Replace uses of assert with exception-raising statements

[Ryan J Ollos]

It was noted in comment:7:ticket:11295 that we should consider replacing uses of assert, which will be removed when running the Python interpreter in optimized (-O) mode, with raises. Here is an example: tags/trac-1.0.5/trac/web/session.py@:242-244#L241​.

There are a few possible paths we could pursue:

1. Replace all uses of assert.
2. Use assert to protect against coding errors, raise exceptions for errors that can be triggered by a user in the scenario of proper-functioning code.

Whichever approach we take, it should be captured in TracDev/CodingStyle.

[Ryan J Ollos]

​th:comment:5:ticket:12292 suggests replacing the following assert with raising a TracError: ​browser:/tags/trac-1.0.6/trac/util/presentation.py@:176#L167​ .

[Ryan J Ollos]

Found this error in the logs.

2015-07-21 20:42:28,547 Trac[main] ERROR: Internal Server Error:
Traceback (most recent call last):
  File "build/bdist.linux-i686/egg/trac/web/main.py", line 550, in _dispatch_request
    dispatcher.dispatch(req)
  File "build/bdist.linux-i686/egg/trac/web/main.py", line 253, in dispatch
    self._post_process_request(req, *resp)
  File "build/bdist.linux-i686/egg/trac/web/main.py", line 384, in _post_process_request
    resp = f.post_process_request(req, *resp)
  File "/srv/trac-hacks.org/pve/lib/python2.6/site-packages/TracVote-0.3.0-py2.6.egg/tracvote/__init__.py", line 391, in post_process_request
    self.render_voter(req)
  File "/srv/trac-hacks.org/pve/lib/python2.6/site-packages/TracVote-0.3.0-py2.6.egg/tracvote/__init__.py", line 598, in render_voter
    resource = resource_from_path(self.env, path)
  File "/srv/trac-hacks.org/pve/lib/python2.6/site-packages/TracVote-0.3.0-py2.6.egg/tracvote/__init__.py", line 93, in resource_from_path
    if _resource_exists(env, resource) in (None, True):
  File "/srv/trac-hacks.org/pve/lib/python2.6/site-packages/TracVote-0.3.0-py2.6.egg/tracvote/__init__.py", line 74, in _resource_exists
    return resource_exists(env, resource)
  File "build/bdist.linux-i686/egg/trac/resource.py", line 454, in resource_exists
    return manager.resource_exists(resource)
  File "build/bdist.linux-i686/egg/trac/ticket/api.py", line 604, in resource_exists
    if self.env.db_query("SELECT id FROM ticket WHERE id=%s", (id_,)):
  File "build/bdist.linux-i686/egg/trac/db/api.py", line 124, in execute
    return db.execute(query, params)
  File "build/bdist.linux-i686/egg/trac/db/api.py", line 184, in __exit__
    self.db.close()
  File "build/bdist.linux-i686/egg/trac/db/pool.py", line 51, in close
    self._pool._return_cnx(cnx, self._key, self._tid)
  File "build/bdist.linux-i686/egg/trac/db/pool.py", line 166, in _return_cnx
    assert (tid, key) in self._active
AssertionError:

I'm unsure of the exact conditions leading to the error, but since this can occur in production it would at least be more clear if we raised a TracError with a message: tags/trac-1.0.7/trac/db/pool.py@:166#L163​

[Ryan J Ollos]

Replying to Ryan J Ollos:

​th:comment:5:ticket:12292 suggests replacing the following assert with raising a TracError: ​browser:/tags/trac-1.0.6/trac/util/presentation.py@:176#L167​ .

Found in the logs today:

2015-11-08 16:26:46,105 Trac[main] ERROR: Internal Server Error: <RequestWithSession "GET '/search?wiki=on&q=%5Bdownload%3A&page=91&noquickjump=1'">, referrer None
Traceback (most recent call last):
  File "build/bdist.linux-i686/egg/trac/web/main.py", line 554, in _dispatch_request
    dispatcher.dispatch(req)
  File "build/bdist.linux-i686/egg/trac/web/main.py", line 247, in dispatch
    resp = chosen_handler.process_request(req)
  File "build/bdist.linux-i686/egg/trac/search/web_ui.py", line 107, in process_request
    data.update(self._prepare_results(req, filters, results))
  File "build/bdist.linux-i686/egg/trac/search/web_ui.py", line 218, in _prepare_results
    results = Paginator(results, page - 1, self.RESULTS_PER_PAGE)
  File "build/bdist.linux-i686/egg/trac/util/presentation.py", line 205, in __init__
    items, num_items, num_pages = paginate(items, page, max_per_page)
  File "build/bdist.linux-i686/egg/trac/util/presentation.py", line 176, in paginate
    assert start < count, 'Page %d out of range' % page
AssertionError: Page 90 out of range

The proposed fix may be similar to ​[th 14785].

[Ryan J Ollos]

Proposed changes to address the issue in comment:4 in log:rjollos.git:t12004_paginate_exception.

[Ryan J Ollos]

comment:5 changes committed to 1.0-stable in [14364:14365], merged to trunk in [14366].