@app.route('xxxx')
def main():
 ...
 data = {
 'a':'a',
 'b':'b'
 }
 mycol.insert_one(data)
 return data

Traceback (most recent call last):
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/app.py", line 2464, in __call__
 return self.wsgi_app(environ, start_response)
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/app.py", line 2450, in wsgi_app
 response = self.handle_exception(e)
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/app.py", line 1867, in handle_exception
 reraise(exc_type, exc_value, tb)
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
 raise value
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/app.py", line 2447, in wsgi_app
 response = self.full_dispatch_request()
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/app.py", line 1953, in full_dispatch_request
 return self.finalize_request(rv)
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/app.py", line 1968, in finalize_request
 response = self.make_response(rv)
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/app.py", line 2112, in make_response
 rv = jsonify(rv)
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/json/__init__.py", line 370, in jsonify
 dumps(data, indent=indent, separators=separators) + "\n",
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/json/__init__.py", line 211, in dumps
 rv = _json.dumps(obj, **kwargs)
 File "/home/xiang/.local/lib/python3.6/site-packages/simplejson/__init__.py", line 412, in dumps
 **kw).encode(obj)
 File "/home/xiang/.local/lib/python3.6/site-packages/simplejson/encoder.py", line 298, in encode
 chunks = list(chunks)
 File "/home/xiang/.local/lib/python3.6/site-packages/simplejson/encoder.py", line 696, in _iterencode
 for chunk in _iterencode_dict(o, _current_indent_level):
 File "/home/xiang/.local/lib/python3.6/site-packages/simplejson/encoder.py", line 652, in _iterencode_dict
 for chunk in chunks:
 File "/home/xiang/.local/lib/python3.6/site-packages/simplejson/encoder.py", line 716, in _iterencode
 o = _default(o)
 File "/home/xiang/.local/lib/python3.6/site-packages/flask/json/__init__.py", line 100, in default
 return _json.JSONEncoder.default(self, o)
 File "/home/xiang/.local/lib/python3.6/site-packages/simplejson/encoder.py", line 273, in default
 o.__class__.__name__)
TypeError: Object of type ObjectId is not JSON serializable

# 首先在数据中插入一条数据
document = {
  'p_id': 'f75046d73a0c4bf88f0b7342e60722c4',
  'c_name': 'Ken',
  'c_star': '5.0',
  'bought': None,
  'c_time': '2017年3月23日',
  'c_userful': 13,
  'c_comment': ['４歳になるトイプードル用に購入。', '今までエサは床に食器直置きで与えていましたが、食後にむせていることがたびたび目撃されたので', '「もしかして食べづらいのでは？」と思い調べてみたところ、この商品にたどり着きました。', '足が長いので食事中は頭が完全に下に向いており、胃や足腰に負担がかかっていたと思われます。'],
  'gmt_create': datetime.now().strftime('%Y-%m-%d %H:%M:%S'),
}
collection.insert(document)

# 首先需要限定一个重复的条件,比如我们想除了gmt_create参数以外,全部不一样就算不重复, 先限定一个条件
result_find = {
  'p_id': 'f75046d73a0c4bf88f0b7342e60722c4',
  'c_name': 'Ken',
  'c_star': '5.0',
  'bought': None,
  'c_time': '2017年3月23日',
  'c_userful': 13,
  'c_comment': ['４歳になるトイプードル用に購入。', '今までエサは床に食器直置きで与えていましたが、食後にむせていることがたびたび目撃されたので',
         '「もしかして食べづらいのでは？」と思い調べてみたところ、この商品にたどり着きました。', '足が長いので食事中は頭が完全に下に向いており、胃や足腰に負担がかかっていたと思われます。']
}
# 执行 update
collection.update(
  result_find,
  {'$setOnInsert': document},
  upsert=True
)