Which key failed in Python KeyError?

Question

If I catch a KeyError, how can I tell what lookup failed?

def poijson2xml(location_node, POI_JSON):   try:     man_json = POI_JSON["FastestMan"]     woman_json = POI_JSON["FastestWoman"]   except KeyError:     # How can I tell what key ("FastestMan" or "FastestWoman") caused the error?     LogErrorMessage ("POIJSON2XML", "Can't find mandatory key in JSON") 

Answer 1

Take the current exception (I used it as e in this case); then for a KeyError the first argument is the key that raised the exception. Therefore we can do:

except KeyError as e:  # One would do it as 'KeyError, e:' in Python 2.     cause = e.args[0] 

With that, you have the offending key stored in cause.

Expanding your sample code, your log might look like this:

def poijson2xml(location_node, POI_JSON):   try:     man_json = POI_JSON["FastestMan"]     woman_json = POI_JSON["FastestWoman"]   except KeyError as e:     LogErrorMessage ("POIJSON2XML", "Can't find mandatory key '"     e.args[0]     "' in JSON") 

It should be noted that e.message works in Python 2 but not Python 3, so it shouldn't be used.