchemist · Accepted Answer · 2015-08-19 18:05:52Z

3

In-place list methods return None to remind you that they operate in place.

Instead of:

return permlist.append(self.get_child_perms(...  # etc.

Try:

permlist.append(self.get_child_perms(...
return permlist

Edited to add: As other comments/answers have pointed out, you'll want to return outside your for loop if you want it to complete.

edited Aug 19, 2015 at 18:05

answered Aug 19, 2015 at 17:58

Two-Bit Alchemist

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Comments

Padraic Cunningham · Accepted Answer · 2015-08-19 19:15:35Z

1

As pointed out append is an inplace operation that returns None, you can avoid using append at all by returning a list comp:

def get_child_perms(self, folder, request, perm_list):
    # Folder contains other folders
    children =  folder.get_children()
    if children:
        # For every sub-folder
        return [self.get_child_perms(subfolder, request, perm_list)
                for subfolder in children]
    return [folder.has_read_permission(request)]

You don't need an else either as you can only return once from the function.

edited Aug 19, 2015 at 19:15

answered Aug 19, 2015 at 18:15

Padraic Cunningham

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5 Comments

Philippe Fisher Over a year ago

This seems to work, but not returning values for nested folders, only returns values of folder that contain only files

Padraic Cunningham Over a year ago

Add a link to your code, I think there is a simpler way to do what you want

Philippe Fisher Over a year ago

dpaste.com/2ZYA3QB here the code for the plugin, but not sure what you want to see, since this is a plugin for django cms, which is based on django-filer.

Padraic Cunningham Over a year ago

Do you actually want boolean values returned based on permissions and only interested in folders?

Padraic Cunningham Over a year ago

I think you need to return [folder.has_read_permission(request)]+ [self.get_child_perms(subfolder..., you will need to flatten the the returned list if you want a flatten list, itertools.chain will do it

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