Evaluating error strings for 'unittest' assert methods.

[John Pote]

I have been writing a very useful test script using the standard Python 
'unittest' module. This works fine and has been a huge help in keeping 
the system I've been writing fully working even when I make changes that 
could break many features of the system. eg major rewrite of the 
interrupt routines. The target system in question is written in 'C' and 
runs on a 50p microcontroller at the end of a serial line.

I like each assert...() to output helpful information when things go 
wrong. So I have put in quite complicated code to generate the error 
string the assert() method uses only when things go wrong. The normal 
case, when everything is working, means that all these error strings are 
constructed only to be discarded immediately when the assert() detects 
the test result is correct and no exception is throw.

To my mind this seems a waste and adding unnecessary delays in the 
running of the whole test script.

So I was wondering if there was some convienient, Python, way of calling 
an assert() method so the error string is only evaluated/constructed if 
the assert() fails and throws an exception. For example,

self.assertEqual( (nBytes,expectedValues), (nBytesRd,valuesRead),
             """Unexpected reg value.
Expected values nBytes:%02x (%s)
"""%(nBytes,' '.join( [ "%04x"%v for v in expectedValues] )) +
"Read values     nBytes:%02x (%s)"%(nBytesRd,' '.join( [ "%04x"%v for v 
in valuesRead] ))
         )

Ideas invited, thanks everyone,
John