Add a new *public* main() method

news/5149.feature

@@ -0,0 +1,7 @@
+Add public, supported replacement for `pip.main` which uses
+`subprocess.check_call` to invoke
+  ${sys.executable} -m pip ...
+with the given arguments. e.g. `pip.main('install', '-U', 'setuptools')` is now
+supported usage.
+
+This means that the behavior may be different from `pip.main` on `pip<10.0.0`.

src/pip/__init__.py

@@ -1 +1,27 @@
+from __future__ import absolute_import
+import subprocess
+import sys
+
 __version__ = "10.0.0.dev0"
+
+
+def main(*args):
+    """
+    This function is included primarily for backwards compatibility.
+
+    Although anything using `pip.main()` in pip<9.0.2 was engaged in a
+    non-supported mode of use, as a pragmatic accommodation, `pip.main()` is
+    being *added* as a *new* piece of functionality with very similar behaviors
+    and call signature.
+
+    Why use check_call instead of check_output?
+    It's behavior is slightly closer to the older `pip.main()` behavior,
+    printing output information directly to stdout.
+
+    check_call was added in py2.5 and is supported through py3.x , so it's more
+    compatible than some alternatives like subprocess.run (added in py3.5)
+    """
+    return subprocess.check_call([sys.executable, '-m', 'pip'] + list(args))
+
+
+__all__ = ('main',)