Problem with dynamic attributes using pytango 9.3.4 - read-parameter() missing 1 argument

Development Python
1

Hi,

I am posting a problem we discovered with PyTango 9.3.4 with dynamic attributes. Dynamic attributes fail with this error: Description - TypeError: read_parameter() missing 1 required positional argument: ‘attr’

Here is an example of a client:

>>> from PyTango import DeviceProxy
>>> ab = DeviceProxy("cm01/metadata/ingest")
>>> ab.read_attribute("EM_voltage")
> /users/blissadm/conda/miniconda/envs/multivisor/lib/python3.7/site-packages/tang/device_proxy.py(155)__check_read_attribute()
-> def __check_read_attribute(dev_attr):
(Pdb) dev_attr
DeviceAttribute(data_format = tango._tango.AttrDataFormat.FMT_UNKNOWN, dim_x = 0, dim_y = 0, has_failed = True, is_empty = True, name = 'EM_voltage', nb_read = 0, nb_written = 0, quality = tango._tango.AttrQuality.ATTR_INVALID, r_dimension = AttributeDimension(dim_x = 0, dim_y = 0), time = TimeVal(tv_nsec = 0, tv_sec = 0, tv_usec = 0), type = tango._tango.CmdArgType(100), value = None, w_dim_x = 0, w_dim_y = 0, w_dimension = AttributeDimension(dim_x = 0, dim_y = 0), w_value = None)

The same call with pytango from conda forge 9.3.3 is fine

>>> from PyTango import DeviceProxy
>>> ab = DeviceProxy("cm01/metadata/ingest")
>>> ab.read_attribute("EM_voltage")
DeviceAttribute(data_format = tango._tango.AttrDataFormat.SCALAR, dim_x = 1, dim_y = 0, has_failed = False, is_empty = False, name = 'EM_voltage', nb_read = 1, nb_written = 1, quality = tango._tango.AttrQuality.ATTR_VALID, r_dimension = AttributeDimension(dim_x = 1, dim_y = 0), time = TimeVal(tv_nsec = 0, tv_sec = 1658238511, tv_usec = 901670), type = tango._tango.CmdArgType.DevString, value = '', w_dim_x = 1, w_dim_y = 0, w_dimension = AttributeDimension(dim_x = 1, dim_y = 0), w_value = '30000')

The problem is on the server side. When a device server is started with pytango 9.3.4, it gives the attribute error. When started with 9.3.3, even a 9.3.4 client works fine.

Any tips or explanations of what we are doing wrong are welcome!

Andy + Antonia

2

Hello,

The problem is related to the fix of PyTango: issue 401…

The general solution is, that when you add dynamic attribute, for f_get, g_set and f_is_allowed you should use “f_set = self.my_read_method” statement. I guess you do something like “f_set = <My_Class_name>.my_read_method” - this won’t work.

Here there is a related discussion and explanation: PyTango: issue 454…

3

Hi Yury,

thank you for this answer - it solves the problem!

For anyone who encounters this problem the solution is as described by Yury above (with slightly different example names):

replace

<Your_Class_name>.your_read_method

with

self.your_read_method

in the self.add_attribute() call e.g.

self.add_attribute(param, self.read_parameter, self.write_parameter, None)

The same for the write method of course. If you do this then your device server will work with pytango 9.3.4

Q: is this compatible with previous versions of pytango?

Q: why was the syntax with the used in the first place?

The whole thing has something to do with bound and unbound variables in Python but don’t ask me to explain it!

Andy

4
  • yes, statements like
self.add_attribute(param, self.read_parameter, self.write_parameter, None)

are compatible with previous pytango

  • this I do not know.

Actually, for me, usage of unbound methods to read|write attribute of the particular device is at least strange.