在json对象中搜索key

Question

I'm trying to make an area class that sums up all instances of the key 'area' in a json string.

For example e = Area('[1, 2, 3, {"area": 123, "weight": 1000}]') should return e=123 and this Area('["Hello", "World", {"area": 999}, {"area": 1}]') should return 1000.

At the moment I'm just getting 0 returned everytime and I think this may be because I'm initializing sum too early or because my indexing into the string may be off.

import json
class Area:

    def __init__(self, txt):
        self.txt=txt

    def __str__(self):
        sum=0
            for a in self.txt:
                if a == 'area':
                    sum+=int(self.txt[a]})
        return str(sum)

Answer 1

using json.loads is fine but you need to make sure you have a dict which you can do with isinstance , you can use the builtin sum function to do the summing for you.

import json
class Area:
    def __init__(self, txt):
        self.txt = txt
    def __str__(self):
        return str(sum(d.get("area", 0) for d in json.loads(self.txt) 
                  if isinstance(d, dict)))

Output:

In [8]: e = Area('[1, 2, 3, {"area": 123, "weight": 1000}]')

In [9]: print e
123
In [10]: e = Area('["Hello", "World", {"area": 999}, {"area": 1}]') 

In [11]: print e
1000

In your own code you are iterating over the characters in the string as you never call loads so if a == 'area': would never be True as you are comparing "area" to each individual char, as is your code would also error as self.txt[a]} is not valid syntax.

Answer 2

This works.

Then you should test each element of the list to see if it contains an item area , you should bear in mind that not all items in the list will support the in operator, so wrap it in an exception block.

Lastly it does seem odd that you are doing all this in the __str__ method. It might be better to have a method that returns

import json

class Area:
def __init__(self, txt):
    self.txt = json.loads(txt)

def __str__(self):
    sum = 0
    for a in self.txt:
        try:
            if 'area' in a:
                sum += int(a['area'])
        except TypeError:
            pass
    return str(sum)


print Area('[1, 2, 3, {"area": 123, "weight": 1000}]')
print Area('["Hello", "World", {"area": 999}, {"area": 1}]')