make 1 the default value of the parallel-downloads arguement

src/pip/_internal/cli/cmdoptions.py

@@ -770,7 +770,7 @@ def _handle_no_cache_dir(
     dest="parallel_downloads",
     type="int",
     metavar="n",
-    default=None,
+    default=1,
     help=(
         "Use upto <n> threads to download packages in parallel."
         "<n> must be greater than 0"

src/pip/_internal/cli/req_command.py

@@ -118,18 +118,14 @@ def _build_session(
                 ssl_context = None
         else:
             ssl_context = None
-        if "parallel_downloads" in options.__dict__:
-            parallel_downloads = options.parallel_downloads
-        else:
-            parallel_downloads = None
 
         session = PipSession(
             cache=os.path.join(cache_dir, "http-v2") if cache_dir else None,
             retries=retries if retries is not None else options.retries,
             trusted_hosts=options.trusted_hosts,
             index_urls=self._get_index_urls(options),
             ssl_context=ssl_context,
-            parallel_downloads=parallel_downloads,
+            parallel_downloads=options.parallel_downloads,
         )
 
         # Handle custom ca-bundles from the user

src/pip/_internal/commands/download.py

@@ -78,9 +78,7 @@ def add_options(self) -> None:
 
     @with_cleanup
     def run(self, options: Values, args: List[str]) -> int:
-        if (options.parallel_downloads is not None) and (
-            options.parallel_downloads < 1
-        ):
+        if options.parallel_downloads < 1:
             raise CommandError("Value of '--parallel-downloads' must be greater than 0")
 
         options.ignore_installed = True

src/pip/_internal/network/session.py

@@ -326,7 +326,7 @@ def __init__(
         trusted_hosts: Sequence[str] = (),
         index_urls: Optional[List[str]] = None,
         ssl_context: Optional["SSLContext"] = None,
-        parallel_downloads: Optional[int] = None,
+        parallel_downloads: int = 1,
         **kwargs: Any,
     ) -> None:
         """
@@ -367,9 +367,7 @@ def __init__(
         # pip._internal.network.BatchDownloader and to set pool_connection in
         # the HTTPAdapter to prevent connection pool from hitting the default(10)
         # limit and throwing 'Connection pool is full' warnings
-        self.parallel_downloads = (
-            parallel_downloads if (parallel_downloads is not None) else 1
-        )
+        self.parallel_downloads = parallel_downloads
         pool_maxsize = max(self.parallel_downloads, 10)
         # Our Insecure HTTPAdapter disables HTTPS validation. It does not
         # support caching so we'll use it for all http:// URLs.