diff --git a/.github/workflows/pythonapp.yml b/.github/workflows/pythonapp.yml
index ed35439..fd8fec2 100644
--- a/.github/workflows/pythonapp.yml
+++ b/.github/workflows/pythonapp.yml
@@ -23,6 +23,7 @@ jobs:
       run: |
         black --diff --check tdworkflow tests
         isort --check-only tdworkflow tests
+        mypy --install-types --non-interactive tdworkflow
     - name: Test with pytest
       run: |
         pytest
diff --git a/setup.cfg b/setup.cfg
index 646b46d..fe637f0 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -33,7 +33,7 @@ setup_requires =
 tdworkflow = py.typed
 
 [options.extras_require]
-dev = black;flake8;isort;pytest;pytest-mock
+dev = black;flake8;isort;pytest;pytest-mock;mypy
 doc = sphinx;sphinx_rtd_theme
 
 [flake8]
diff --git a/tdworkflow/attempt.py b/tdworkflow/attempt.py
index 1c7cb7d..e575a1b 100644
--- a/tdworkflow/attempt.py
+++ b/tdworkflow/attempt.py
@@ -1,6 +1,6 @@
 import dataclasses
 from datetime import datetime
-from typing import Dict
+from typing import Any, Dict, Optional
 
 from .project import Project
 from .resource import Resource
@@ -13,22 +13,22 @@ class Attempt(Resource):
     id: int
     sessionId: int = -1
     sessionUuid: str = ""
-    sessionTime: datetime = None
-    workflow: Workflow = None
-    project: Project = None
+    sessionTime: Optional[datetime] = None
+    workflow: Optional[Workflow] = None
+    project: Optional[Project] = None
     index: int = -1
     retryAttemptName: str = ""
     done: bool = False
     success: bool = False
     cancelRequested: bool = False
-    params: Dict = None
-    createdAt: datetime = None
-    finishedAt: datetime = None
+    params: Optional[Dict[str, Any]] = None
+    createdAt: Optional[datetime] = None
+    finishedAt: Optional[datetime] = None
     status: str = ""
 
-    def __post_init__(self):
+    def __post_init__(self) -> None:
         self.id = int(self.id)
-        self.sessionTime = parse_iso8601(self.sessionTime)
+        self.sessionTime = parse_iso8601(self.sessionTime)  # type: ignore
         if self.project and isinstance(self.project, dict):
             self.project = Project(**self.project)
         if self.workflow and isinstance(self.workflow, dict):
@@ -36,38 +36,38 @@ def __post_init__(self):
         self.done = bool(self.done)
         self.success = bool(self.success)
         self.cancelRequested = bool(self.cancelRequested)
-        self.createdAt = parse_iso8601(self.createdAt)
-        self.finishedAt = parse_iso8601(self.finishedAt)
+        self.createdAt = parse_iso8601(self.createdAt)  # type: ignore
+        self.finishedAt = parse_iso8601(self.finishedAt)  # type: ignore
         self.status = self.status
 
     @property
-    def session_id(self):
+    def session_id(self) -> int:
         return self.sessionId
 
     @property
-    def session_uuid(self):
+    def session_uuid(self) -> str:
         return self.sessionUuid
 
     @property
-    def session_time(self):
+    def session_time(self) -> Optional[datetime]:
         return self.sessionTime
 
     @property
-    def retry_attempt_name(self):
+    def retry_attempt_name(self) -> str:
         return self.retryAttemptName
 
     @property
-    def cancel_requested(self):
+    def cancel_requested(self) -> bool:
         return self.cancelRequested
 
     @property
-    def finished_at(self):
+    def finished_at(self) -> Optional[datetime]:
         return self.finishedAt
 
-    def finished(self):
+    def finished(self) -> bool:
         return bool(self.finished_at)
 
-    def update(self, **args):
+    def update(self, **args: Any) -> None:
         other_attempt = Attempt(**args)
         self.id = other_attempt.id
         self.sessionId = other_attempt.sessionId
diff --git a/tdworkflow/client.py b/tdworkflow/client.py
index 08431a9..f257fac 100644
--- a/tdworkflow/client.py
+++ b/tdworkflow/client.py
@@ -6,7 +6,7 @@
 import time
 import uuid
 from datetime import datetime
-from typing import Any, BinaryIO, Dict, List, Optional, Tuple, Union
+from typing import Any, BinaryIO, Callable, Dict, List, Optional, Tuple, Union, cast
 
 import requests
 from requests.adapters import HTTPAdapter
@@ -29,6 +29,8 @@
 
 
 class WorkflowAPI:
+    get: Callable
+
     def workflows(
         self,
         name_pattern: Optional[str] = None,
@@ -52,7 +54,7 @@ def workflows(
         :return: List of Workflow
         :rtype: List[Workflow]
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[str, bool, int, None]]
         if name_pattern:
             params["name_pattern"] = name_pattern
         if search_project_name:
@@ -83,6 +85,10 @@ def workflow(self, workflow: Union[int, Workflow]) -> Workflow:
 
 
 class ProjectAPI:
+    get: Callable
+    put: Callable
+    delete: Callable
+
     def project(self, project: Union[int, Project]) -> Project:
         """Get a project
 
@@ -114,7 +120,7 @@ def projects(
         :return: List of Project
         :rtype: List[Project]
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[str, int, None]]
         if name:
             params["name"] = name
         if name_pattern:
@@ -147,7 +153,7 @@ def project_workflows(
         :return: List of Workflow
         :rtype: List[Workflow]
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[str, int, None]]
         if workflow:
             workflow_name = (
                 workflow.name if isinstance(workflow, Workflow) else workflow
@@ -277,7 +283,7 @@ def project_schedules(
         :param last_id: List schedules whose id is grater than this id for pagination
         :return: List of Schedule
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[str, int, None]]
         if workflow:
             workflow_name = (
                 workflow.name if isinstance(workflow, Workflow) else workflow
@@ -393,7 +399,7 @@ def project_sessions(
         :param page_size: Number of sessions to return
         :return: List of Session
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[str, int, None]]
         if workflow:
             workflow_name = (
                 workflow.name if isinstance(workflow, Workflow) else workflow
@@ -412,6 +418,10 @@ def project_sessions(
 
 
 class AttemptAPI:
+    get: Callable
+    put: Callable
+    post: Callable
+
     def attempts(
         self,
         project: Optional[Union[str, Project]] = None,
@@ -435,7 +445,7 @@ def attempts(
         :return: List of Attempt object
         :rtype: List[Attempt]
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[str, bool, int, None]]
         if project:
             project_name = project.name if isinstance(project, Project) else project
             params.update({"project": project_name})
@@ -443,7 +453,7 @@ def attempts(
             workflow_name = (
                 workflow.name if isinstance(workflow, Workflow) else workflow
             )
-            params.upadte({"workflow": workflow_name})
+            params.update({"workflow": workflow_name})
         if include_retried:
             params.update({"include_retried": include_retried})
         if last_id:
@@ -474,7 +484,11 @@ def attempt(
             raise ValueError(f"Unable to find attempt id {attempt_id}")
 
         if inplace:
-            attempt.update(**r)
+            if isinstance(attempt, int):
+                raise ValueError(f"Unable to use inplace with integer value {attempt=}")
+            else:
+                attempt.update(**r)
+            return None
         else:
             return Attempt.from_api_repr(**r)
 
@@ -518,7 +532,9 @@ def start_attempt(
         :return:
         """
         workflow_id = workflow.id if isinstance(workflow, Workflow) else workflow
-        _params = {"workflowId": workflow_id}
+        _params = {
+            "workflowId": workflow_id
+        }  # type: Dict[str, Union[str, int, Dict, None]]
         workflow_params = workflow_params if workflow_params else {}
         _params.update({"params": workflow_params})
         if not session_time:
@@ -548,6 +564,7 @@ def kill_attempt(
         self.post(f"attempts/{attempt_id}/kill", content=True)
         if inplace:
             self.attempt(attempt, inplace=True)
+            return None
         else:
             return self.attempt(attempt)
 
@@ -563,6 +580,9 @@ def wait_attempt(
         :return: Latest status of Attempt
         :rtype: Attempt
         """
+        if isinstance(attempt, int):
+            attempt = cast(Attempt, self.attempt(attempt))
+
         while not attempt.done:
             time.sleep(wait_interval)
             self.attempt(attempt, inplace=True)
@@ -571,6 +591,9 @@ def wait_attempt(
 
 
 class ScheduleAPI:
+    get: Callable
+    post: Callable
+
     def schedules(self, last_id: Optional[int] = None) -> List[Schedule]:
         """List schedules
 
@@ -614,7 +637,7 @@ def backfill_schedule(
         :param count: Count
         :return: ScheduleAttempt
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[str, int, None]]
         if from_time:
             params["fromTime"] = to_iso8601(from_time)
         if attempt_name:
@@ -673,7 +696,7 @@ def skip_schedule(
         :param dry_run: Flag for dry run
         :return: New Schedule
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[str, datetime, bool, None]]
         if from_time:
             params["fromTime"] = to_iso8601(from_time)
         if next_time:
@@ -690,6 +713,8 @@ def skip_schedule(
 
 
 class SessionAPI:
+    get: Callable
+
     def sessions(
         self, last_id: Optional[int] = None, page_size: Optional[int] = None
     ) -> List[Session]:
@@ -752,6 +777,8 @@ def session_attempts(
 
 
 class LogAPI:
+    get: Callable
+
     def log_files(
         self,
         attempt: Union[Attempt, int],
@@ -765,7 +792,7 @@ def log_files(
         :param direct_download: Flag for direct download
         :return: List of LogFile
         """
-        params = {}
+        params = {}  # type: Dict[str, Union[int, str, bool, None]]
         if task:
             params["task"] = task
         if direct_download:
@@ -778,7 +805,9 @@ def log_files(
         else:
             return []
 
-    def log_file(self, attempt: Union[Attempt, int], file: Union[LogFile, str]) -> str:
+    def log_file(
+        self, attempt: Union[Attempt, int], file: Union[LogFile, str]
+    ) -> Union[bytes, str]:
         """Get a log string for an attempt
 
         :param attempt: Target Attempt id or Attempt object
@@ -796,7 +825,7 @@ def log_file(self, attempt: Union[Attempt, int], file: Union[LogFile, str]) -> s
         else:
             raise ValueError(f"Unable to get file: {file_name}")
 
-    def logs(self, attempt: Union[Attempt, int]) -> List[str]:
+    def logs(self, attempt: Union[Attempt, int]) -> List[Union[bytes, str]]:
         """Get log string list for an attempt
 
         :param attempt: Attempt ID or Attempt object
@@ -900,7 +929,7 @@ def __init__(
         self.api_base = f"{scheme}://{self.endpoint}/api/"
 
     @property
-    def http(self):
+    def http(self) -> requests.Session:
         """
         :return: Established session
         :rtype: requests.Session
@@ -923,7 +952,7 @@ def get(
         """
         url = f"{self.api_base}{path}"
         r = self.http.get(url, params=params)
-        logger.debug(f"{r.status_code}\n{r.content}")
+        logger.debug(f"{r.status_code!r}\n{r.content!r}")
 
         if not 200 <= r.status_code < 300:
             exceptions.raise_response_error(r)
@@ -948,7 +977,7 @@ def post(
         """
         url = f"{self.api_base}{path}"
         r = self.http.post(url, json=body)
-        logger.debug(f"{r.status_code}\n{r.content}")
+        logger.debug(f"{r.status_code!r}\n{r.content!r}")
 
         if not 200 <= r.status_code < 300:
             exceptions.raise_response_error(r)
@@ -958,10 +987,12 @@ def post(
         elif r.content and "application/json" in r.headers.get("Content-Type", ""):
             return r.json()
 
+        return None
+
     def put(
         self,
         path: str,
-        data: Optional[Union[Dict, List[Tuple], BinaryIO]] = None,
+        data: Optional[Union[str, Dict, List[Tuple], BinaryIO]] = None,
         _json: Optional[Dict[str, str]] = None,
         params: Optional[Dict[str, str]] = None,
     ) -> Optional[Dict[str, str]]:
@@ -970,7 +1001,7 @@ def put(
         :param path: Treasure Workflow API path
         :type path: str
         :param data: Content body
-        :type data: Optional[Union[Dict, List[Tuple], BinaryIO]], oprional
+        :type data: Optional[str, Union[Dict, List[Tuple], BinaryIO]], optional
         :param _json: Content body as JSON
         :type _json: Optional[Dict[str, str]], optional
         :param params: Query parameters
@@ -986,8 +1017,8 @@ def put(
         if not _json and data and hasattr(data, "read"):
             headers["Content-Type"] = "application/gzip"
 
-        r = self.http.put(url, data=data, headers=headers, params=params)
-        logger.debug(f"{r.status_code}\n{r.content}")
+        r = self.http.put(url, data=data, headers=headers, params=params)  # type: ignore
+        logger.debug(f"{r.status_code!r}\n{r.content!r}")
 
         if not 200 <= r.status_code < 300:
             exceptions.raise_response_error(r)
@@ -995,6 +1026,8 @@ def put(
         if r.content and "application/json" in r.headers.get("Content-Type", ""):
             return r.json()
 
+        return None
+
     def delete(
         self, path: str, params: Optional[Dict[str, str]] = None
     ) -> Optional[Dict[str, str]]:
@@ -1010,10 +1043,12 @@ def delete(
         url = f"{self.api_base}{path}"
 
         r = self.http.delete(url, params=params)
-        logger.debug(f"{r.status_code}\n{r.content}")
+        logger.debug(f"{r.status_code!r}\n{r.content!r}")
 
         if not 200 <= r.status_code < 300:
             exceptions.raise_response_error(r)
 
         if r.content and "application/json" in r.headers.get("Content-Type", ""):
             return r.json()
+
+        return None
diff --git a/tdworkflow/exceptions.py b/tdworkflow/exceptions.py
index 520ce1a..53bb672 100644
--- a/tdworkflow/exceptions.py
+++ b/tdworkflow/exceptions.py
@@ -1,3 +1,5 @@
+from typing import NoReturn, Union
+
 import requests
 
 
@@ -5,9 +7,10 @@ class HttpError(Exception):
     pass
 
 
-def raise_response_error(r: requests.Response):
+def raise_response_error(r: requests.Response) -> Union[NoReturn, None]:
     try:
         r.raise_for_status()
+        return None
     except requests.exceptions.HTTPError as e:
         response = {}
         if r.content and "application/json" in r.headers.get("Content-Type", ""):
diff --git a/tdworkflow/log.py b/tdworkflow/log.py
index ea79ecf..bb2e674 100644
--- a/tdworkflow/log.py
+++ b/tdworkflow/log.py
@@ -1,6 +1,6 @@
 import dataclasses
 from datetime import datetime
-from typing import Dict
+from typing import Any, Dict, Optional
 
 from .resource import Resource
 from .util import parse_iso8601
@@ -10,30 +10,30 @@
 class LogFile(Resource):
     fileName: str
     taskName: str
-    direct: Dict
+    direct: Dict[Any, Any]
     fileSize: int
     agentId: str
-    fileTime: datetime = None
+    fileTime: Optional[datetime] = None
 
-    def __post_init__(self):
-        self.fileTime = parse_iso8601(self.fileTime)
+    def __post_init__(self) -> None:
+        self.fileTime = parse_iso8601(self.fileTime)  # type: ignore
 
     @property
-    def file_name(self):
+    def file_name(self) -> str:
         return self.fileName
 
     @property
-    def taks_name(self):
+    def taks_name(self) -> str:
         return self.taskName
 
     @property
-    def file_time(self):
+    def file_time(self) -> Optional[datetime]:
         return self.fileTime
 
     @property
-    def file_size(self):
+    def file_size(self) -> int:
         return self.fileSize
 
     @property
-    def agent_id(self):
+    def agent_id(self) -> str:
         return self.agentId
diff --git a/tdworkflow/project.py b/tdworkflow/project.py
index 1c1b1f6..bba6742 100644
--- a/tdworkflow/project.py
+++ b/tdworkflow/project.py
@@ -1,5 +1,6 @@
 import dataclasses
 from datetime import datetime
+from typing import Optional
 
 from .resource import Resource
 from .util import parse_iso8601
@@ -12,32 +13,32 @@ class Project(Resource):
     revision: str = ""
     archiveType: str = ""
     archiveMd5: str = ""
-    createdAt: datetime = None
-    deletedAt: datetime = None
-    updatedAt: datetime = None
+    createdAt: Optional[datetime] = None
+    deletedAt: Optional[datetime] = None
+    updatedAt: Optional[datetime] = None
 
-    def __post_init__(self):
+    def __post_init__(self) -> None:
         self.id = int(self.id)
-        self.createdAt = parse_iso8601(self.createdAt)
-        self.deletedAt = parse_iso8601(self.deletedAt)
-        self.updatedAt = parse_iso8601(self.updatedAt)
+        self.createdAt = parse_iso8601(self.createdAt)  # type: ignore
+        self.deletedAt = parse_iso8601(self.deletedAt)  # type: ignore
+        self.updatedAt = parse_iso8601(self.updatedAt)  # type: ignore
 
     @property
-    def archive_type(self):
+    def archive_type(self) -> str:
         return self.archiveType
 
     @property
-    def archive_md5(self):
+    def archive_md5(self) -> str:
         return self.archive_md5
 
     @property
-    def created_at(self):
+    def created_at(self) -> Optional[datetime]:
         return self.createdAt
 
     @property
-    def deleted_at(self):
+    def deleted_at(self) -> Optional[datetime]:
         return self.deletedAt
 
     @property
-    def updated_at(self):
+    def updated_at(self) -> Optional[datetime]:
         return self.updatedAt
diff --git a/tdworkflow/revision.py b/tdworkflow/revision.py
index 225bde4..2db4c6b 100644
--- a/tdworkflow/revision.py
+++ b/tdworkflow/revision.py
@@ -1,6 +1,6 @@
 import dataclasses
 from datetime import datetime
-from typing import Dict
+from typing import Any, Dict, Optional
 
 from .resource import Resource
 from .util import parse_iso8601
@@ -9,22 +9,22 @@
 @dataclasses.dataclass
 class Revision(Resource):
     revision: str
-    createdAt: datetime = None
+    createdAt: Optional[datetime] = None
     archiveType: str = ""
     archiveMd5: str = ""
-    userInfo: Dict = None
+    userInfo: Optional[Dict[str, Any]] = None
 
-    def __post_init__(self):
-        self.createdAt = parse_iso8601(self.createdAt)
+    def __post_init__(self) -> None:
+        self.createdAt = parse_iso8601(self.createdAt)  # type: ignore
 
     @property
-    def archive_type(self):
+    def archive_type(self) -> str:
         return self.archiveType
 
     @property
-    def archive_md5(self):
+    def archive_md5(self) -> str:
         return self.archive_md5
 
     @property
-    def created_at(self):
+    def created_at(self) -> Optional[datetime]:
         return self.createdAt
diff --git a/tdworkflow/schedule.py b/tdworkflow/schedule.py
index 5d67e44..defdea4 100644
--- a/tdworkflow/schedule.py
+++ b/tdworkflow/schedule.py
@@ -1,6 +1,6 @@
 import dataclasses
 from datetime import datetime
-from typing import Dict, List
+from typing import Any, Dict, List, Optional, Union
 
 from .attempt import Attempt
 from .project import Project
@@ -8,45 +8,47 @@
 from .util import parse_iso8601
 from .workflow import Workflow
 
+NextSchedule = Dict[str, Union[int, str, Dict[str, Any]]]
+
 
 @dataclasses.dataclass
 class Schedule(Resource):
     id: int
     project: Project
     workflow: Workflow
-    createdAt: datetime = None
-    updatedAt: datetime = None
-    disabledAt: datetime = None
-    nextScheduleTime: Dict = None
-    nextRunTime: datetime = None
+    createdAt: Optional[datetime] = None
+    updatedAt: Optional[datetime] = None
+    disabledAt: Optional[datetime] = None
+    nextScheduleTime: Optional[NextSchedule] = None
+    nextRunTime: Optional[datetime] = None
 
-    def __post_init__(self):
+    def __post_init__(self) -> None:
         self.id = int(self.id)
-        self.project = Project(**self.project)
-        self.workflow = Workflow(**self.workflow)
-        self.createdAt = parse_iso8601(self.createdAt)
-        self.updatedAt = parse_iso8601(self.updatedAt)
-        self.disabledAt = parse_iso8601(self.disabledAt)
-        self.nextRunTime = parse_iso8601(self.nextRunTime)
+        self.project = Project(**self.project)  # type: ignore
+        self.workflow = Workflow(**self.workflow)  # type: ignore
+        self.createdAt = parse_iso8601(self.createdAt)  # type: ignore
+        self.updatedAt = parse_iso8601(self.updatedAt)  # type: ignore
+        self.disabledAt = parse_iso8601(self.disabledAt)  # type: ignore
+        self.nextRunTime = parse_iso8601(self.nextRunTime)  # type: ignore
 
     @property
-    def created_at(self):
+    def created_at(self) -> Optional[datetime]:
         return self.createdAt
 
     @property
-    def updated_at(self):
+    def updated_at(self) -> Optional[datetime]:
         return self.updatedAt
 
     @property
-    def disabled_at(self):
+    def disabled_at(self) -> Optional[datetime]:
         return self.disabledAt
 
     @property
-    def next_run_time(self):
+    def next_run_time(self) -> Optional[datetime]:
         return self.nextRunTime
 
     @property
-    def next_schedule_time(self):
+    def next_schedule_time(self) -> Optional[NextSchedule]:
         return self.nextScheduleTime
 
 
@@ -54,11 +56,11 @@ def next_schedule_time(self):
 class ScheduleAttempt(Resource):
     id: int
     attempts: List[Attempt]
-    project: Project = None
-    workflow: Workflow = None
+    project: Optional[Project] = None
+    workflow: Optional[Workflow] = None
 
-    def __post_init__(self):
+    def __post_init__(self) -> None:
         self.id = int(self.id)
-        self.attempts = [Attempt(**att) for att in self.attempts]
-        self.project = Project(**self.project)
-        self.workflow = Project(**self.workflow)
+        self.attempts = [Attempt(**att) for att in self.attempts]  # type: ignore
+        self.project = Project(**self.project)  # type: ignore
+        self.workflow = Project(**self.workflow)  # type: ignore
diff --git a/tdworkflow/session.py b/tdworkflow/session.py
index 37c587a..9bbc4bc 100644
--- a/tdworkflow/session.py
+++ b/tdworkflow/session.py
@@ -1,5 +1,6 @@
 import dataclasses
 from datetime import datetime
+from typing import Optional
 
 from .attempt import Attempt
 from .project import Project
@@ -14,24 +15,24 @@ class Session(Resource):
     project: Project
     workflow: Workflow
     sessionUuid: str
-    sessionTime: datetime = None
-    lastAttempt: Attempt = None
+    sessionTime: Optional[datetime] = None
+    lastAttempt: Optional[Attempt] = None
 
-    def __post_init__(self):
+    def __post_init__(self) -> None:
         self.id = int(self.id)
-        self.project = Project(**self.project)
-        self.workflow = Workflow(**self.workflow)
-        self.sessionTime = parse_iso8601(self.sessionTime)
-        self.lastAttempt = Attempt(**self.lastAttempt)
+        self.project = Project(**self.project)  # type: ignore
+        self.workflow = Workflow(**self.workflow)  # type: ignore
+        self.sessionTime = parse_iso8601(self.sessionTime)  # type: ignore
+        self.lastAttempt = Attempt(**self.lastAttempt)  # type: ignore
 
     @property
-    def session_uuid(self):
+    def session_uuid(self) -> str:
         return self.sessionUuid
 
     @property
-    def session_time(self):
+    def session_time(self) -> Optional[datetime]:
         return self.session_time
 
     @property
-    def last_attempt(self):
+    def last_attempt(self) -> Optional[Attempt]:
         return self.lastAttempt
diff --git a/tdworkflow/task.py b/tdworkflow/task.py
index 93a22fa..a0fcd64 100644
--- a/tdworkflow/task.py
+++ b/tdworkflow/task.py
@@ -1,7 +1,7 @@
 import dataclasses
 import json
 from datetime import datetime
-from typing import Dict, List
+from typing import Any, Dict, List, Optional
 
 from .resource import Resource
 from .util import parse_iso8601
@@ -11,66 +11,66 @@
 class Task(Resource):
     id: int
     state: str
-    updatedAt: datetime = None
+    updatedAt: Optional[datetime] = None
     fullName: str = ""
-    parentId: int = None
-    upstreams: List[int] = None
-    retryAt: datetime = None
-    config: Dict = None
-    exportParams: Dict = None
-    storeParams: Dict = None
-    stateParams: Dict = None
-    error: Dict = None
-    startedAt: datetime = None
+    parentId: Optional[int] = None
+    upstreams: Optional[List[int]] = None
+    retryAt: Optional[datetime] = None
+    config: Optional[Dict[str, Any]] = None
+    exportParams: Optional[Dict[str, Any]] = None
+    storeParams: Optional[Dict[str, Any]] = None
+    stateParams: Optional[Dict[str, Any]] = None
+    error: Optional[Dict[str, Any]] = None
+    startedAt: Optional[datetime] = None
     cancelRequested: bool = False
     isGroup: bool = False
 
-    def __post_init__(self):
+    def __post_init__(self) -> None:
         self.id = int(self.id)
-        self.updatedAt = parse_iso8601(self.updatedAt)
+        self.updatedAt = parse_iso8601(self.updatedAt)  # type: ignore
         self.parentId = int(self.parentId) if self.parentId else None
-        self.upstreams = [int(_id) for _id in self.upstreams]
-        self.retryAt = parse_iso8601(self.retryAt)
-        self.startedAt = parse_iso8601(self.startedAt)
+        self.upstreams = [int(_id) for _id in self.upstreams] if self.upstreams else []
+        self.retryAt = parse_iso8601(self.retryAt)  # type: ignore
+        self.startedAt = parse_iso8601(self.startedAt)  # type: ignore
 
     @property
-    def updated_at(self):
+    def updated_at(self) -> Optional[datetime]:
         return self.updatedAt
 
     @property
-    def full_name(self):
+    def full_name(self) -> str:
         return self.fullName
 
     @property
-    def parent_id(self):
+    def parent_id(self) -> Optional[int]:
         return self.parentId
 
     @property
-    def retry_at(self):
+    def retry_at(self) -> Optional[datetime]:
         return self.retryAt
 
     @property
-    def export_params(self):
+    def export_params(self) -> Optional[Dict[str, Any]]:
         return self.exportParams
 
     @property
-    def store_params(self):
+    def store_params(self) -> Optional[Dict[str, Any]]:
         return self.storeParams
 
     @property
-    def state_params(self):
+    def state_params(self) -> Optional[Dict[str, Any]]:
         return self.stateParams
 
     @property
-    def started_at(self):
+    def started_at(self) -> Optional[datetime]:
         return self.startedAt
 
     @property
-    def cancel_requested(self):
+    def cancel_requested(self) -> bool:
         return self.cancelRequested
 
     @property
-    def group(self):
+    def group(self) -> bool:
         return self.isGroup
 
 
diff --git a/tdworkflow/util.py b/tdworkflow/util.py
index df09ba1..d9c64a6 100644
--- a/tdworkflow/util.py
+++ b/tdworkflow/util.py
@@ -5,7 +5,7 @@
 import tarfile
 from datetime import datetime, timedelta, timezone
 from pathlib import Path
-from typing import List, Optional
+from typing import List, Optional, Union
 
 logger = logging.getLogger(__name__)
 
@@ -34,15 +34,15 @@ def archive_files(target_dir: str, exclude_patterns: List[str]) -> io.BytesIO:
     return _bytes
 
 
-def parse_iso8601(target: str) -> Optional[datetime]:
+def parse_iso8601(target: Optional[str]) -> Optional[datetime]:
     if not target:
         return None
 
     return datetime.fromisoformat(target.replace("Z", "+00:00"))
 
 
-def to_iso8601(dt: datetime) -> str:
-    if not datetime:
+def to_iso8601(dt: Optional[Union[str, datetime]]) -> str:
+    if not dt:
         return ""
 
     if isinstance(dt, datetime):
diff --git a/tdworkflow/workflow.py b/tdworkflow/workflow.py
index 54ae76f..c530058 100644
--- a/tdworkflow/workflow.py
+++ b/tdworkflow/workflow.py
@@ -1,5 +1,6 @@
 import dataclasses
 from datetime import datetime
+from typing import Any, Dict, Optional
 
 from .project import Project
 from .resource import Resource
@@ -10,29 +11,29 @@
 class Workflow(Resource):
     id: int
     name: str
-    project: Project = None
+    project: Optional[Project] = None
     timezone: str = ""
-    config: dict = None
+    config: Optional[Dict[str, Any]] = None
     revision: str = ""
-    createdAt: datetime = None
-    deletedAt: datetime = None
-    updatedAt: datetime = None
+    createdAt: Optional[datetime] = None
+    deletedAt: Optional[datetime] = None
+    updatedAt: Optional[datetime] = None
 
-    def __post_init__(self):
+    def __post_init__(self) -> None:
         self.id = int(self.id)
-        self.project = Project(**self.project) if self.project else None
-        self.createdAt = parse_iso8601(self.createdAt)
-        self.deletedAt = parse_iso8601(self.deletedAt)
-        self.updatedAt = parse_iso8601(self.updatedAt)
+        self.project = Project(**self.project) if self.project else None  # type: ignore
+        self.createdAt = parse_iso8601(self.createdAt)  # type: ignore
+        self.deletedAt = parse_iso8601(self.deletedAt)  # type: ignore
+        self.updatedAt = parse_iso8601(self.updatedAt)  # type: ignore
 
     @property
-    def created_at(self):
+    def created_at(self) -> Optional[datetime]:
         return self.createdAt
 
     @property
-    def deleted_at(self):
+    def deleted_at(self) -> Optional[datetime]:
         return self.deletedAt
 
     @property
-    def updated_at(self):
+    def updated_at(self) -> Optional[datetime]:
         return self.updatedAt