from __future__ import <anything> results in a Syntax Error

[TimDumol]

from future import * statements must be the first statements in a file. However, the old Sage Notebook inserts synchronization code before the file, and the new SageNB inserts prompt changing code first. Both of the aforementioned changes break the code.

CC: @qed777 @williamstein

Component: notebook

Author: Tim Dumol

Reviewer: Alex Leone

Merged: sagenb-0.7

Issue created by migration from https://trac.sagemath.org/ticket/7207

[TimDumol]

Fixed formatting to relocate future imports to the top of the file.

[TimDumol]

Author: Tim Dumol

[TimDumol]

comment:1

Attachment: trac_7207-sagenb-future-import.patch.gz

[TimDumol]

comment:2

Would anyone mind reviewing this? It just moves the displayhook_hack to another file, and adds some tests and a tad of refactoring.

[williamstein]

comment:3

Needs work, since this has a subtle bug, which I bet you can easily fix. See below.

Using regular expressions is unfortunately not rock solid. For example, this input "mysteriously" gives a SyntaxError:

print """
from __future__ import division"""

outputs:

Syntax Error:
    from __future__ import division"""

whereas the similar

print """
from __xfuture__ import division"""

works fine.

I think the right fix is to require that the even in the notebook the from `future` import ... lines appear at the very top. This would make it possible to use the same method you already used.

[TimDumol]

Uses stdlib's ast to parse the file instead of regexp's.

[TimDumol]

comment:4

Attachment: trac_7207-sagenb-future-import.2.patch.gz

This version of the patch uses ast (from stdlib). It should prevent errors such as what you described.