I can get the code but when get access token by code, it throws error 400. I have follow the steps of /api/rest/authorization/use-authorization-code.html
I also use the python sample code to run and get the same error.
You're still not sending a valid request. Please refer to the example request in the Grant - Authorization Code documentation. You must send the request exactly as documented. This includes sending a properly form-encoded body. Your examples have all been visibly different from the example in the documentation thus far.
Also, you could try configuring the auth code grant using Postman's Authorization OAuth2 configuration. There's nothing wrong with doing it manually, but Postman has tools to help with this.