Handling Errors/Exceptions in a Loop

Question

I have two corresponding lists:

One is a list of items
The other is a list of labels for those items

I want to write a function that check if each item in the 1st list is a JSON object. If it is, it should remain in the list but if not, then it and its corresponding label should be deleted.

I wrote the following script to do this:

import json 
def check_json (list_of_items, list_of_labels):
    for item in list_of items:
        try:
            json.loads(item)
            break
        except ValueError:
            index_item = list_of_items.index(item)
            list_of_labels.remove(index_item)
            list_of_items.remove(index_item)

However, it does not remove items that are not JSON objects.

chepner · Accepted Answer · 2017-10-25 17:45:06Z

Don't try to modify the list you are iterating over; it breaks the iterator. Instead, build and return new lists.

import json 
def check_json (list_of_items, list_of_labels):
    new_items = []
    new_labels = []
    for item, label in zip(list_of items, list_of_labels):
        try:
            json.loads(item)
        except ValueError:
            continue
        new_items.append(item)
        new_labels.append(label)
    return new_items, new_labels

If you insist modifying the original arguments:

def check_json (list_of_items, list_of_labels):
    new_items = []
    new_labels = []
    for item, label in zip(list_of items, list_of_labels):
        try:
            json.loads(item)
        except ValueError:
            continue
        new_items.append(item)
        new_labels.append(label)
    list_of_items[:] = new_items
    list_of_labels[:] = new_labels

but note that this isn't really any more efficient; it just provides a different interface.

blue note · Accepted Answer · 2017-10-25 17:48:24Z

0

You can create a list comprehension and unzip it:

def is_json(item):
    try:
        json.loads(item)
        return True
    except ValueError:
        return False


temp = [(item, label) for item, label in zip(items, labels) if is_json(item)]
items, labels = zip(*temp)

answered Oct 25, 2017 at 17:48

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