python - 将输出序列化为 JSON - ValueError : Circular reference detected

Question

我正在尝试将我的 mysql 查询结果输出到 JSON。 我在序列化 datetime.datetime 字段时遇到问题，所以我写了一个小函数来做到这一点:

def date_handler(obj):
    if hasattr(obj, 'isoformat'):
        return obj.isoformat()
    else:
        return obj

然后在我刚刚运行的主代码中:

products_json = []
for code in best_matching_codes:
    cur = db.cursor()
    query = "SELECT * FROM %s WHERE code LIKE '%s'" % (PRODUCTS_TABLE_NAME, product_code)
    cur.execute(query)
    columns = [desc[0] for desc in cur.description]
    rows = cur.fetchall()
    for row in rows:
        products_json.append(dict((k,v) for k,v in zip(columns,row)))   

return json.dumps(products_json, default = date_handler)

但是，由于我编写了 date_handler 函数，我收到“ValueError: Circular reference detected”

127.0.0.1 - - [10/Jan/2013 00:42:18] "GET /1/product?code=9571%2F702 HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/flask/app.py", line 1701, in __call__
return self.wsgi_app(environ, start_response)
  File "/Library/Python/2.7/site-packages/flask/app.py", line 1689, in wsgi_app
response = self.make_response(self.handle_exception(e))
  File "/Library/Python/2.7/site-packages/flask/app.py", line 1687, in wsgi_app
response = self.full_dispatch_request()
  File "/Library/Python/2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
rv = self.handle_user_exception(e)
  File "/Library/Python/2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
rv = self.dispatch_request()
  File "/Library/Python/2.7/site-packages/flask/app.py", line 1344, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/pisarzp/Desktop/SusyChoosy/susyAPI/test1.py", line 69, in product_search
return json.dumps(products_json, default = date_handler)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 238, in dumps
**kw).encode(obj)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py", line 201, in encode
chunks = self.iterencode(o, _one_shot=True)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py", line 264, in iterencode
return _iterencode(o, 0)
ValueError: Circular reference detected

我弄坏了什么？有没有更好的方法将输出序列化为 JSON？

Answer 1

您作为 default 参数传递的函数只会被 json 模块本身不可序列化的对象调用。它必须返回一个可序列化的对象，或者引发一个 TypeError。

如果它不是您正在修复的一种类型(日期)，您的版本将返回与您传递的相同对象。这导致了循环引用错误(这是一种误导，因为在 date_handler 处理后，圆在一个对象和它自己之间)。

您可以通过更改 date_handler 以在其 else block 中引发异常来开始解决此问题。这仍然可能会失败，但您可能可以使用如下代码找出导致问题的数据结构中的对象是什么:

def date_handler(obj):
    if hasattr(obj, 'isoformat'):
        return obj.isoformat()
    else:
        raise TypeError(
            "Unserializable object {} of type {}".format(obj, type(obj))
        )