Small edit to add backoff_factor to client (#759)

* Add backoff_factor

As defined in `urllib3` docs

* Update `Retry` to use `backoff_factor`

* Fix typo

* Fix linting

---------

Co-authored-by: Jason Munro <jmunro@lbl.gov>

mp_api/client/core/client.py

@@ -135,13 +135,15 @@ def _create_session(api_key, include_user_agent, headers):
             platform_info = f"{platform.system()}/{platform.release()}"
             session.headers["user-agent"] = f"{pymatgen_info} ({python_info} {platform_info})"
 
-        max_retry_num = MAPIClientSettings().MAX_RETRIES
+        settings = MAPIClientSettings()
+        max_retry_num = settings.MAX_RETRIES
         retry = Retry(
             total=max_retry_num,
             read=max_retry_num,
             connect=max_retry_num,
             respect_retry_after_header=True,
             status_forcelist=[429],  # rate limiting
+            backoff_factor=settings.BACKOFF_FACTOR
         )
         adapter = HTTPAdapter(max_retries=retry)
         session.mount("http://", adapter)

mp_api/client/core/settings.py

@@ -63,6 +63,11 @@ class MAPIClientSettings(BaseSettings):
         _MAX_RETRIES, description="Maximum number of retries for requests."
     )
 
+    BACKOFF_FACTOR: float = Field(
+        0.1,
+        description="A backoff factor to apply between retry attempts. To disable, set to 0.",
+    )
+
     MUTE_PROGRESS_BARS: bool = Field(
         _MUTE_PROGRESS_BAR,
         description="Whether to mute progress bars when data is retrieved.",