how to pass 'a{sv}' dbus signature to udisks2.Mount() from python?

Question

dbus api uses a special format to describe complex parameters.

Since dbus specification wasn't written with Python in mind, it's a far fetch to find out what parameter structure you exactly have to pass.

In my example I want to call the Mount() method of the Filesystem object. This method got the signature a{sv}.

Mount() is defined like this

org.freedesktop.UDisks2.Filesystem
...
The Mount() method
Mount (IN  a{sv} options,
       OUT s     mount_path);

source: http://storaged.org/doc/udisks2-api/latest/gdbus-org.freedesktop.UDisks2.Filesystem.html#gdbus-method-org-freedesktop-UDisks2-Filesystem.Mount

The complete code to mount a partition is this:

bus = dbus.SystemBus()
device = "/org/freedesktop/UDisks2/block_devices/sdi1"
obj = bus.get_object('org.freedesktop.UDisks2', device)
obj.Mount(..., dbus_interface="org.freedesktop.UDisks2.Filesystem")

Where ... is the parameters in question.

JPT · Accepted Answer · 2021-04-05 10:40:28Z

The answer is separated into different layers:

parameter structure
key names
legal values

The parameter structure for dbus is defined here: https://dbus.freedesktop.org/doc/dbus-specification.html#type-system

We learn from it that a{sv} is an ARRAY that contains one (or multiple?) DICT (list of key-value pairs). The key is STRING, the value is VARIANT which is data of any type preceded by a type code.

Thankfully we don't have to deal with low-level details. Python is going to deal with that.

So the solution simply is:

obj.Mount(dict(key="value", key2="value2"), 
dbus_interface="org.freedesktop.UDisks2.Filesystem")

The actual key names are defined in udisks docs

IN a{sv} options:   Options - known options (in addition to standard options) 
                    includes fstype (of type 's') and options (of type 's').
    
OUT s mount_path:   The filesystem path where the device was mounted.

from http://storaged.org/doc/udisks2-api/latest/gdbus-org.freedesktop.UDisks2.Filesystem.html#gdbus-method-org-freedesktop-UDisks2-Filesystem.Mount

while standard options refers to

Option name, Value type, Description
auth.no_user_interaction, 'b', If set to TRUE, then no user interaction will happen when checking if the method call is authorized.

from http://storaged.org/doc/udisks2-api/latest/udisks-std-options.html

So, adding the key names we have

obj.Mount(dict(fstype="value", options="value2"), 
dbus_interface="org.freedesktop.UDisks2.Filesystem")

Regarding the values I think you have to study the sections Filesystem Independent Mount Options and Filesystem Dependent Mount Options from https://linux.die.net/man/8/mount

So the final solution looks like

obj.Mount(dict(fstype="vfat", options="ro"), 
dbus_interface="org.freedesktop.UDisks2.Filesystem")

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