Python Language Tutorial => Avoiding KeyError Exceptions

Example

One common pitfall when using dictionaries is to access a non-existent key. This typically results in a KeyError exception

mydict = {}
mydict['not there']

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'not there'

One way to avoid key errors is to use the dict.get method, which allows you to specify a default value to return in the case of an absent key.

value = mydict.get(key, default_value)

Which returns mydict[key] if it exists, but otherwise returns default_value. Note that this doesn't add key to mydict. So if you want to retain that key value pair, you should use mydict.setdefault(key, default_value), which does store the key value pair.

mydict = {}
print(mydict)
# {}
print(mydict.get("foo", "bar"))
# bar
print(mydict)
# {}
print(mydict.setdefault("foo", "bar"))
# bar
print(mydict)
# {'foo': 'bar'}

An alternative way to deal with the problem is catching the exception

try:
    value = mydict[key]
except KeyError:
    value = default_value

You could also check if the key is in the dictionary.

if key in mydict:
    value = mydict[key]
else:
    value = default_value

Do note, however, that in multi-threaded environments it is possible for the key to be removed from the dictionary after you check, creating a race condition where the exception can still be thrown.

Another option is to use a subclass of dict, collections.defaultdict, that has a default_factory to create new entries in the dict when given a new_key.

PDF - Download Python Language for free

Previous Next

Python Language

Fastest Entity Framework Extensions

Example

Got any Python Language Question?

Python Language

Python Language Dictionary Avoiding KeyError Exceptions

Fastest Entity Framework Extensions

Example

Got any Python Language Question?