Backport of the print function, using a __future__ import.

This work is substantially Anthony Baxter's, from issue
1633807.  I just freshened it, made a few minor tweaks,
and added the test cases.  I also created issue 2412,
which is to check for 2to3's behavior with the print
function.  I also added myself to ACKS.

2 files changed, 135 insertions, 0 deletions

diff --git a/Lib/__future__.py b/Lib/__future__.py
index d8e14d1..ea14bf3 100644
--- a/Lib/__future__.py
+++ b/Lib/__future__.py
@@ -53,6 +53,7 @@ all_feature_names = [
     "division",
     "absolute_import",
     "with_statement",
+    "print_function",
 ]
 
 __all__ = ["all_feature_names"] + all_feature_names
@@ -66,6 +67,7 @@ CO_GENERATOR_ALLOWED = 0        # generators (obsolete, was 0x1000)
 CO_FUTURE_DIVISION   = 0x2000   # division
 CO_FUTURE_ABSOLUTE_IMPORT = 0x4000 # perform absolute imports by default
 CO_FUTURE_WITH_STATEMENT  = 0x8000   # with statement
+CO_FUTURE_PRINT_FUNCTION  = 0x10000   # print function
 
 class _Feature:
     def __init__(self, optionalRelease, mandatoryRelease, compiler_flag):
@@ -114,3 +116,7 @@ absolute_import = _Feature((2, 5, 0, "alpha", 1),
 with_statement = _Feature((2, 5, 0, "alpha", 1),
                           (2, 6, 0, "alpha", 0),
                           CO_FUTURE_WITH_STATEMENT)
+
+print_function = _Feature((2, 6, 0, "alpha", 2),
+                          (3, 0, 0, "alpha", 0),
+                          CO_FUTURE_PRINT_FUNCTION)
diff --git a/Lib/test/test_print.py b/Lib/test/test_print.py
new file mode 100644
index 0000000..db09c9c
--- /dev/null
+++ b/Lib/test/test_print.py
@@ -0,0 +1,129 @@
+"""Test correct operation of the print function.
+"""
+
+from __future__ import print_function
+
+import unittest
+from test import test_support
+
+import sys
+try:
+    # 3.x
+    from io import StringIO
+except ImportError:
+    # 2.x
+    from StringIO import StringIO
+
+from contextlib import contextmanager
+
+NotDefined = object()
+
+# A dispatch table all 8 combinations of providing
+#  sep, end, and file
+# I use this machinery so that I'm not just passing default
+#  values to print, I'm eiher passing or not passing in the
+#  arguments
+dispatch = {
+    (False, False, False):
+     lambda args, sep, end, file: print(*args),
+    (False, False, True):
+     lambda args, sep, end, file: print(file=file, *args),
+    (False, True,  False):
+     lambda args, sep, end, file: print(end=end, *args),
+    (False, True,  True):
+     lambda args, sep, end, file: print(end=end, file=file, *args),
+    (True,  False, False):
+     lambda args, sep, end, file: print(sep=sep, *args),
+    (True,  False, True):
+     lambda args, sep, end, file: print(sep=sep, file=file, *args),
+    (True,  True,  False):
+     lambda args, sep, end, file: print(sep=sep, end=end, *args),
+    (True,  True,  True):
+     lambda args, sep, end, file: print(sep=sep, end=end, file=file, *args),
+    }
+
+@contextmanager
+def stdout_redirected(new_stdout):
+    save_stdout = sys.stdout
+    sys.stdout = new_stdout
+    try:
+        yield None
+    finally:
+        sys.stdout = save_stdout
+
+# Class used to test __str__ and print
+class ClassWith__str__:
+    def __init__(self, x):
+        self.x = x
+    def __str__(self):
+        return self.x
+
+class TestPrint(unittest.TestCase):
+    def check(self, expected, args,
+            sep=NotDefined, end=NotDefined, file=NotDefined):
+        # Capture sys.stdout in a StringIO.  Call print with args,
+        #  and with sep, end, and file, if they're defined.  Result
+        #  must match expected.
+
+        # Look up the actual function to call, based on if sep, end, and file
+        #  are defined
+        fn = dispatch[(sep is not NotDefined,
+                       end is not NotDefined,
+                       file is not NotDefined)]
+
+        t = StringIO()
+        with stdout_redirected(t):
+            fn(args, sep, end, file)
+
+        self.assertEqual(t.getvalue(), expected)
+
+    def test_print(self):
+        def x(expected, args, sep=NotDefined, end=NotDefined):
+            # Run the test 2 ways: not using file, and using
+            #  file directed to a StringIO
+
+            self.check(expected, args, sep=sep, end=end)
+
+            # When writing to a file, stdout is expected to be empty
+            o = StringIO()
+            self.check('', args, sep=sep, end=end, file=o)
+
+            # And o will contain the expected output
+            self.assertEqual(o.getvalue(), expected)
+
+        x('\n', ())
+        x('a\n', ('a',))
+        x('None\n', (None,))
+        x('1 2\n', (1, 2))
+        x('1   2\n', (1, ' ', 2))
+        x('1*2\n', (1, 2), sep='*')
+        x('1 s', (1, 's'), end='')
+        x('a\nb\n', ('a', 'b'), sep='\n')
+        x('1.01', (1.0, 1), sep='', end='')
+        x('1*a*1.3+', (1, 'a', 1.3), sep='*', end='+')
+        x('a\n\nb\n', ('a\n', 'b'), sep='\n')
+        x('\0+ +\0\n', ('\0', ' ', '\0'), sep='+')
+
+        x('a\n b\n', ('a\n', 'b'))
+        x('a\n b\n', ('a\n', 'b'), sep=None)
+        x('a\n b\n', ('a\n', 'b'), end=None)
+        x('a\n b\n', ('a\n', 'b'), sep=None, end=None)
+
+        x('*\n', (ClassWith__str__('*'),))
+        x('abc 1\n', (ClassWith__str__('abc'), 1))
+
+        # 2.x unicode tests
+        x(u'1 2\n', ('1', u'2'))
+        x(u'u\1234\n', (u'u\1234',))
+        x(u'  abc 1\n', (' ', ClassWith__str__(u'abc'), 1))
+
+        # errors
+        self.assertRaises(TypeError, print, '', sep=3)
+        self.assertRaises(TypeError, print, '', end=3)
+        self.assertRaises(AttributeError, print, '', file='')
+
+def test_main():
+    test_support.run_unittest(TestPrint)
+
+if __name__ == "__main__":
+    test_main()