return a flattened list from a list comprehension of a nested list

Question

I am trying to create a flattened list of all users that belong to a linux group.
The structure returning from grp.getgrall() is:

[
  grp.struct_group(
                  gr_name='plugdev',
                  gr_passwd='x',
                  gr_gid=46,
                  gr_mem=[
                            'rolf',
                            'public'
                          ])
]

where the members of the group are in the form of a list in gr_mem=[]

I can use the following:

import grp
plugdev_members = []
for group in grp.getgrall():
    if "plugdev" in group.gr_name:
        for user in group.gr_mem:
            plugdev_members.append(user)

which returns a flat list:

['rolf','public']

Using list comprehension:

plugdev_members = [
                    mem for mem in [
                                    m.gr_mem for m in grp.getgrall()
                                       if "plugdev" in m.gr_name
                                   ]
                  ] 
or

plugdev_members = [
                    m.gr_mem for m in grp.getgrall() 
                      if "plugdev" in m.gr_name
                   ]

I get a list within a list:

[['rolf', 'public']]

I'm sure that there must be a way to return a flattened list using list comprehension, I'm just not seeing it. I know that I need a second loop within the first, as per the first example but I'm just not getting it right.
Any tips would be appreciated

score 1 · Accepted Answer · answered Mar 07 '17 at 09:51

1

You are missing a second loop inside the first one:

plugdev_members = [m.gr_mem for m in grp.getgrall() if "plugdev" in m.gr_name]
#                  ^----- This is a list, so pugdev_members will be a list of lists

Instead:

plugdev_members = [x for m in grp.getgrall() for x in m.gr_mem if "plugdev" in m.gr_name]

answered Mar 07 '17 at 09:51

Carles Mitjans

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The old 'inverse the operation' of an equation, switcheroo. I never did crack that! Many thanks, just what I was looking for. – Rolf of Saxony Mar 07 '17 at 10:05
1

@RolfofSaxony That's exactly what was in the duplicated questions linked. – Peter Wood Mar 07 '17 at 11:42
@PeterWood Ok! Finally got it `x = [item for sublist in grp.getgrall() for item in sublist.gr_mem if "plugdev" in sublist.gr_name]`. Thanks! – Rolf of Saxony Mar 08 '17 at 11:25

return a flattened list from a list comprehension of a nested list

1 Answers1