You can try to do json.loads(), which will throw a ValueError if the string you pass can't be decoded as JSON.

In general, the "Pythonic" philosophy for this kind of situation is called EAFP, for Easier to Ask for Forgiveness than Permission.

The correct answer is: stop NOT wanting to catch the ValueError.

Example Python script returns a boolean if a string is valid json:

import json

def is_json(myjson):
    try:
        json_object = json.loads(myjson)
    except ValueError as e:
        return False
    return True

print(is_json('{}'))              # prints True
print(is_json('{asdf}'))          # prints False
print(is_json('{"age":100}'))     # prints True
print(is_json('{'age':100 }'))    # prints False
print(is_json('{"age":100 }'))    # prints True

More compact way should be like this. Json lib process only str, bytes or bytearray structures, so only consider them. Instead of if len(text)==0, if not text is much faster for long strings, we don't want to know length of text. Json lib may raise JsonDecoderError. First last characters of text can be checked back-reference with regex but I tried possible edge cases such as '{]' and '[}', they don't fail.

def is_json(text: str) -> bool:
    from json import loads, JSONDecodeError
 
    if not isinstance(text, (str, bytes, bytearray)):
        return False
    if not text:
        return False
    text = text.strip()
    if text[0] in {'{', '['} and text[-1] in {'}', ']'}:
        try:
            loads(text)
        except (ValueError, TypeError, JSONDecodeError):
            return False
        else:
            return True
    else:
        return False

EDIT: We should check whether text is empty or not, not to raise IndexError.

def is_json(text: str) -> bool:
    if not isinstance(text, (str, bytes, bytearray)):
        return False
    if not text:
        return False
    text = text.strip()
    if text:
        if text[0] in {'{', '['} and text[-1] in {'}', ']'}:
            try:
                loads(text)
            except (ValueError, TypeError, JSONDecodeError):
                return False
            else:
                return True
        else:
            return False
    return False

You can use try except clause to check if a string is actually json:

import json
line = '<what you think is json>'
try:
    json_line = json.loads(line)
except ValueError:
    print("not a json")

In your above code the last line is not a valid JSON. You can use this tool JSONLint to verify if your JSON is a valid JSON.

There is no way to check if an object is serializable to a json format. However, you can (even if it is not really EAFP compliant) try and if it fails go for a fallback process:

try:
    json.dumps(my_object)
except TypeError:
    print("Unable to serialize the object")

A way to be sure would be to implement your own JSONEncoder if you are entirely sure of the data processed.