diff --git a/odc/geo/cog/_az.py b/odc/geo/cog/_az.py
index 9995a0ab..0b51c1c8 100644
--- a/odc/geo/cog/_az.py
+++ b/odc/geo/cog/_az.py
@@ -2,6 +2,7 @@
 from typing import Any, Union
 
 import dask
+from dask.delayed import Delayed
 
 from ._mpu import mpu_write
 from ._multipart import MultiPartUploadBase
@@ -33,6 +34,11 @@ def max_part(self) -> int:
 
 
 class AzMultiPartUpload(AzureLimits, MultiPartUploadBase):
+    """
+    Azure Blob Storage multipart upload.
+    """
+    # pylint: disable=too-many-instance-attributes
+
     def __init__(
         self, account_url: str, container: str, blob: str, credential: Any = None
     ):
@@ -94,10 +100,11 @@ def finalise(self, parts: list[dict[str, Any]]) -> str:
         self.blob_client.commit_block_list(block_list)
         return self.blob_client.get_blob_properties().etag
 
-    def cancel(self):
+    def cancel(self, other: str = ""):
         """
         Cancel the upload by clearing the block list.
         """
+        assert other == ""
         self.block_ids.clear()
 
     @property
@@ -118,7 +125,7 @@ def started(self) -> bool:
         """
         return bool(self.block_ids)
 
-    def writer(self, kw: dict[str, Any], client: Any = None):
+    def writer(self, kw: dict[str, Any], *, client: Any = None):
         """
         Return a stateless writer compatible with Dask.
         """
@@ -130,12 +137,12 @@ def upload(
         *,
         mk_header: Any = None,
         mk_footer: Any = None,
-        user_kw: dict[str, Any] = None,
+        user_kw: dict[str, Any] | None = None,
         writes_per_chunk: int = 1,
         spill_sz: int = 20 * (1 << 20),
         client: Any = None,
         **kw,
-    ) -> dask.delayed.Delayed:
+    ) -> Delayed:
         """
         Upload chunks to Azure Blob Storage with multipart uploads.
 
diff --git a/odc/geo/cog/_multipart.py b/odc/geo/cog/_multipart.py
index c9060bee..0fc9b4c8 100644
--- a/odc/geo/cog/_multipart.py
+++ b/odc/geo/cog/_multipart.py
@@ -7,9 +7,11 @@
 """
 
 from abc import ABC, abstractmethod
-from typing import Any, Union
+from typing import Any, Union, TYPE_CHECKING
 
-import dask.bag
+if TYPE_CHECKING:
+    # pylint: disable=import-outside-toplevel,import-error
+    import dask.bag
 
 
 class MultiPartUploadBase(ABC):
@@ -18,34 +20,28 @@ class MultiPartUploadBase(ABC):
     @abstractmethod
     def initiate(self, **kwargs) -> str:
         """Initiate a multipart upload and return an identifier."""
-        pass
 
     @abstractmethod
     def write_part(self, part: int, data: bytes) -> dict[str, Any]:
         """Upload a single part."""
-        pass
 
     @abstractmethod
     def finalise(self, parts: list[dict[str, Any]]) -> str:
         """Finalise the upload with a list of parts."""
-        pass
 
     @abstractmethod
     def cancel(self, other: str = ""):
         """Cancel the multipart upload."""
-        pass
 
     @property
     @abstractmethod
     def url(self) -> str:
         """Return the URL of the upload target."""
-        pass
 
     @property
     @abstractmethod
     def started(self) -> bool:
         """Check if the multipart upload has been initiated."""
-        pass
 
     @abstractmethod
     def writer(self, kw: dict[str, Any], *, client: Any = None) -> Any:
@@ -55,7 +51,6 @@ def writer(self, kw: dict[str, Any], *, client: Any = None) -> Any:
         :param kw: Additional parameters for the writer.
         :param client: Dask client for distributed execution.
         """
-        pass
 
     @abstractmethod
     def upload(
@@ -64,7 +59,7 @@ def upload(
         *,
         mk_header: Any = None,
         mk_footer: Any = None,
-        user_kw: dict[str, Any] = None,
+        user_kw: dict[str, Any] | None = None,
         writes_per_chunk: int = 1,
         spill_sz: int = 20 * (1 << 20),
         client: Any = None,
@@ -82,4 +77,3 @@ def upload(
         :param client: Dask client for distributed execution.
         :return: A Dask delayed object representing the finalised upload.
         """
-        pass
diff --git a/odc/geo/cog/_tifffile.py b/odc/geo/cog/_tifffile.py
index fae90408..3ab86e21 100644
--- a/odc/geo/cog/_tifffile.py
+++ b/odc/geo/cog/_tifffile.py
@@ -24,6 +24,7 @@
 from ..types import Shape2d, SomeNodata, Unset, shape_
 from ._mpu import mpu_write
 from ._mpu_fs import MPUFileSink
+from ._multipart import MultiPartUploadBase
 
 from ._shared import (
     GDAL_COMP,
@@ -736,22 +737,26 @@ def save_cog_with_dask(
     # Determine output type and initiate uploader
     parsed_url = urlparse(dst)
     if parsed_url.scheme == "s3":
-        if have.s3:
+        if have.botocore:
             from ._s3 import S3MultiPartUpload, s3_parse_url
 
             bucket, key = s3_parse_url(dst)
-            uploader = S3MultiPartUpload(bucket, key, **aws)
+            uploader: MultiPartUploadBase = S3MultiPartUpload(bucket, key, **aws)
         else:
             raise RuntimeError("Please install `boto3` to use S3")
     elif parsed_url.scheme == "az":
         if have.azure:
             from ._az import AzMultiPartUpload
 
+            assert azure is not None
+            assert "account_url" in azure
+            assert "credential" in azure
+
             uploader = AzMultiPartUpload(
-                account_url=azure.get("account_url"),
+                account_url=azure["account_url"],
                 container=parsed_url.netloc,
                 blob=parsed_url.path.lstrip("/"),
-                credential=azure.get("credential"),
+                credential=azure["credential"],
             )
         else:
             raise RuntimeError("Please install `azure-storage-blob` to use Azure")