covering_design.py & design_catalog.py: Fix AttributeError

[vklein]

1. Fix the following bug.

   sage: C = designs.best_known_covering_design_from_LJCR(7, 3, 2)
   ---------------------------------------------------------------------------
   AttributeError                            Traceback (most recent call last)
   <ipython-input-1-a3d9f9e592f1> in <module>()
   ----> 1 C = designs.best_known_covering_design_from_LJCR(Integer(7), Integer(3), Integer(2))
   
   /home/slabbe/GitBox/sage/local/lib/python2.7/site-packages/sage/combinat/designs/covering_design.pyc in best_known_covering_design_www(v, k, t, verbose)
       525     if verbose:
       526         print("Looking up the bounds at %s" % url)
   --> 527     with urlopen(url) as f:
       528         s = bytes_to_str(f.read())
       529     if 'covering not in database' in s:  # not found
   
   AttributeError: addinfourl instance has no attribute '__exit__'
   

Found by Sebastien Labbé see #26592

The root cause is that the object returned by urlopen doesn't have an __exit__ method in python2 and therefore it can not be used with a with ... as ... syntax.

2. Fix a doctest error in design_catalog.py:

sage -t --long src/sage/combinat/designs/design_catalog.py
**********************************************************************
File "src/sage/combinat/designs/design_catalog.py", line 23, in sage.combinat.designs.design_catalog
Failed example:
    C                            # optional - internet
Expected:
    (7,3,2)-covering design of size 7
    Lower bound: 7
    Method: lex covering
    Submitted on: 1996-12-01 00:00:00
Got:
    (7, 3, 2)-covering design of size 7
    Lower bound: 7
    Method: lex covering
    Submitted on: 1996-12-01 00:00:00
**********************************************************************

[slabbe]

Is this a solution:

from six.moves.urllib.request import urlopen

? This is what I used in #25535.

[vklein]

Replying to slabbe:

Is this a solution:

from six.moves.urllib.request import urlopen

? This is what I used in #25535.

No because it is already used in covering_design.py. In #25535 you use with ... as with a file objet, this works nicely.

---

New commits:

​1e93e49

Trac #27821: Fix AttributeError in ...

[slabbe]

As mentionned ​here, two files were having the same issue. We are almost there.

There is still one small issue:

$ sage -tp --long --optional=sage,optional,external src/sage/combinat/designs/covering_design.py  src/sage/combinat/designs/design_catalog.py

gives

too many failed tests, not using stored timings
Running doctests with ID 2019-05-13-17-17-32-23591643.
Git branch: 27821
Using --optional=4ti2,bliss,cbc,ccache,cmake,cryptominisat,dot2tex,e_antic,external,glucose,latte_int,lidia,lrslib,memlimit,mpir,ninja_build,normaliz,notedown,pandoc_attributes,pycosat,pynormaliz,python2,qhull,rst2ipynb,sage,topcom
External software to be detected: cplex,ffmpeg,graphviz,gurobi,imagemagick,internet,latex,macaulay2,magma,maple,mathematica,matlab,octave,pandoc,scilab
Doctesting 2 files using 8 threads.
sage -t --long src/sage/combinat/designs/design_catalog.py
**********************************************************************
File "src/sage/combinat/designs/design_catalog.py", line 23, in sage.combinat.designs.design_catalog
Failed example:
    C                            # optional - internet
Expected:
    (7,3,2)-covering design of size 7
    Lower bound: 7
    Method: lex covering
    Submitted on: 1996-12-01 00:00:00
Got:
    (7, 3, 2)-covering design of size 7
    Lower bound: 7
    Method: lex covering
    Submitted on: 1996-12-01 00:00:00
**********************************************************************
1 item had failures:
   1 of   5 in sage.combinat.designs.design_catalog
    [4 tests, 1 failure, 0.73 s]
sage -t --long src/sage/combinat/designs/covering_design.py
    [49 tests, 0.73 s]
----------------------------------------------------------------------
sage -t --long src/sage/combinat/designs/design_catalog.py  # 1 doctest failed
----------------------------------------------------------------------
Total time for all tests: 0.8 seconds
    cpu time: 0.1 seconds
    cumulative wall time: 1.5 seconds
External software detected for doctesting: internet

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​138870c

Trac #27821: Fix a doctest in design_catalog.py

[vklein]

The other doctest bug of design_catalog.py has been fixed.

[slabbe]

It works! Thank you.