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")
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.
BaseException.args is documented, but not in enough detail that its uses are obvious.
Not sure if you're using any modules to assist you - if the JSON is coming in as a dict, one can use dict.get() towards a useful end.
def POIJSON2DOM (location_node, POI_JSON):
man_JSON = POI_JSON.get("FastestMan", 'No Data for fastest man')
woman_JSON = POI_JSON.get("FastestWoman", 'No Data for fastest woman')
#work with the answers as you see fit
dict.get() takes two arguments - the first being the key you want, the second being the value to return if that key does not exist.
KeyError.If you import the sys module you can get exception info with sys.exc_info()
like this:
def POIJSON2DOM (location_node, POI_JSON):
try:
man_JSON = POI_JSON["FastestMan"]
woman_JSON = POI_JSON["FastestWoman"]
except KeyError:
# you can inspect these variables for error information
err_type, err_value, err_traceback = sys.exc_info()
REDI.LogErrorMessage ("POIJSON2DOM", "Can't find mandatory key in JSON")
sys.exc_info() was a reasonable approach.
except Foo as e, and it's a race condition in multithreaded code. What if two threads raise a KeyError before either of them can call sys.exc_info? Both threads would see the same output, so one of them could be wrong. The except Foo as e syntax is an atomic operation.
trystatement.