ValueError: Invalid usage of jsonify

ValueError from jsonify

$ flask --app app.py run
Traceback (most recent call last):
  File "/path/to/app.py", line 20, in stats
    return jsonify(User())
ValueError: View return value is not JSON serializable

Why this happens

jsonify converts Python objects to JSON. If you pass a non-serializable object (like a custom class without a serializer) or misuse its signature (e.g., multiple positional args that don’t map cleanly), Flask will raise a ValueError or produce an invalid response.

Fix

Serialize complex objects into dicts/lists, or implement a to_dict/schema. Pass keyword args or a single dict/list to jsonify to keep shape predictable.

Wrong code

# app.py
from flask import Flask, jsonify
app = Flask(__name__)

class User:  # not serializable by default
    def __init__(self, name):
        self.name = name

@app.get('/stats')
def stats():
    return jsonify(User('Ada'))

Fixed code

# app.py
from flask import Flask, jsonify
app = Flask(__name__)

class User:
    def __init__(self, name):
        self.name = name
    def to_dict(self):
        return {'name': self.name}

@app.get('/stats')
def stats():
    user = User('Ada')
    return jsonify(user=user.to_dict(), ok=True)

Tips

• Always return primitive JSON types (dict, list, str, int, float, bool, None).
• Use marshmallow or pydantic for complex serialization needs.