Merge branch Davi0kProgramsThings:fix/refactoring into branch bitfinexcom:master. (#238)

# Description
<!--- Describe your changes in detail -->
PR includes some global refactoring in preparation for the v3.0.0 stable release.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
-

## Related Issue
<!--- If suggesting a new feature or change, please discuss it in an issue first -->
<!--- If fixing a bug, there should be an issue describing it with steps to reproduce -->
<!--- Please link to the issue here: -->
PR fixes the following issue: -

## Type of change
<!-- Select the most suitable choice and remove the others from the checklist -->

- [X] Bug fix (non-breaking change which fixes an issue);

# Checklist:

- [X] I've done a self-review of my code;
- [X] I've made corresponding changes to the documentation;
- [X] I've made sure my changes generate no warnings;
- [X] mypy returns no errors when run on the root package;
<!-- If you use pre-commit hooks you can always check off the following tasks -->
- [X] I've run black to format my code;
- [X] I've run isort to format my code's import statements;
- [X] flake8 reports no errors when run on the entire code base;

bfxapi/rest/_interface/middleware.py

@@ -0,0 +1,129 @@
+import hashlib
+import hmac
+import json
+from datetime import datetime
+from enum import IntEnum
+from typing import TYPE_CHECKING, Any, List, Optional
+
+import requests
+
+from bfxapi._utils.json_decoder import JSONDecoder
+from bfxapi._utils.json_encoder import JSONEncoder
+from bfxapi.exceptions import InvalidCredentialError
+from bfxapi.rest.exceptions import RequestParametersError, UnknownGenericError
+
+if TYPE_CHECKING:
+    from requests.sessions import _Params
+
+
+class _Error(IntEnum):
+    ERR_UNK = 10000
+    ERR_GENERIC = 10001
+    ERR_PARAMS = 10020
+    ERR_AUTH_FAIL = 10100
+
+
+class Middleware:
+    __TIMEOUT = 30
+
+    def __init__(
+        self, host: str, api_key: Optional[str] = None, api_secret: Optional[str] = None
+    ):
+        self.__host = host
+
+        self.__api_key = api_key
+
+        self.__api_secret = api_secret
+
+    def get(self, endpoint: str, params: Optional["_Params"] = None) -> Any:
+        headers = {"Accept": "application/json"}
+
+        if self.__api_key and self.__api_secret:
+            headers = {**headers, **self.__get_authentication_headers(endpoint)}
+
+        request = requests.get(
+            url=f"{self.__host}/{endpoint}",
+            params=params,
+            headers=headers,
+            timeout=Middleware.__TIMEOUT,
+        )
+
+        data = request.json(cls=JSONDecoder)
+
+        if isinstance(data, list) and len(data) > 0 and data[0] == "error":
+            self.__handle_error(data)
+
+        return data
+
+    def post(
+        self,
+        endpoint: str,
+        body: Optional[Any] = None,
+        params: Optional["_Params"] = None,
+    ) -> Any:
+        _body = body and json.dumps(body, cls=JSONEncoder) or None
+
+        headers = {"Accept": "application/json", "Content-Type": "application/json"}
+
+        if self.__api_key and self.__api_secret:
+            headers = {
+                **headers,
+                **self.__get_authentication_headers(endpoint, _body),
+            }
+
+        request = requests.post(
+            url=f"{self.__host}/{endpoint}",
+            data=_body,
+            params=params,
+            headers=headers,
+            timeout=Middleware.__TIMEOUT,
+        )
+
+        data = request.json(cls=JSONDecoder)
+
+        if isinstance(data, list) and len(data) > 0 and data[0] == "error":
+            self.__handle_error(data)
+
+        return data
+
+    def __handle_error(self, error: List[Any]) -> None:
+        if error[1] == _Error.ERR_PARAMS:
+            raise RequestParametersError(
+                "The request was rejected with the following parameter"
+                f"error: <{error[2]}>"
+            )
+
+        if error[1] == _Error.ERR_AUTH_FAIL:
+            raise InvalidCredentialError(
+                "Cannot authenticate with given API-KEY and API-SECRET."
+            )
+
+        if not error[1] or error[1] == _Error.ERR_UNK or error[1] == _Error.ERR_GENERIC:
+            raise UnknownGenericError(
+                "The server replied to the request with a generic error with "
+                f"the following message: <{error[2]}>."
+            )
+
+    def __get_authentication_headers(self, endpoint: str, data: Optional[str] = None):
+        assert (
+            self.__api_key and self.__api_secret
+        ), "API-KEY and API-SECRET must be strings."
+
+        nonce = str(round(datetime.now().timestamp() * 1_000_000))
+
+        if not data:
+            message = f"/api/v2/{endpoint}{nonce}"
+        else:
+            message = f"/api/v2/{endpoint}{nonce}{data}"
+
+        signature = hmac.new(
+            key=self.__api_secret.encode("utf8"),
+            msg=message.encode("utf8"),
+            digestmod=hashlib.sha384,
+        )
+
+        return {
+            "bfx-nonce": nonce,
+            "bfx-signature": signature.hexdigest(),
+            "bfx-apikey": self.__api_key,
+        }