How to read a multiline device property?

Development Python
1

In a device server, I’m using device properties as described in the docs:


class MyDeviceServer(Device):
	multiline_property = device_property(dtype=str)
	...

	# Somewhere in code:
	lines = self.multiline_property # Only gets the first line of the property

The problem is that if the property is a multiline string (i.e. there are multiple strings separated by newlines), the above code only retrieves the first line. Is there a way to read the entire value content of a property, not just the first line?

Thanks!

2

Hi Dušan,

It’s usually done this way:

class MyDeviceServer(Device):

        # Note that dtype is now (str,) (i.e. tango.DevVarStringArray)
	multiline_property = device_property(dtype=(str,)) 
	…

	# Somewhere in code:
	lines = self.multiline_property
        # Or
        string = '\n'.join(self.multiline_property)

Cheers,

3

Vincent, thank you! Your solution works. Note, however, that for some reason, using dtype=(str,) or [str] results in the following error message at device server startup:

[quote]PyDS: DeviceServer: An error occured in the constructor:
‘NoneType’ object is not iterable[/quote]
The device server then proceeds to work and run normally, but the array dtype is still causing this error message - I’m not sure why, but it happens even with an empty device server code as soon as you use a (str,) property …

4

Hi Dušan,

I guess you edited the property value with Jive, right?
The problem is that Jive is not able to distinguish a string of multiple lines from a list of strings :-(.

Picking up from your first example:


class MyDeviceServer(Device):

	multiline_property = device_property(dtype=str) 

	def init_device(self):
            Device.init_device()
	    print('{!r}'.format(self.multiline_property))

… and if you use PyTango to write the property value like this:


import tango
dev = tango.DeviceProxy('my/device/server')
dev.put_device_property(dict(multiline_property='line1\nline2'))
# force the server to reread the properties by initializing it
dev.init()

You should see on the server output that the string is correct.

5

Tiago, thank you for your proposal and for pointing this out. You are right in that I was using Jive to edit the property. I don’t think that I’ll be able to rely on DeviceProxy for value setting, though, because the end user may use Jive as well.

Anyway, I figured out how to get rid of the “‘NoneType’ object is not iterable” error mentioned above. You must assign a default value to your property:

multiline_property = device_property(dtype=[str], default_value="")

After this, multiline properties work normally with no erroneous output.