gpython: Don't inherit __future__ when executing scripts

Because we use execfile to emulate "python script.py" invocation, future
enabled in gpython main script are also enabled in "script.py" and because
we use print_function feature, it was also applied in "script.py".
On python 2 when "script.py" uses print statements they were considered as
SyntaxError.

Hopefuly, compile has a `dont_inherit` flag which does exactly what we need
here: compile the script without inheriting futures enabled, so we use
compile with this flag.

/reviewed-by @kirr
/reviewed-on nexedi/pygolang!8

gpython/__init__.py

@@ -212,7 +212,7 @@ def _execfile(path, globals=None, locals=None):
     import six
     with open(path, "rb") as f:
         src = f.read()
-    code = compile(src, path, 'exec')
+    code = compile(src, path, 'exec', dont_inherit=True)
     six.exec_(code, globals, locals)
 
 # _version_info_str converts version_info -> str.

gpython/gpython_test.py

@@ -208,6 +208,16 @@ def test_pymain():
         b"- error::SyntaxWarning::*\n" + \
         b"- ignore::Warning::*\n"), _
 
+
+def test_pymain_print_function_future():
+    if PY2:
+        _ = pyout([], stdin=b'print "print", "is", "a", "statement"\n')
+        assert _ == b"print is a statement\n"
+        _ = pyout(['print_statement.py'], cwd=testprog)
+        assert _ == b"print is a statement\n"
+    _ = pyout(['future_print_function.py'], cwd=testprog)
+    assert _ == b"print is a function with print_function future\n"
+
 # verify that pymain sets sys.path in exactly the same way as underlying python does.
 @gpython_only
 def test_pymain_syspath():

gpython/testprog/future_print_function.py

+# -*- coding: utf-8 -*-
+# Copyright (C) 2020  Nexedi SA and Contributors.
+#                     Jérome Perrin <jerome@nexedi.com>
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+"""Program future_print_function enable print_function future and use print
+as a function."""
+
+from __future__ import print_function
+
+print("print", "is", "a", "function", "with", "print_function", "future")

gpython/testprog/print_statement.py

+# -*- coding: utf-8 -*-
+# Copyright (C) 2020  Nexedi SA and Contributors.
+#                     Jérome Perrin <jerome@nexedi.com>
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+"""Program print_statement uses print as a statement, which is allowed on
+python2 when not using print_statement future."""
+
+print "print", "is", "a", "statement"