bpo-38662: Invoke pip via runpy, in ensurepip

Lib/ensurepip/__init__.py

@@ -1,6 +1,7 @@
 import os
 import os.path
 import sys
+import runpy
 import tempfile
 from importlib import resources
 
@@ -26,9 +27,18 @@ def _run_pip(args, additional_paths=None):
     if additional_paths is not None:
         sys.path = additional_paths + sys.path
 
-    # Install the bundled software
-    import pip._internal
-    return pip._internal.main(args)
+    # Invoke pip as if it's the main module, and catch the exit.
+    backup_argv = sys.argv[:]
+    sys.argv[1:] = args
+    try:
+        # run_module() alters sys.modules and sys.argv, but restore them at exit
+        runpy.run_module("pip", run_name="__main__", alter_sys=True)
+    except SystemExit as exc:
+        return exc.code
+    finally:
+        sys.argv[:] = backup_argv
+
+    raise SystemError("pip have not exited, that should never happen")
 
 
 def version():

Misc/NEWS.d/next/Library/2020-03-10-15-32-31.bpo-38662.o1DMXj.rst

@@ -0,0 +1,4 @@
+The ``ensurepip`` module now invokes ``pip`` via the ``runpy`` module.
+Hence it is no longer tightly coupled with the internal API of the bundled
+``pip`` version, allowing easier updates to a newer ``pip`` version both
+internally and for distributors.