如何使用lambda函数在Python for循环中捕获KeyError？

Question

Is it possible to catch a KeyError for x['uri'] for this particular scenario.

joined_rooms = ('coala/coala', 'coala/coala-bears')
room_data = [{'id': '130', 'uri': 'coala/coala'},
             {'id': '234', 'name': 'Nitanshu'},
             {'id': '897', 'uri': 'coala/coala-bears'}]

for room in filter(lambda x: x['uri'] in joined_rooms, room_data):
    #do stuff

Doing this:

try:
    for room in filter(lambda x: x['uri'] in joined_rooms, room_data):
        #do stuff
except KeyError:
    pass

will skip all the items after KeyError is raised.

Is there any way to handle this apart from migrating to the classic nested for loops with if condition ?

Answer 1

Python dict s have a get method for just such an occasion. It takes two arguments, first is the key you're looking for, second is the default value to use if the key is not present in the dictionary. Try this:

for room in filter(lambda x:x.get('uri',None) in joined_rooms, room_data):
    #do stuff

The problem here, although it might not be a problem in your case, is that you're left with needing to supply a default value that will never appear in joined_rooms .

Answer 2

As mentioned, you can use dict.get to return a default value if a key does not exist in a dictionary. If not provided, this default value is None .

As an alternative, you can use a generator expression. This will be more efficient than filter + lambda expression. It may also be more readable:

for room in (x for x in room_data if x.get('iri') in joined_rooms):
    # do something

Another perfectly valid solution is to use an if statement within your for loop:

for room in room_data:
    if 'iri' in room:
        # do something