I did this on Windows 10:

P:\>python --version
Python 3.7.2

P:\>echo print 1, 2 > Test.py

P:\>python Test.py
  File "Test.py", line 1
    print 1, 2
          ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(1, 2)?

P:\>2to3 -w Test.py
RefactoringTool: Skipping optional fixer: buffer
RefactoringTool: Skipping optional fixer: idioms
RefactoringTool: Skipping optional fixer: set_literal
RefactoringTool: Skipping optional fixer: ws_comma
RefactoringTool: Refactored Test.py
--- Test.py     (original)
+++ Test.py     (refactored)
@@ -1 +1 @@
-print 1, 2
+print(1, 2)
RefactoringTool: Files that were modified:
RefactoringTool: Test.py

P:\>python Test.py
1 2

P:\>2to3 -w Test.py
RefactoringTool: Skipping optional fixer: buffer
RefactoringTool: Skipping optional fixer: idioms
RefactoringTool: Skipping optional fixer: set_literal
RefactoringTool: Skipping optional fixer: ws_comma
RefactoringTool: Refactored Test.py
--- Test.py     (original)
+++ Test.py     (refactored)
@@ -1 +1 @@
-print(1, 2)
+print((1, 2))
RefactoringTool: Files that were modified:
RefactoringTool: Test.py

P:\>python Test.py
(1, 2)

Note how "print 1, 2" first becomes "print(1, 2)" (expected), then becomes "print((1, 2))" in the following run. This changes the output of Test.py

This seems to be same as issue35417 and some resolution on detecting these type of cases at issue10375.

Because what's being printed is a tuple, I think it's not exactly the same as issue35417, because in fact this is the correct behavior for 2to3, note that in Python 2:

Python 2.7.15 (default, Jul 21 2018, 11:13:03) 
>>> print 1, 2 
1 2
>>> print(1, 2)
(1, 2)

And in Python 3:

Python 3.7.2 (default, Feb  9 2019, 13:18:43) 
>>> print(1, 2)
1 2
>>> print((1, 2))
(1, 2)

I think this bug report is based on an understandable misunderstanding of what 2to3 does - 2to3 is not intended to be idempotent or to generate code the works for both Python 2 and Python 3, it's intended to translate Python 2 code into Python 3, so passing it something that is *already Python 3 code* you are not guaranteed to get a meaningful output from it.

In this case, it first translates `print 1, 2` (Python 2) into `print(1, 2)` (Python 3), then when you run it a second time, it translates `print(1, 2)` (Python 2) into `print((1, 2))` (Python 3) - in both cases it's doing the right thing.

@bers I hope that this has helped to clarify the situation. Thank you for taking the time to report this.

Yes, understood! Thanks for the explanation.

messages: + msg336675
stage: resolved