Python 3 preparation: Handle changes to "raw_input()" and "input()"

[wluebbe]

The Py2 function raw_input() is called input() in Py3.
The Py2 input() function (which implicitly evaluates the input) is gone.

The tool 2to3 renames raw_input to input.
But the code has to depend on the Python version!

There are 3 effected (usage of raw_input()) modules.

This ticket is tracked as a dependency of meta-ticket ticket:16052.

[wluebbe]

Instead of

+import six
...
...
-            line = raw_input()
+            line = six.moves.input()

I would prefer

+from six.moves import input
...
...
-            line = raw_input()
+            line = input()

The reasoning is

• I see the goal as not only to enable Sage for Python 2 AND 3 but by writing clean Python 3 code that is backwards compatible with Python 2.
• Usage of compatibility libraries like six (or future) should appear (if possible) only at the top of a module. The rest of the module should have only pure Python 3 code.
• This will ease the cleaning of the code when somewhere in the (far) future Python 2 support will be dropped.
• We will have to explain in the documentation how the write Python 3 code in Sage while maintaining compatibility with Python 2. And it would by very helpful to have automated checks ...

By the way all tests pass :-)

[aapitzsch]

Replying to wluebbe:

+from six.moves import input
...
...
-            line = raw_input()
+            line = input()

would override Python2's input() which behaves different, as written in the ticket description. That's why I went the other way.

Cleaning is easy, too: s/six.moves.//g

[wluebbe]

I did not find any uses of Python 2 input() in Sage, so there is no name conflict. This old input() has been removed from Python 3 as it is not secure because eval() is called implicitly on an unchecked string.

[aapitzsch]

Users might import one of the effected files and therefore change the meaning of input() in their scripts. Although I doubt anybody will import these.

[wluebbe]

I still hold that the comment

• I see the goal as not only to enable Sage for Python 2 AND 3 but by writing clean Python 3 code that is backwards compatible with Python 2.

is valid and important.

But this change effects only 3 modules, the code is fine and it tests OK.

But we should discuss on sage-devel how to approach the other and bigger tickets of stage 2 (usage of migration tools, which compatibility library, ...). I will try to initiate such a wider discussion of the Sage Py2/3 migration strategy. We will advance faster when there are more collaborators and ideas. A common understanding and strategy could prevent wasted efforts.