Python: Handling JSON Files

📢 This article was translated by gemini-2.5-flash

Intro

Got some Record Tree JSON files from a senior dev. So I quickly coded up this thing to make life easier and jot down what I always forget. ~~(Hey, fitting that this is article #256!)~~

I even used it in PixivDownloader before, but completely blanked on it.

Reading JSON Files

After reading, the returned value changes based on the JSON file structure. It’ll be either a Python list or a dictionary.

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import json

with open(FILE_PATH, "r", encoding="utf-8") as f:
    jsonParse = json.load(f)

Now you can mess with the jsonParse variable.

Lists

If your JSON file looks like this:

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[
    {
        "a": "b"
    },
    {
        "c": "d"
    }
]

Then, jsonParse will be an outer list, with each item inside being a dictionary.

Dictionaries

If your JSON file looks like this:

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{
    "a": {
        "c": "d"
    }
}

Then, jsonParse will be a two-level nested dictionary.

Operations

Once you know the type, you just treat it like any Python dictionary or list. For example, given this file:

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[
  {
    "a": 1,
    "b": 2
  },
  {
    "c": 3,
    "d": 4
  }
]

Code to print 1:

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import json

with open(FILE_PATH, "r", encoding="utf-8") as f:
    s = json.load(f)
    print(s[0]["a"])
    # Or
    print(s[0].get("a"))

Code to change 4 to 100:

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import json

with open(FILE_PATH, "r", encoding="utf-8") as f:
    s = json.load(f)
    s[1]["d"] = 100

Add an item to make the file look like this:

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[
  {
    "a": 1,
    "b": 2
  },
  {
    "c": 3,
    "d": 4
  },
  {
    "e": 5
  }
]

Basically, just append a dictionary to the list. Here’s the code:

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import json

with open(FILE_PATH, "r", encoding="utf-8") as f:
    s = json.load(f)
    s.append({"e": 5})

Iteration

For lists, use list iteration:

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for i in s:
    print(i)

For dictionaries, use dictionary iteration:

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for key, value in s.items():
    print(key, value)

for key in s:
    print(key)

Saving

Use json.dump:

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with open(FILE_PATH, "r", encoding="utf-8") as f:
    s = json.load(f)
    # Make some changes to variable s

with open(FILE_PATH, "w", encoding="utf-8") as f:
    json.dump(s, f, ensure_ascii=False, indent=2)

ensure_ascii=False makes sure non-ASCII chars (like Chinese) are written correctly. indent=2 pretty-prints it with 2-space indents.