Testing ca_nothing for boolean needs to be deprecated

As discussed in DiamondLightSource/aioca#45 it would appear that allowing a ca_nothing value to return False to a boolean test triggers unexpected behaviour: it turns out that Exception subclasses are expected by core Python libraries to return True when tested with __bool__.

The obvious fix is to delete the implementation of ca_nothing.__bool__ here:

cothread/src/cothread/catools.py

Lines 103 to 105 in bb62450

    
           def __bool__(self): 
        
               return self.ok 
        
           __nonzero__ = __bool__   # For python 2

but unfortunately this is a breaking change for any code which is testing bool(value) rather than value.ok. The only reasonable solution is probably to raise a one-shot deprecation warning when this function is called.

Fortunately I don't think the boolean test feature of ca_nothing is documented, but this still needs to be flagged as a breaking change.

Fortunately I don't think the boolean test feature of ca_nothing is documented, but this still needs to be flagged as a breaking change.

Unfortunately not only is this documented behaviour, it is recommended:

cothread/docs/user/catools.rst

Lines 218 to 223 in bb62450

    
               set.  The return code can be tested for boolean success, so for example it 
        
               is possible to write:: 
        
                   if not caput(pv, value, throw=False): 
        
                       # process caput error

The patch described in DiamondLightSource/aioca#45 (comment) should probably be applied to cothread.

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Testing `ca_nothing` for boolean needs to be deprecated #67

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	def __bool__(self):
	return self.ok
	__nonzero__ = __bool__ # For python 2

	set. The return code can be tested for boolean success, so for example it
	is possible to write::

	if not caput(pv, value, throw=False):
	# process caput error

Testing ca_nothing for boolean needs to be deprecated #67

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Araneidae commented on Jul 9, 2024

Araneidae commented on Dec 2, 2024

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Testing `ca_nothing` for boolean needs to be deprecated #67