Add "python -m setuptools" invocation method

changelog.d/2080.change.rst

@@ -0,0 +1 @@
+Add support for running `python -m setuptools cmd` as a setup.py alternative.

setuptools/__main__.py

@@ -0,0 +1,10 @@
+"""
+Command-line interface for running setup.py or new-fashioned setup.cfg after
+setuptools is monkey-patched into distutils.
+"""
+
+if __name__ == '__main__':
+    import sys
+    from setuptools.launch import run_setup
+    sys.argv[0] = 'setuptools'
+    run_setup()

setuptools/launch.py

@@ -6,30 +6,43 @@
 # Note that setuptools gets imported implicitly by the
 # invocation of this script using python -m setuptools.launch
 
-import tokenize
+import io
+import os
 import sys
+import tokenize
+
+
+def _open_setup_script(setup_script, fallback):
+    if fallback and not os.path.exists(setup_script):
+        # Supply a default setup.py
+        return io.StringIO(u"from setuptools import setup; setup()")
+
+    return getattr(tokenize, 'open', open)(setup_script)
+
+
+def run_setup(setup_script='setup.py', fallback=True):
+    namespace = {
+        '__file__': setup_script,
+        '__name__': '__main__',
+        '__doc__': None
+    }
+
+    with _open_setup_script(setup_script, fallback) as f:
+        code = f.read().replace(r'\r\n', r'\n')
+
+    exec(compile(code, setup_script, 'exec'), namespace)
 
 
-def run():
+def run_script():
     """
     Run the script in sys.argv[1] as if it had
     been invoked naturally.
     """
-    __builtins__
     script_name = sys.argv[1]
-    namespace = dict(
-        __file__=script_name,
-        __name__='__main__',
-        __doc__=None,
-    )
     sys.argv[:] = sys.argv[1:]
 
-    open_ = getattr(tokenize, 'open', open)
-    script = open_(script_name).read()
-    norm_script = script.replace('\\r\\n', '\\n')
-    code = compile(norm_script, script_name, 'exec')
-    exec(code, namespace)
+    run_setup(script_name, fallback=False)
 
 
 if __name__ == '__main__':
-    run()
+    run_script()