Conditional statement to check if a JSON key is null with Python 3

Question

This has been updated to clarify the question so it can better help others in the future.

I am attempting to use an if statement to test if the key classes exists in a JSON structure using Python. to check if a key exists in a JSON structure using Python. I am able to get the key but need help finding out what the condition should be to check if it exists.

I successfully was able to return the value of the key class when it is exsits in the JSON structure using the following code:

#Parameter: json_data - The JSON structure it is checking
def parse_classes(json_data):
    lst = list()
    if('classes' is in json_data['images'][0]['classifiers'][0]): #This doesn't work
        for item in json_data['images'][0]['classifiers'][0]['classes']: 
            lst.append(item['class'])
    else:
        lst = None

    return(lst)

Example json_data:

{"images": ["classifiers": ["classes": ["class": "street"]]]}

My issue seems to be on line 4 and the issue seems to be that the conditional statement is incorrect. What do I need to change about the if-statement to make it work correctly?

`for item in json_data['images'][0]['classifiers'][0]['classes']`: shouldn't it be `for item in classes` ? — Jean-François Fabre, Jul 25 '17 at 20:54
This is a follow-up question to https://stackoverflow.com/questions/45310791/retrieve-multiple-json-items-python?noredirect=1#comment77589110_45310791 — PM 2Ring, Jul 25 '17 at 20:56
you need to check `len(json_data['images'])` , then `len(json_data['images'][0]['classifiers'])` — PRMoureu, Jul 25 '17 at 20:57
you're screenshot doesn't match the code you posted (quotes vs no quotes for classes). I first edited it but cmd answer made me rollback. — Jean-François Fabre, Jul 25 '17 at 20:57
Although I figured it out, I just updated my question in case someone in the future needs help. If you think it is better and improved and you want to, would you mind removing your down vote and if you want to upvote do that too please. Thanks a lot! @AlanLeuthard — James, Sep 10 '17 at 18:36

cmd · Accepted Answer · 2017-07-25T21:25:37.697

3

I think you mean if instead of for:

def parse_classes(json_data):
    lst = set()
    if 'classes' in json_data['images'][0]['classifiers'][0]:
        for item in json_data['images'][0]['classifiers'][0]['classes']:
            lst.add(item['class'])
    else:
        print("")
    return lst

or defensively

def parse_classes(json_data):
    lst = set()
    if (json_data.get('images')
            and json_data['images'][0].get('classifiers')
            and json_data['images'][0]['classifiers'][0].get('classes')):
        for item in json_data['images'][0]['classifiers'][0].get('classes', []):
            lst.add(item['class'])
    return lst if lst else None

if you want all class in all classifiers in all images

def parse_classes(json_data):
    lst = set()
    for image in json_data.get('images', []):
        for classifier in image.get('classifiers', []):
            for item in classifier.get('classes', []):
                lst.add(item['class'])
    return lst if lst else None

edited Jul 25 '17 at 21:25

answered Jul 25 '17 at 20:55

cmd

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Still says `IndexError: List index out of range` – James Jul 25 '17 at 20:56
It works for the json data you have given. Do you have the data for when this fails? – cmd Jul 25 '17 at 20:59
`json_data['images'][0]['classifiers'][0]` must be empty in one of the sub-lists. cannot reproduce – Jean-François Fabre Jul 25 '17 at 20:59
@cmd I added the code. With your second thing it still says list index out of range. – James Jul 25 '17 at 21:03
will need to see full error and/or data it errors on. – cmd Jul 25 '17 at 21:03
`def parse_classes(json_data): lst = set() if json_data.get('images') and json_data['images'][0].get('classifiers'): for item in json_data['images'][0]['classifiers'][0].get('classes', []): lst.add(item['class']) else: lst = None return lst` – James Jul 25 '17 at 21:06
that works it is your second one but in the else statement replace print with lst=None – James Jul 25 '17 at 21:06
Thank you so much! – James Jul 25 '17 at 21:08
@Brendan updated to return None instead of empty set – cmd Jul 25 '17 at 21:08
@Brendan as a note: lists usually imply there can be more then one element, you may want to iterate over more then just the `[0]`th element. – cmd Jul 25 '17 at 21:13

Conditional statement to check if a JSON key is null with Python 3

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