Can't migrate database from 0.15.4 to 0.17.0 using Python 3

[rsxm]

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if any
• I have reproduced the issue with at least the latest released version of superset
• I have checked the issue tracker for the same issue and I haven't found one similar

Superset version

Migrating from 0.15.4 to 0.17.0 using MySQL database backend and Python 3.5.

Expected results

Succesfull migration.

Actual results

INFO  [alembic.runtime.migration] Running upgrade db0c65b146bd -> a99f2f7c195a, rewriting url from shortner with new format
Traceback (most recent call last):
  File "/Users/canary/anaconda3/envs/super0.17.0/bin/superset", line 6, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/Users/canary/Development/supersource/superset/bin/superset", line 14, in <module>
    manager.run()
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/flask_script/__init__.py", line 412, in run
    result = self.handle(sys.argv[0], sys.argv[1:])
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/flask_script/__init__.py", line 383, in handle
    res = handle(*args, **config)
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/flask_script/commands.py", line 216, in __call__
    return self.run(*args, **kwargs)
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/flask_migrate/__init__.py", line 200, in upgrade
    command.upgrade(config, revision, sql=sql, tag=tag)
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/alembic/command.py", line 254, in upgrade
    script.run_env()
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/alembic/script/base.py", line 416, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/alembic/util/pyfiles.py", line 93, in load_python_file
    module = load_module_py(module_id, path)
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/alembic/util/compat.py", line 64, in load_module_py
    module_id, path).load_module(module_id)
  File "<frozen importlib._bootstrap_external>", line 396, in _check_name_wrapper
  File "<frozen importlib._bootstrap_external>", line 817, in load_module
  File "<frozen importlib._bootstrap_external>", line 676, in load_module
  File "<frozen importlib._bootstrap>", line 268, in _load_module_shim
  File "<frozen importlib._bootstrap>", line 693, in _load
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 673, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/Users/canary/Development/supersource/superset/migrations/env.py", line 100, in <module>
    run_migrations_online()
  File "/Users/canary/Development/supersource/superset/migrations/env.py", line 93, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/alembic/runtime/environment.py", line 817, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/Users/canary/anaconda3/envs/super0.17.0/lib/python3.5/site-packages/alembic/runtime/migration.py", line 323, in run_migrations
    step.migration_fn(**kw)
  File "/Users/canary/Development/supersource/superset/migrations/versions/a99f2f7c195a_rewriting_url_from_shortner_with_new_.py", line 62, in upgrade
    d = cast_form_data(d)
  File "/Users/canary/Development/supersource/superset/legacy.py", line 77, in cast_form_data
    for k in d.keys():
RuntimeError: dictionary changed size during iteration

Steps to reproduce

1. create Python 3 virtualenv
2. pip install superset==0.15.4
3. initialise the demo database with a MySQL backend, and add new slices with filters.
4. pip install -U superset==0.17.0
5. superset db upgrade

[rsxm]

See #2342 for fix.

[rsxm]

This Python 2/3 compatibility bug affects two migrations:

• a99f2f7c195a_rewriting_url_from_shortner_with_new_.py
• db0c65b146bd_update_slice_model_json.py

As well as the get_form_data inner function of the approve view.

[mistercrunch]

will be fixed in 1.7.1