Change argument name to better reflect purpose.

Also assert on provided path, since it should have been created in all
cases.

src/pip/_internal/cli/req_command.py

@@ -141,7 +141,7 @@ class RequirementCommand(IndexGroupCommand):
 
     @staticmethod
     def make_requirement_preparer(
-        temp_directory,           # type: TempDirectory
+        temp_build_dir,           # type: TempDirectory
         options,                  # type: Values
         req_tracker,              # type: RequirementTracker
         download_dir=None,        # type: str
@@ -151,8 +151,10 @@ class RequirementCommand(IndexGroupCommand):
         """
         Create a RequirementPreparer instance for the given parameters.
         """
+        temp_build_dir_path = temp_build_dir.path
+        assert temp_build_dir_path is not None
         return RequirementPreparer(
-            build_dir=temp_directory.path,
+            build_dir=temp_build_dir_path,
             src_dir=options.src_dir,
             download_dir=download_dir,
             wheel_download_dir=wheel_download_dir,

src/pip/_internal/commands/download.py

@@ -125,7 +125,7 @@ class DownloadCommand(RequirementCommand):
             )
 
             preparer = self.make_requirement_preparer(
-                temp_directory=directory,
+                temp_build_dir=directory,
                 options=options,
                 req_tracker=req_tracker,
                 download_dir=options.download_dir,

src/pip/_internal/commands/install.py

@@ -358,7 +358,7 @@ class InstallCommand(RequirementCommand):
                     wheel_cache
                 )
                 preparer = self.make_requirement_preparer(
-                    temp_directory=directory,
+                    temp_build_dir=directory,
                     options=options,
                     req_tracker=req_tracker,
                 )

src/pip/_internal/commands/wheel.py

@@ -136,7 +136,7 @@ class WheelCommand(RequirementCommand):
                 )
 
                 preparer = self.make_requirement_preparer(
-                    temp_directory=directory,
+                    temp_build_dir=directory,
                     options=options,
                     req_tracker=req_tracker,
                     wheel_download_dir=options.wheel_dir,