--help shows username only option. 

While this command can retrieve GPOs for machine account also.

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# /usr/local/samba/bin/samba-tool gpo list --help
Usage: samba-tool gpo list <username> [options]

List GPOs for an account.
-------------------------------

-------------------------------
# /usr/local/samba/bin/samba-tool gpo list test-user-1
GPOs for user test-user-1
    test-Users-GPO-1 {8D86E985-594F-4D32-95E6-272647F9CDF4}
    Default Domain Policy {31B2F340-016D-11D2-945F-00C04FB984F9}
# /usr/local/samba/bin/samba-tool gpo list amitk-machine-2
GPOs for user amitk-machine-2
    test-Computers-GPO-1 {0C7EBE47-2264-4A4C-868C-B31CDDC999B5}
    Default Domain Policy {31B2F340-016D-11D2-945F-00C04FB984F9}
    Default Domain Policy {31B2F340-016D-11D2-945F-00C04FB984F9}
-------------------------------

Ask is:
1. We should update the help to include machinename, something as:
-------------------------------
# /usr/local/samba/bin/samba-tool gpo list --help
Usage: samba-tool gpo list <username|machinename> [options]

List GPOs for an account.
-------------------------------

2. Code uses an variable name 'username', this can be changed to 'accountname' to depict clear meaning.
python/samba/netcmd/gpo.py
class cmd_list(GPOCommand):
takes_args = ['username']