If aioca is not installed, making an epics_signal_r (or rw, or x .. etc) will fail with a non-descriptive error message:
AttributeError: 'function' object has no attribute 'value'
This is unexpected behaviour and the error is obscure.
Description, with examples
If aioca or p4p are not installed, ophyd/v2/epics.py has:
try:
from ._aioca import CaSignalBackend
except ImportError as ca_error:
def CaSignalBackend(*args, ca_error=ca_error, **kwargs): # type: ignore
raise NotImplementedError("CA support not available") from ca_error
and equivalent for p4p. ._aioca and ._p4p contain import p4p, and import aioca, which will fail with the import error. So if my venv doesn't have aioca for example, CaSignalBackend will be a function, not a class.
The issue with this, is later on if you want to make backends (as epics_signal_r does, or any epics signal factory)...
def _transport_pv(pv: str) -> Tuple[EpicsTransport, str]:
split = pv.split("://", 1)
if len(split) > 1:
# We got something like pva://mydevice, so use specified comms mode
transport_str, pv = split
transport = EpicsTransport[transport_str]
else:
# No comms mode specified, use the default
transport = _default_epics_transport
return transport, pv
def _make_backend(
datatype: Optional[Type[T]], read_pv: str, write_pv: str
) -> SignalBackend[T]:
r_transport, r_pv = _transport_pv(read_pv)
w_transport, w_pv = _transport_pv(write_pv)
transport = get_unique({read_pv: r_transport, write_pv: w_transport}, "transports")
return transport.value(datatype, r_pv, w_pv)
_transport_pv should return a tuple of an enum and string. However,if I don't have aioca installed, CaSignalBackend is replaced with a function, which apparently isn't stored properly in the enum. This means that the call to transport.value fails, because transport is then just a function, instead of an Enum. In ipython...
In [19]: EpicsTransport.pva
Out[19]: <EpicsTransport.pva: <class 'ophyd.v2._p4p.PvaSignalBackend'>>
In [20]: EpicsTransport.ca
Out[20]: <function ophyd.v2.epics.CaSignalBackend(*args, ca_error=ModuleNotFoundError("No module named 'aioca'"), **kwargs)>
In [21]: _transport_pv("pva://abc")
Out[21]: (<EpicsTransport.pva: <class 'ophyd.v2._p4p.PvaSignalBackend'>>, 'abc')
In [22]: _transport_pv("abc")
Out[22]:
(<function ophyd.v2.epics.CaSignalBackend(*args, ca_error=ModuleNotFoundError("No module named 'aioca'"), **kwargs)>,
'abc')
How to fix this
In this PR I mentioned to avoid typing errors, if aioca/p4p are not installed, instead of replacing CaSignalBackend and PvaSignalBackend with functions we should replace them with classes. I think this is now a better idea as it would solve this issue. But am open to discussion on anything else we should do.
Summary of Problem
If aioca is not installed, making an epics_signal_r (or rw, or x .. etc) will fail with a non-descriptive error message:
This is unexpected behaviour and the error is obscure.
Description, with examples
If aioca or p4p are not installed, ophyd/v2/epics.py has:
and equivalent for p4p. ._aioca and ._p4p contain import p4p, and import aioca, which will fail with the import error. So if my venv doesn't have aioca for example, CaSignalBackend will be a function, not a class.
The issue with this, is later on if you want to make backends (as epics_signal_r does, or any epics signal factory)...
_transport_pv should return a tuple of an enum and string. However,if I don't have aioca installed, CaSignalBackend is replaced with a function, which apparently isn't stored properly in the enum. This means that the call to
transport.valuefails, becausetransportis then just a function, instead of an Enum. In ipython...How to fix this
In this PR I mentioned to avoid typing errors, if aioca/p4p are not installed, instead of replacing CaSignalBackend and PvaSignalBackend with functions we should replace them with classes. I think this is now a better idea as it would solve this issue. But am open to discussion on anything else we should do.