diff --git a/swh/lister/bitbucket/lister.py b/swh/lister/bitbucket/lister.py
--- a/swh/lister/bitbucket/lister.py
+++ b/swh/lister/bitbucket/lister.py
@@ -7,9 +7,9 @@
 import iso8601
 
 from datetime import datetime, timezone
-from typing import Any
+from typing import Any, Dict, List, Optional
 from urllib import parse
-
+from requests import Response
 
 from swh.lister.bitbucket.models import BitBucketModel
 from swh.lister.core.indexing_lister import IndexingHttpLister
@@ -26,14 +26,16 @@
     instance = 'bitbucket'
     default_min_bound = datetime.fromtimestamp(0, timezone.utc)  # type: Any
 
-    def __init__(self, url=None, override_config=None, per_page=100):
+    def __init__(self, url: Optional[str] = None,
+                 override_config: Optional[bool] = None,
+                 per_page: int = 100) -> None:
         super().__init__(url=url, override_config=override_config)
         per_page = self.config.get('per_page', per_page)
 
         self.PATH_TEMPLATE = '%s&pagelen=%s' % (
             self.PATH_TEMPLATE, per_page)
 
-    def get_model_from_repo(self, repo):
+    def get_model_from_repo(self, repo: Dict) -> Dict[str, Any]:
         return {
             'uid': repo['uuid'],
             'indexable': iso8601.parse_date(repo['created_on']),
@@ -44,7 +46,8 @@
             'origin_type': repo['scm'],
         }
 
-    def get_next_target_from_response(self, response):
+    def get_next_target_from_response(self, response: Response
+                                      ) -> Optional[datetime]:
         """This will read the 'next' link from the api response if any
            and return it as a datetime.
 
@@ -60,23 +63,28 @@
         if next_ is not None:
             next_ = parse.urlparse(next_)
             return iso8601.parse_date(parse.parse_qs(next_.query)['after'][0])
+        return None
 
-    def transport_response_simplified(self, response):
+    def transport_response_simplified(self, response: Response
+                                      ) -> List[Dict[str, Any]]:
         repos = response.json()['values']
         return [self.get_model_from_repo(repo) for repo in repos]
 
-    def request_uri(self, identifier):
-        identifier = parse.quote(identifier.isoformat())
-        return super().request_uri(identifier or '1970-01-01')
+    def request_uri(self, identifier: datetime) -> str:  # type: ignore
+        identifier_str = parse.quote(identifier.isoformat())
+        return super().request_uri(identifier_str or '1970-01-01')
 
-    def is_within_bounds(self, inner, lower=None, upper=None):
+    def is_within_bounds(self, inner: int, lower: Optional[int] = None,
+                         upper: Optional[int] = None) -> bool:
         # values are expected to be datetimes
-        if lower is None and upper is None:
-            ret = True
-        elif lower is None:
-            ret = inner <= upper
-        elif upper is None:
-            ret = inner >= lower
+        if lower is None:
+            if upper is None:
+                ret = True
+            else:
+                ret = inner <= upper
         else:
-            ret = lower <= inner <= upper
+            if upper is None:
+                ret = inner >= lower
+            else:
+                ret = lower <= inner <= upper
         return ret
diff --git a/swh/lister/cgit/lister.py b/swh/lister/cgit/lister.py
--- a/swh/lister/cgit/lister.py
+++ b/swh/lister/cgit/lister.py
@@ -8,8 +8,10 @@
 
 from bs4 import BeautifulSoup
 from requests import Session
-from requests.adapters import HTTPAdapter
 
+from requests.adapters import HTTPAdapter
+from requests.structures import CaseInsensitiveDict
+from typing import Dict, Generator, Optional, Any
 from .models import CGitModel
 
 from swh.core.utils import grouper
@@ -54,7 +56,9 @@
     LISTER_NAME = 'cgit'
     url_prefix_present = True
 
-    def __init__(self, url=None, instance=None, override_config=None):
+    def __init__(self, url: Optional[str] = None,
+                 instance: Optional[str] = None,
+                 override_config: Optional[str] = None) -> None:
         """Lister class for CGit repositories.
 
         Args:
@@ -75,11 +79,11 @@
         self.instance = instance
         self.session = Session()
         self.session.mount(self.url, HTTPAdapter(max_retries=3))
-        self.session.headers = {
+        self.session.headers = CaseInsensitiveDict({
             'User-Agent': USER_AGENT,
-        }
+        })
 
-    def run(self):
+    def run(self) -> Dict[str, Any]:
         status = 'uneventful'
         total = 0
         for repos in grouper(self.get_repos(), 10):
@@ -94,7 +98,7 @@
 
         return {'status': status}
 
-    def get_repos(self):
+    def get_repos(self) -> Generator[str, None, None]:
         """Generate git 'project' URLs found on the current CGit server
 
         """
@@ -116,7 +120,7 @@
                 # no pager, or no next page
                 next_page = None
 
-    def build_model(self, repo_url):
+    def build_model(self, repo_url: str) -> Optional[Dict[str, str]]:
         """Given the URL of a git repo project page on a CGit server,
         return the repo description (dict) suitable for insertion in the db.
         """
@@ -124,7 +128,7 @@
         urls = [x['href'] for x in bs.find_all('a', {'rel': 'vcs-git'})]
 
         if not urls:
-            return
+            return None
 
         # look for the http/https url, if any, and use it as origin_url
         for url in urls:
@@ -138,11 +142,11 @@
         return {'uid': repo_url,
                 'name': bs.find('a', title=re.compile('.+'))['title'],
                 'origin_type': 'git',
-                'instance': self.instance,
+                'instance': str(self.instance),
                 'origin_url': origin_url,
                 }
 
-    def get_and_parse(self, url):
+    def get_and_parse(self, url: str) -> BeautifulSoup:
         "Get the given url and parse the retrieved HTML using BeautifulSoup"
         return BeautifulSoup(self.session.get(url).text,
                              features='html.parser')
diff --git a/swh/lister/core/indexing_lister.py b/swh/lister/core/indexing_lister.py
--- a/swh/lister/core/indexing_lister.py
+++ b/swh/lister/core/indexing_lister.py
@@ -12,10 +12,14 @@
 from .lister_transports import ListerHttpTransport
 from .lister_base import ListerBase
 
+from requests import Response
+from typing import Any, Dict, List, Tuple, Optional, Generic, TypeVar
+
 logger = logging.getLogger(__name__)
+T = TypeVar('T')
 
 
-class IndexingLister(ListerBase):
+class IndexingLister(ListerBase, Generic[T]):
     """Lister* intermediate class for any service that follows the pattern:
 
     - The service must report at least one stable unique identifier, known
@@ -55,7 +59,9 @@
     """
 
     @abc.abstractmethod
-    def get_next_target_from_response(self, response):
+    def get_next_target_from_response(
+            self, response: Response
+    ) -> T:
         """Find the next server endpoint identifier given the entire response.
 
         Implementation of this method depends on the server API spec
@@ -71,7 +77,8 @@
 
     # You probably don't need to override anything below this line.
 
-    def filter_before_inject(self, models_list):
+    def filter_before_inject(
+            self, models_list: List[Dict[str, Any]]) -> List[Dict[str, str]]:
         """Overrides ListerBase.filter_before_inject
 
         Bounds query results by this Lister's set max_index.
@@ -100,7 +107,9 @@
             retlist = retlist.filter(self.MODEL.indexable <= end)
         return retlist
 
-    def db_partition_indices(self, partition_size):
+    def db_partition_indices(
+            self, partition_size: int
+    ) -> List[Tuple[Optional[int], Optional[int]]]:
         """Describe an index-space compartmentalization of the db table
            in equal sized chunks. This is used to describe min&max bounds for
            parallelizing fetch tasks.
@@ -165,6 +174,7 @@
         t = self.db_session.query(func.min(self.MODEL.indexable)).first()
         if t:
             return t[0]
+        return None
 
     def db_last_index(self):
         """Look in the db for the largest indexable value
@@ -175,6 +185,7 @@
         t = self.db_session.query(func.max(self.MODEL.indexable)).first()
         if t:
             return t[0]
+        return None
 
     def disable_deleted_repo_tasks(self, start, end, keep_these):
         """Disable tasks for repos that no longer exist between start and end.
@@ -254,6 +265,7 @@
 class IndexingHttpLister(ListerHttpTransport, IndexingLister):
     """Convenience class for ensuring right lookup and init order
         when combining IndexingLister and ListerHttpTransport."""
+
     def __init__(self, url=None, override_config=None):
         IndexingLister.__init__(self, override_config=override_config)
         ListerHttpTransport.__init__(self, url=url)
diff --git a/swh/lister/core/lister_base.py b/swh/lister/core/lister_base.py
--- a/swh/lister/core/lister_base.py
+++ b/swh/lister/core/lister_base.py
@@ -2,6 +2,11 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+from requests import Response
+from .abstractattribute import AbstractAttribute
+from swh.scheduler import get_scheduler, utils
+from swh.core.utils import grouper
+from swh.core import config
 import abc
 import datetime
 import gzip
@@ -13,17 +18,13 @@
 
 from sqlalchemy import create_engine, func
 from sqlalchemy.orm import sessionmaker
-from typing import Any, Dict, List, Type, Union
-
-from swh.core import config
-from swh.core.utils import grouper
-from swh.scheduler import get_scheduler, utils
-
-from .abstractattribute import AbstractAttribute
-
+from typing import Any, Dict, List, Type, Union, Optional
+from typing import Generic, TypeVar, Tuple
 
 logger = logging.getLogger(__name__)
 
+T = TypeVar('T')
+
 
 def utcnow():
     return datetime.datetime.now(tz=datetime.timezone.utc)
@@ -37,7 +38,7 @@
         return repr(self.response)
 
 
-class ListerBase(abc.ABC, config.SWHConfig):
+class ListerBase(abc.ABC, config.SWHConfig, Generic[T]):
     """Lister core base class.
         Generally a source code hosting service provides an API endpoint
         for listing the set of stored repositories. A Lister is the discovery
@@ -73,7 +74,7 @@
     LISTER_NAME = AbstractAttribute(
         "Lister's name")  # type: Union[AbstractAttribute, str]
 
-    def transport_request(self, identifier):
+    def transport_request(self, identifier: Union[str, int]):
         """Given a target endpoint identifier to query, try once to request it.
 
         Implementation of this method determines the network request protocol.
@@ -93,7 +94,7 @@
         """
         pass
 
-    def transport_response_to_string(self, response):
+    def transport_response_to_string(self, response: Response):
         """Convert the server response into a formatted string for logging.
 
         Implementation of this method depends on the shape of the network
@@ -106,7 +107,7 @@
         """
         pass
 
-    def transport_response_simplified(self, response):
+    def transport_response_simplified(self, response: Response):
         """Convert the server response into list of a dict for each repo in the
             response, mapping columns in the lister's MODEL class to repo data.
 
@@ -122,7 +123,7 @@
         """
         pass
 
-    def transport_quota_check(self, response):
+    def transport_quota_check(self, response: Response):
         """Check server response to see if we're hitting request rate limits.
 
         Implementation of this method depends on the server communication
@@ -137,7 +138,8 @@
         """
         pass
 
-    def filter_before_inject(self, models_list):
+    def filter_before_inject(
+            self, models_list: List[Dict]) -> List[Dict]:
         """Filter models_list entries prior to injection in the db.
         This is ran directly after `transport_response_simplified`.
 
@@ -152,7 +154,8 @@
         """
         return models_list
 
-    def do_additional_checks(self, models_list):
+    def do_additional_checks(
+            self, models_list: List[Dict]) -> Union[List[Dict], bool]:
         """Execute some additional checks on the model list (after the
         filtering).
 
@@ -169,7 +172,9 @@
         """
         return models_list
 
-    def is_within_bounds(self, inner, lower=None, upper=None):
+    def is_within_bounds(
+            self, inner: int,
+            lower: Optional[int] = None, upper: Optional[int] = None) -> bool:
         """See if a sortable value is inside the range [lower,upper].
 
         MAY BE OVERRIDDEN, for example if the server indexable* key is
@@ -188,7 +193,7 @@
             if lower is None and upper is None:
                 return True
             elif lower is None:
-                ret = inner <= upper
+                ret = inner <= upper  # type: ignore
             elif upper is None:
                 ret = inner >= lower
             else:
@@ -238,7 +243,7 @@
     MAX_RETRIES = 7
     CONN_SLEEP = 10
 
-    def __init__(self, override_config=None):
+    def __init__(self, override_config=None) -> None:
         self.backoff = self.INITIAL_BACKOFF
         logger.debug('Loading config from %s' % self.CONFIG_BASE_FILENAME)
         self.config = self.parse_config_file(
@@ -258,17 +263,18 @@
         self.mk_session = sessionmaker(bind=self.db_engine)
         self.db_session = self.mk_session()
 
-    def reset_backoff(self):
+    def reset_backoff(self) -> None:
         """Reset exponential backoff timeout to initial level."""
         self.backoff = self.INITIAL_BACKOFF
 
-    def back_off(self):
+    def back_off(self) -> int:
         """Get next exponential backoff timeout."""
         ret = self.backoff
         self.backoff *= 10
         return ret
 
-    def safely_issue_request(self, identifier):
+    def safely_issue_request(self, identifier: Union[str, int]
+                             ) -> Optional[Response]:
         """Make network request with retries, rate quotas, and response logs.
 
         Protocol is handled by the implementation of the transport_request
@@ -315,7 +321,7 @@
 
         return r
 
-    def db_query_equal(self, key, value):
+    def db_query_equal(self, key: Any, value: Any):
         """Look in the db for a row with key == value
 
         Args:
@@ -396,7 +402,7 @@
         kw = {'priority': priority} if priority else {}
         return utils.create_task_dict(_type, _policy, url=origin_url, **kw)
 
-    def string_pattern_check(self, a, b, c=None):
+    def string_pattern_check(self, a, b, c=None) -> None:
         """When comparing indexable types in is_within_bounds, complex strings
             may not be allowed to differ in basic structure. If they do, it
             could be a sign of not understanding the data well. For instance,
@@ -418,8 +424,8 @@
             a_pattern = re.sub('[a-zA-Z0-9]',
                                '[a-zA-Z0-9]',
                                re.escape(a))
-            if (isinstance(b, str) and (re.match(a_pattern, b) is None)
-               or isinstance(c, str) and (re.match(a_pattern, c) is None)):
+            if (isinstance(b, str) and (re.match(a_pattern, b) is None) or
+                    isinstance(c, str) and (re.match(a_pattern, c) is None)):
                 logger.debug(a_pattern)
                 raise TypeError('incomparable string patterns detected')
 
@@ -456,7 +462,7 @@
         """
         tasks = {}
 
-        def _task_key(m):
+        def _task_key(m) -> str:
             return '%s-%s' % (
                 m['type'],
                 json.dumps(m['arguments'], sort_keys=True)
@@ -481,7 +487,8 @@
                 ir, m, _ = tasks[_task_key(task)]
                 ir.task_id = task['id']
 
-    def ingest_data(self, identifier, checks=False):
+    def ingest_data(self, identifier: Union[str, int], checks: bool = False
+                    ) -> Tuple[Optional[Response], Union[Dict, List]]:
         """The core data fetch sequence. Request server endpoint. Simplify and
             filter response list of repositories. Inject repo information into
             local db. Queue loader tasks for linked repositories.
@@ -506,7 +513,7 @@
         self.schedule_missing_tasks(models_list, injected)
         return response, injected
 
-    def save_response(self, response):
+    def save_response(self, response: Response):
         """Log the response from a server request to a cache dir.
 
         Args:
diff --git a/swh/lister/core/lister_transports.py b/swh/lister/core/lister_transports.py
--- a/swh/lister/core/lister_transports.py
+++ b/swh/lister/core/lister_transports.py
@@ -12,7 +12,8 @@
 import requests
 import xmltodict
 
-from typing import Optional, Union
+from typing import Optional, Union, Dict, Any, List
+from requests import Response
 
 from swh.lister import USER_AGENT_TEMPLATE, __version__
 
@@ -39,7 +40,7 @@
 
     EXPECTED_STATUS_CODES = (200, 429, 403, 404)
 
-    def request_headers(self):
+    def request_headers(self) -> Dict[str, Any]:
         """Returns dictionary of any request headers needed by the server.
 
         MAY BE OVERRIDDEN if request headers are needed.
@@ -48,7 +49,7 @@
             'User-Agent': USER_AGENT_TEMPLATE % self.lister_version
         }
 
-    def request_instance_credentials(self):
+    def request_instance_credentials(self) -> List[Dict[str, Any]]:
         """Returns dictionary of any credentials configuration needed by the
         forge instance to list.
 
@@ -81,23 +82,23 @@
             list of credential dicts for the current lister.
 
         """
-        all_creds = self.config.get('credentials')
+        all_creds = self.config.get('credentials')  # type: ignore
         if not all_creds:
             return []
-        lister_creds = all_creds.get(self.LISTER_NAME, {})
-        creds = lister_creds.get(self.instance, [])
+        lister_creds = all_creds.get(self.LISTER_NAME, {})  # type: ignore
+        creds = lister_creds.get(self.instance, [])  # type: ignore
         return creds
 
-    def request_uri(self, identifier):
+    def request_uri(self, identifier: Union[str, int]) -> str:
         """Get the full request URI given the transport_request identifier.
 
         MAY BE OVERRIDDEN if something more complex than the PATH_TEMPLATE is
         required.
         """
-        path = self.PATH_TEMPLATE % identifier
+        path = self.PATH_TEMPLATE % identifier  # type: ignore
         return self.url + path
 
-    def request_params(self, identifier):
+    def request_params(self, identifier: Union[str, int]) -> Dict[str, Any]:
         """Get the full parameters passed to requests given the
         transport_request identifier.
 
@@ -108,14 +109,15 @@
         is needed.
 
         """
-        params = {}
+        params: Dict[str, Any] = {}
         params['headers'] = self.request_headers() or {}
         creds = self.request_instance_credentials()
         if not creds:
             return params
         auth = random.choice(creds) if creds else None
         if auth:
-            params['auth'] = (auth['username'], auth['password'])
+            params['auth'] = (auth['username'],
+                              auth['password'])
         return params
 
     def transport_quota_check(self, response):
@@ -152,7 +154,9 @@
         self.session = requests.Session()
         self.lister_version = __version__
 
-    def _transport_action(self, identifier, method='get'):
+    def _transport_action(
+            self, identifier: Union[str, int],
+            method: str = 'get') -> Response:
         """Permit to ask information to the api prior to actually executing
            query.
 
@@ -176,13 +180,13 @@
                 raise FetchError(response)
             return response
 
-    def transport_head(self, identifier):
+    def transport_head(self, identifier: str) -> Response:
         """Retrieve head information on api.
 
         """
         return self._transport_action(identifier, method='head')
 
-    def transport_request(self, identifier):
+    def transport_request(self, identifier: Union[str, int]) -> Response:
         """Implements ListerBase.transport_request for HTTP using Requests.
 
         Retrieve get information on api.
@@ -190,7 +194,7 @@
         """
         return self._transport_action(identifier)
 
-    def transport_response_to_string(self, response):
+    def transport_response_to_string(self, response: Response) -> str:
         """Implements ListerBase.transport_response_to_string for HTTP given
             Requests responses.
         """
diff --git a/swh/lister/core/page_by_page_lister.py b/swh/lister/core/page_by_page_lister.py
--- a/swh/lister/core/page_by_page_lister.py
+++ b/swh/lister/core/page_by_page_lister.py
@@ -7,6 +7,8 @@
 
 from .lister_transports import ListerHttpTransport
 from .lister_base import ListerBase
+from requests import Response
+from typing import Optional, Tuple, List, Dict, Any, Union
 
 
 class PageByPageLister(ListerBase):
@@ -38,7 +40,8 @@
 
     """
     @abc.abstractmethod
-    def get_next_target_from_response(self, response):
+    def get_next_target_from_response(self, response: Response
+                                      ) -> Optional[int]:
         """Find the next server endpoint page given the entire response.
 
         Implementation of this method depends on the server API spec
@@ -58,7 +61,8 @@
         pass
 
     @abc.abstractmethod
-    def get_pages_information(self):
+    def get_pages_information(
+            self) -> Tuple[Optional[int], Optional[int], Optional[int]]:
         """Find the total number of pages.
 
         Implementation of this method depends on the server API spec
@@ -79,7 +83,9 @@
 
     # You probably don't need to override anything below this line.
 
-    def do_additional_checks(self, models_list):
+    def do_additional_checks(self,
+                             models_list: List[Dict[str, Any]]
+                             ) -> Union[List[Dict[str, Any]], bool]:
         """Potentially check for existence of repositories in models_list.
 
         This will be called only if check_existence is flipped on in
@@ -92,7 +98,9 @@
                 return False
         return models_list
 
-    def run(self, min_bound=None, max_bound=None, check_existence=False):
+    def run(self, min_bound: Optional[int] = None,
+            max_bound: Optional[int] = None,
+            check_existence: bool = False):
         """Main entry function. Sequentially fetches repository data from the
            service according to the basic outline in the class
            docstring. Continually fetching sublists until either there
@@ -130,7 +138,8 @@
                 break
             status = 'eventful'
 
-            next_page = self.get_next_target_from_response(response)
+            next_page = self.get_next_target_from_response(
+                response)  # type: ignore
 
             # termination condition
 
@@ -159,6 +168,8 @@
        combining PageByPageLister and ListerHttpTransport.
 
     """
-    def __init__(self, url=None, override_config=None):
+
+    def __init__(self, url: Optional[str] = None,
+                 override_config: Optional[bool] = None) -> None:
         PageByPageLister.__init__(self, override_config=override_config)
         ListerHttpTransport.__init__(self, url=url)
diff --git a/swh/lister/core/simple_lister.py b/swh/lister/core/simple_lister.py
--- a/swh/lister/core/simple_lister.py
+++ b/swh/lister/core/simple_lister.py
@@ -9,6 +9,7 @@
 
 from swh.core import utils
 from .lister_base import ListerBase
+from requests import Response
 
 
 logger = logging.getLogger(__name__)
@@ -29,7 +30,7 @@
        db (see fn:`ingest_data`).
     """
 
-    def list_packages(self, response: Any) -> List[Any]:
+    def list_packages(self, response: Response) -> List[Any]:
         """Listing packages method.
 
         """
diff --git a/swh/lister/cran/lister.py b/swh/lister/cran/lister.py
--- a/swh/lister/cran/lister.py
+++ b/swh/lister/cran/lister.py
@@ -8,13 +8,15 @@
 import pkg_resources
 import subprocess
 
-from typing import List, Mapping, Tuple
+from typing import List, Mapping, Tuple, Any, Dict, Optional, Union
 
 from swh.lister.cran.models import CRANModel
 
 from swh.lister.core.simple_lister import SimpleLister
 from swh.scheduler.utils import create_task_dict
 
+from requests import Response
+
 
 logger = logging.getLogger(__name__)
 
@@ -27,8 +29,10 @@
     LISTER_NAME = 'cran'
     instance = 'cran'
 
-    def task_dict(self, origin_type, origin_url, version=None, html_url=None,
-                  policy=None, **kwargs):
+    def task_dict(self, origin_type: str, origin_url: str,
+                  version: Optional[str] = None,
+                  html_url: Optional[str] = None, policy: Optional[str] = None,
+                  **kwargs) -> Dict[str, Any]:
         """Return task format dict. This creates tasks with args and kwargs
         set, for example::
 
@@ -54,7 +58,7 @@
             }], retries_left=3
         )
 
-    def safely_issue_request(self, identifier):
+    def safely_issue_request(self, identifier: Union[str, int]) -> None:
         """Bypass the implementation. It's now the `list_packages` which
         returns data.
 
@@ -65,7 +69,7 @@
         """
         return None
 
-    def list_packages(self, response) -> List[Mapping[str, str]]:
+    def list_packages(self, response: Response) -> List[Mapping[str, str]]:
         """Runs R script which uses inbuilt API to return a json response
            containing data about the R packages.
 
diff --git a/swh/lister/debian/lister.py b/swh/lister/debian/lister.py
--- a/swh/lister/debian/lister.py
+++ b/swh/lister/debian/lister.py
@@ -13,7 +13,8 @@
 from debian.deb822 import Sources
 from sqlalchemy.orm import joinedload, load_only
 from sqlalchemy.schema import CreateTable, DropTable
-from typing import Mapping, Optional
+from typing import Mapping, Optional, Dict, Any, Union
+from requests import Response
 
 from swh.lister.debian.models import (
     AreaSnapshot, Distribution, DistributionSnapshot, Package,
@@ -58,7 +59,7 @@
         self.date = override_config.get('date', date) or datetime.datetime.now(
             tz=datetime.timezone.utc)
 
-    def transport_request(self, identifier):
+    def transport_request(self, identifier: Union[str, int]) -> Response:
         """Subvert ListerHttpTransport.transport_request, to try several
         index URIs in turn.
 
@@ -88,13 +89,13 @@
         self.decompressor = decompressors.get(compression)
         return response
 
-    def request_uri(self, identifier):
+    def request_uri(self, identifier: Union[str, int]):
         # In the overridden transport_request, we pass
         # ListerBase.transport_request() the full URI as identifier, so we
         # need to return it here.
         return identifier
 
-    def request_params(self, identifier):
+    def request_params(self, identifier: Union[str, int]) -> Dict[str, Any]:
         # Enable streaming to allow wrapping the response in the decompressor
         # in transport_response_simplified.
         params = super().request_params(identifier)
@@ -108,13 +109,14 @@
         Checksums-Sha1, Checksums-Sha256), to return a files dict mapping
         filenames to their checksums.
         """
+        data = response.raw
         if self.decompressor:
-            data = self.decompressor(response.raw)
+            data = self.decompressor(data)
         else:
             data = response.raw
 
         for src_pkg in Sources.iter_paragraphs(data.readlines()):
-            files = defaultdict(dict)
+            files: Dict[str, Any] = defaultdict(dict)
 
             for field in src_pkg._multivalued_fields:
                 if field.startswith('checksums-'):
diff --git a/swh/lister/github/lister.py b/swh/lister/github/lister.py
--- a/swh/lister/github/lister.py
+++ b/swh/lister/github/lister.py
@@ -5,11 +5,13 @@
 
 import re
 
-from typing import Any
+from typing import Any, Dict, List, Tuple, Optional
 
 from swh.lister.core.indexing_lister import IndexingHttpLister
 from swh.lister.github.models import GitHubModel
 
+from requests import Response
+
 
 class GitHubLister(IndexingHttpLister):
     PATH_TEMPLATE = '/repositories?since=%d'
@@ -20,7 +22,7 @@
     instance = 'github'  # There is only 1 instance of such lister
     default_min_bound = 0  # type: Any
 
-    def get_model_from_repo(self, repo):
+    def get_model_from_repo(self, repo: Dict[str, Any]) -> Dict[str, Any]:
         return {
             'uid': repo['id'],
             'indexable': repo['id'],
@@ -32,7 +34,7 @@
             'fork': repo['fork'],
         }
 
-    def transport_quota_check(self, response):
+    def transport_quota_check(self, response: Response) -> Tuple[bool, int]:
         x_rate_limit_remaining = response.headers.get('X-RateLimit-Remaining')
         if not x_rate_limit_remaining:
             return False, 0
@@ -42,17 +44,21 @@
             return True, delay
         return False, 0
 
-    def get_next_target_from_response(self, response):
+    def get_next_target_from_response(self,
+                                      response: Response) -> Optional[int]:
         if 'next' in response.links:
             next_url = response.links['next']['url']
-            return int(self.API_URL_INDEX_RE.match(next_url).group(1))
+            return int(
+                self.API_URL_INDEX_RE.match(next_url).group(1))  # type: ignore
+        return None
 
-    def transport_response_simplified(self, response):
+    def transport_response_simplified(self, response: Response
+                                      ) -> List[Dict[str, Any]]:
         repos = response.json()
         return [self.get_model_from_repo(repo)
                 for repo in repos if repo and 'id' in repo]
 
-    def request_headers(self):
+    def request_headers(self) -> Dict[str, Any]:
         """(Override) Set requests headers to send when querying the GitHub API
 
         """
@@ -60,7 +66,8 @@
         headers['Accept'] = 'application/vnd.github.v3+json'
         return headers
 
-    def disable_deleted_repo_tasks(self, index, next_index, keep_these):
+    def disable_deleted_repo_tasks(self, index: int,
+                                   next_index: int, keep_these: int):
         """ (Overrides) Fix provided index value to avoid erroneously disabling
         some scheduler tasks
         """
diff --git a/swh/lister/gitlab/lister.py b/swh/lister/gitlab/lister.py
--- a/swh/lister/gitlab/lister.py
+++ b/swh/lister/gitlab/lister.py
@@ -9,6 +9,9 @@
 from ..core.page_by_page_lister import PageByPageHttpLister
 from .models import GitLabModel
 
+from typing import Any, Dict, List, Tuple, Union, MutableMapping, Optional
+from requests import Response
+
 
 class GitLabLister(PageByPageHttpLister):
     # Template path expecting an integer that represents the page id
@@ -17,8 +20,10 @@
     MODEL = GitLabModel
     LISTER_NAME = 'gitlab'
 
-    def __init__(self, url=None, instance=None,
-                 override_config=None, sort='asc', per_page=20):
+    def __init__(self, url: Optional[str] = None,
+                 instance: Optional[str] = None,
+                 override_config: Optional[bool] = None,
+                 sort: str = 'asc', per_page: int = 20) -> None:
         super().__init__(url=url, override_config=override_config)
         if instance is None:
             instance = parse_url(self.url).host
@@ -26,10 +31,10 @@
         self.PATH_TEMPLATE = '%s&sort=%s&per_page=%s' % (
             self.PATH_TEMPLATE, sort, per_page)
 
-    def uid(self, repo):
+    def uid(self, repo: Dict[str, Any]) -> str:
         return '%s/%s' % (self.instance, repo['path_with_namespace'])
 
-    def get_model_from_repo(self, repo):
+    def get_model_from_repo(self, repo: Dict[str, Any]) -> Dict[str, Any]:
         return {
             'instance': self.instance,
             'uid': self.uid(repo),
@@ -40,7 +45,8 @@
             'origin_type': 'git',
         }
 
-    def transport_quota_check(self, response):
+    def transport_quota_check(self, response: Response
+                              ) -> Tuple[bool, Union[int, float]]:
         """Deal with rate limit if any.
 
         """
@@ -53,22 +59,26 @@
                 return True, delay
         return False, 0
 
-    def _get_int(self, headers, key):
+    def _get_int(self, headers: MutableMapping[str, Any],
+                 key: str) -> Optional[int]:
         _val = headers.get(key)
         if _val:
             return int(_val)
+        return None
 
-    def get_next_target_from_response(self, response):
+    def get_next_target_from_response(
+            self, response: Response) -> Optional[int]:
         """Determine the next page identifier.
 
         """
         return self._get_int(response.headers, 'x-next-page')
 
-    def get_pages_information(self):
+    def get_pages_information(self) -> Tuple[Optional[int],
+                                             Optional[int], Optional[int]]:
         """Determine pages information.
 
         """
-        response = self.transport_head(identifier=1)
+        response = self.transport_head(identifier=1)  # type: ignore
         if not response.ok:
             raise ValueError(
                 'Problem during information fetch: %s' % response.status_code)
@@ -77,6 +87,7 @@
                 self._get_int(h, 'x-total-pages'),
                 self._get_int(h, 'x-per-page'))
 
-    def transport_response_simplified(self, response):
+    def transport_response_simplified(self, response: Response
+                                      ) -> List[Dict[str, Any]]:
         repos = response.json()
         return [self.get_model_from_repo(repo) for repo in repos]
diff --git a/swh/lister/gnu/lister.py b/swh/lister/gnu/lister.py
--- a/swh/lister/gnu/lister.py
+++ b/swh/lister/gnu/lister.py
@@ -10,6 +10,8 @@
 from swh.lister.gnu.models import GNUModel
 from swh.lister.gnu.tree import GNUTree
 
+from typing import Any, Dict, List, Mapping, Union
+from requests import Response
 
 logger = logging.getLogger(__name__)
 
@@ -19,11 +21,12 @@
     LISTER_NAME = 'gnu'
     instance = 'gnu'
 
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args, **kwargs: Mapping[str, str]) -> None:
         super().__init__(*args, **kwargs)
         self.gnu_tree = GNUTree('https://ftp.gnu.org/tree.json.gz')
 
-    def task_dict(self, origin_type, origin_url, **kwargs):
+    def task_dict(self, origin_type: str,
+                  origin_url: str, **kwargs) -> Dict[str, Any]:
         """Return task format dict
 
         This is overridden from the lister_base as more information is
@@ -58,7 +61,7 @@
             retries_left=3,
         )
 
-    def safely_issue_request(self, identifier):
+    def safely_issue_request(self, identifier: Union[str, int]) -> None:
         """Bypass the implementation. It's now the GNUTree which deals with
         querying the gnu mirror.
 
@@ -69,7 +72,7 @@
         """
         return None
 
-    def list_packages(self, response):
+    def list_packages(self, response: Response) -> List[Dict[str, Any]]:
         """List the actual gnu origins (package name) with their name, url and
            associated tarballs.
 
@@ -96,7 +99,7 @@
         """
         return list(self.gnu_tree.projects.values())
 
-    def get_model_from_repo(self, repo):
+    def get_model_from_repo(self, repo: Dict[str, Any]) -> Dict[str, Any]:
         """Transform from repository representation to model
 
         """
diff --git a/swh/lister/npm/lister.py b/swh/lister/npm/lister.py
--- a/swh/lister/npm/lister.py
+++ b/swh/lister/npm/lister.py
@@ -6,6 +6,9 @@
 from swh.lister.npm.models import NpmModel
 from swh.scheduler.utils import create_task_dict
 
+from typing import Dict, Optional, List, Any, Mapping
+from requests import Response
+
 
 class NpmListerBase(IndexingHttpLister):
     """List packages available in the npm registry in a paginated way
@@ -15,14 +18,16 @@
     LISTER_NAME = 'npm'
     instance = 'npm'
 
-    def __init__(self, url='https://replicate.npmjs.com',
-                 per_page=1000, override_config=None):
+    def __init__(self, url: str = 'https://replicate.npmjs.com',
+                 per_page: int = 1000,
+                 override_config: Optional[str] = None) -> None:
         super().__init__(url=url, override_config=override_config)
         self.per_page = per_page + 1
-        self.PATH_TEMPLATE += '&limit=%s' % self.per_page
+        self.PATH_TEMPLATE: str = self.PATH_TEMPLATE + '&limit=%s' % (
+            self.per_page)
 
     @property
-    def ADDITIONAL_CONFIG(self):
+    def ADDITIONAL_CONFIG(self) -> Dict[str, str]:
         """(Override) Add extra configuration
 
         """
@@ -30,7 +35,7 @@
         default_config['loading_task_policy'] = ('str', 'recurring')
         return default_config
 
-    def get_model_from_repo(self, repo_name):
+    def get_model_from_repo(self, repo_name: str) -> Dict[str, str]:
         """(Override) Transform from npm package name to model
 
         """
@@ -45,7 +50,9 @@
             'origin_type': 'npm',
         }
 
-    def task_dict(self, origin_type, origin_url, **kwargs):
+    def task_dict(self, origin_type: str,
+                  origin_url: str,
+                  **kwargs: Mapping[str, str]) -> Dict[str, Any]:
         """(Override) Return task dict for loading a npm package into the
         archive.
 
@@ -58,7 +65,7 @@
         return create_task_dict(task_type, task_policy,
                                 url=origin_url)
 
-    def request_headers(self):
+    def request_headers(self) -> Dict[str, str]:
         """(Override) Set requests headers to send when querying the npm
         registry.
 
@@ -67,7 +74,7 @@
         headers['Accept'] = 'application/json'
         return headers
 
-    def string_pattern_check(self, inner, lower, upper=None):
+    def string_pattern_check(self, inner: int, lower: int, upper: int = None):
         """ (Override) Inhibit the effect of that method as packages indices
         correspond to package names and thus do not respect any kind
         of fixed length string pattern
@@ -82,14 +89,16 @@
     """
     PATH_TEMPLATE = '/_all_docs?startkey="%s"'
 
-    def get_next_target_from_response(self, response):
+    def get_next_target_from_response(
+            self, response: Response) -> Optional[str]:
         """(Override) Get next npm package name to continue the listing
 
         """
         repos = response.json()['rows']
         return repos[-1]['id'] if len(repos) == self.per_page else None
 
-    def transport_response_simplified(self, response):
+    def transport_response_simplified(
+            self, response: Response) -> List[Dict[str, str]]:
         """(Override) Transform npm registry response to list for model manipulation
 
         """
@@ -110,14 +119,16 @@
     def CONFIG_BASE_FILENAME(self):  # noqa: N802
         return 'lister_npm_incremental'
 
-    def get_next_target_from_response(self, response):
+    def get_next_target_from_response(
+            self, response: Response) -> Optional[str]:
         """(Override) Get next npm package name to continue the listing.
 
         """
         repos = response.json()['results']
         return repos[-1]['seq'] if len(repos) == self.per_page else None
 
-    def transport_response_simplified(self, response):
+    def transport_response_simplified(
+            self, response: Response) -> List[Dict[str, str]]:
         """(Override) Transform npm registry response to list for model
         manipulation.
 
@@ -127,7 +138,8 @@
             repos = repos[:-1]
         return [self.get_model_from_repo(repo['id']) for repo in repos]
 
-    def filter_before_inject(self, models_list):
+    def filter_before_inject(
+            self, models_list: List[Dict[str, str]]) -> List[Dict[str, str]]:
         """(Override) Filter out documents in the CouchDB database
         not related to a npm package.
 
diff --git a/swh/lister/packagist/lister.py b/swh/lister/packagist/lister.py
--- a/swh/lister/packagist/lister.py
+++ b/swh/lister/packagist/lister.py
@@ -7,7 +7,7 @@
 import logging
 import random
 
-from typing import Any, Dict, List, Mapping
+from typing import Any, Dict, List, Mapping, Optional
 
 from swh.scheduler import utils
 from swh.lister.core.simple_lister import SimpleLister
@@ -57,7 +57,7 @@
     PAGE = 'https://packagist.org/packages/list.json'
     instance = 'packagist'
 
-    def __init__(self, override_config=None):
+    def __init__(self, override_config: Optional[bool] = None) -> None:
         ListerOnePageApiTransport .__init__(self)
         SimpleLister.__init__(self, override_config=override_config)
 
diff --git a/swh/lister/phabricator/lister.py b/swh/lister/phabricator/lister.py
--- a/swh/lister/phabricator/lister.py
+++ b/swh/lister/phabricator/lister.py
@@ -14,6 +14,8 @@
 from swh.lister.core.indexing_lister import IndexingHttpLister
 from swh.lister.phabricator.models import PhabricatorModel
 
+from typing import Any, Dict, List, Optional, Union
+from requests import Response
 
 logger = logging.getLogger(__name__)
 
@@ -25,13 +27,15 @@
     MODEL = PhabricatorModel
     LISTER_NAME = 'phabricator'
 
-    def __init__(self, url=None, instance=None, override_config=None):
+    def __init__(self, url: Optional[str] = None,
+                 instance: Optional[str] = None,
+                 override_config: Optional[bool] = None) -> None:
         super().__init__(url=url, override_config=override_config)
         if not instance:
             instance = urllib.parse.urlparse(self.url).hostname
         self.instance = instance
 
-    def request_params(self, identifier):
+    def request_params(self, identifier: Union[str, int]) -> Dict[str, Any]:
         """Override the default params behavior to retrieve the api token
 
         Credentials are stored as:
@@ -52,7 +56,7 @@
         return {'headers': self.request_headers() or {},
                 'params': {'api.token': api_token}}
 
-    def request_headers(self):
+    def request_headers(self) -> Dict[str, Any]:
         """
         (Override) Set requests headers to send when querying the
         Phabricator API
@@ -61,7 +65,8 @@
         headers['Accept'] = 'application/json'
         return headers
 
-    def get_model_from_repo(self, repo):
+    def get_model_from_repo(
+            self, repo: Dict[str, Any]) -> Optional[Dict[str, Any]]:
         url = get_repo_url(repo['attachments']['uris']['uris'])
         if url is None:
             return None
@@ -76,12 +81,15 @@
             'instance': self.instance,
         }
 
-    def get_next_target_from_response(self, response):
+    def get_next_target_from_response(
+            self, response: Response) -> Optional[int]:
         body = response.json()['result']['cursor']
         if body['after'] and body['after'] != 'null':
             return int(body['after'])
+        return None
 
-    def transport_response_simplified(self, response):
+    def transport_response_simplified(
+            self, response: Response) -> List[Optional[Dict[str, Any]]]:
         repos = response.json()
         if repos['result'] is None:
             raise ValueError(
@@ -89,7 +97,8 @@
         repos = repos['result']['data']
         return [self.get_model_from_repo(repo) for repo in repos]
 
-    def filter_before_inject(self, models_list):
+    def filter_before_inject(
+            self, models_list: List[Dict[str, str]]) -> List[Dict[str, str]]:
         """
         (Overrides) IndexingLister.filter_before_inject
         Bounds query results by this Lister's set max_index.
@@ -97,7 +106,8 @@
         models_list = [m for m in models_list if m is not None]
         return super().filter_before_inject(models_list)
 
-    def disable_deleted_repo_tasks(self, index, next_index, keep_these):
+    def disable_deleted_repo_tasks(
+            self, index: int, next_index: int, keep_these: str):
         """
         (Overrides) Fix provided index value to avoid:
 
@@ -117,7 +127,7 @@
         return super().disable_deleted_repo_tasks(index, next_index,
                                                   keep_these)
 
-    def db_first_index(self):
+    def db_first_index(self) -> Optional[int]:
         """
         (Overrides) Filter results by Phabricator instance
 
@@ -128,8 +138,9 @@
         t = t.filter(self.MODEL.instance == self.instance).first()
         if t:
             return t[0]
+        return None
 
-    def db_last_index(self):
+    def db_last_index(self) -> Optional[int]:
         """
         (Overrides) Filter results by Phabricator instance
 
@@ -140,8 +151,9 @@
         t = t.filter(self.MODEL.instance == self.instance).first()
         if t:
             return t[0]
+        return None
 
-    def db_query_range(self, start, end):
+    def db_query_range(self, start: int, end: int) -> List:
         """
         (Overrides) Filter the results by the Phabricator instance to
         avoid disabling loading tasks for repositories hosted on a
@@ -155,14 +167,14 @@
         return retlist.filter(self.MODEL.instance == self.instance)
 
 
-def get_repo_url(attachments):
+def get_repo_url(attachments: List[Dict[str, Any]]) -> Optional[int]:
     """
     Return url for a hosted repository from its uris attachments according
     to the following priority lists:
     * protocol: https > http
     * identifier: shortname > callsign > id
     """
-    processed_urls = defaultdict(dict)
+    processed_urls = defaultdict(dict)  # type: Dict[str, Any]
     for uri in attachments:
         protocol = uri['fields']['builtin']['protocol']
         url = uri['fields']['uri']['effective']
diff --git a/swh/lister/pypi/lister.py b/swh/lister/pypi/lister.py
--- a/swh/lister/pypi/lister.py
+++ b/swh/lister/pypi/lister.py
@@ -12,6 +12,9 @@
 from swh.lister.core.simple_lister import SimpleLister
 from swh.lister.core.lister_transports import ListerOnePageApiTransport
 
+from typing import Any, Dict, List, Optional, Mapping
+from requests import Response
+
 
 class PyPILister(ListerOnePageApiTransport, SimpleLister):
     MODEL = PyPIModel
@@ -19,11 +22,13 @@
     PAGE = 'https://pypi.org/simple/'
     instance = 'pypi'  # As of today only the main pypi.org is used
 
-    def __init__(self, override_config=None):
+    def __init__(self, override_config: Optional[str] = None) -> None:
         ListerOnePageApiTransport .__init__(self)
         SimpleLister.__init__(self, override_config=override_config)
 
-    def task_dict(self, origin_type, origin_url, **kwargs):
+    def task_dict(self, origin_type: str,
+                  origin_url: str,
+                  **kwargs: Mapping[str, str]) -> Dict[str, Any]:
         """(Override) Return task format dict
 
         This is overridden from the lister_base as more information is
@@ -35,7 +40,7 @@
         return utils.create_task_dict(
             _type, _policy, url=origin_url)
 
-    def list_packages(self, response):
+    def list_packages(self, response: Response) -> List[Dict]:
         """(Override) List the actual pypi origins from the response.
 
         """
@@ -50,7 +55,7 @@
         """
         return 'https://pypi.org/project/%s/' % repo_name
 
-    def get_model_from_repo(self, repo_name):
+    def get_model_from_repo(self, repo_name: str) -> Dict[str, Any]:
         """(Override) Transform from repository representation to model
 
         """