Returns the result of fn(kwargs), or if there are exceptions, returns alternative instead.

safeResult.py

def safeResult(fn, expectedError = Exception, alternative = None, **kwargs):
    """Returns the result of fn(kwargs), or if there are exceptions, returns alternative instead."""
    result = alternative
    try:
        result = fn( kwargs )
    except expectedError:
        pass
    return result

usage-example.py

class Example(object):
    tree = None

# ...

    def getSourceLang(self):
        fn = lambda x: x["tree"].xpath( "//TS/@sourcelanguage" )[0]
        return safeResult( fn, alternative="und", tree=self.tree )