https://github.com/lightquantumarchive/ee458_exm_py
SJTU EE458, excuse me?
https://github.com/lightquantumarchive/ee458_exm_py
Last synced: about 1 year ago
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SJTU EE458, excuse me?
- Host: GitHub
- URL: https://github.com/lightquantumarchive/ee458_exm_py
- Owner: LightQuantumArchive
- License: apache-2.0
- Created: 2021-04-14T10:27:31.000Z (about 5 years ago)
- Default Branch: master
- Last Pushed: 2021-04-14T11:29:22.000Z (about 5 years ago)
- Last Synced: 2025-03-26T22:42:59.804Z (about 1 year ago)
- Language: Python
- Size: 9.77 KB
- Stars: 1
- Watchers: 2
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE.md
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README
# EE458, excuse me?
Adapted from Java code because I don't have JVM installed locally.
Intended to serve as demo code for 1-N object design model.
## Definition
The definition of objects can be found at [definition.py](definition.py).
An `Advertiser` can hold multiple `Account`s,
An `Account` may only exist in a single `Advertiser`.
`Account.set_owner` may change its owner.
`Advertiser.add_account` may add an `Account` to its accounts, and vice versa.
## Tests
Test code can be found at [tests.py](tests.py). Run test by executing [main.py](main.py)
``` shell
$ python3 main.py
test_owner (tests.TestAddToDifferentAdvertiser) ... FAIL
test_contains (tests.TestAddToDifferentAdvertiser) ... FAIL
test_owner (tests.TestResetOwner) ... FAIL
test_contains (tests.TestResetOwner) ... FAIL
======================================================================
FAIL: test_owner (tests.TestAddToDifferentAdvertiser)
----------------------------------------------------------------------
AssertionError: unexpectedly None : Who's my owner?
======================================================================
FAIL: test_contains (tests.TestAddToDifferentAdvertiser)
----------------------------------------------------------------------
AssertionError: not found in : I should be one of daniel.
======================================================================
FAIL: test_owner (tests.TestResetOwner)
----------------------------------------------------------------------
AssertionError: unexpectedly None : Who's my owner?
======================================================================
FAIL: test_contains (tests.TestResetOwner)
----------------------------------------------------------------------
AssertionError: not found in : I should be one of daniel.
----------------------------------------------------------------------
Ran 4 tests in 0.001s
FAILED (failures=4)
```
So how may I change the owner of an `Account` without setting its owner to `None`(`null`) first?
## Fixed Solution
A fixed solution is given in [fixed_definition.py](fixed_definition.py).
```shell
test_owner (tests.get_test_classes..TestAddToDifferentAdvertiser) ... ok
test_contains (tests.get_test_classes..TestAddToDifferentAdvertiser) ... ok
test_owner (tests.get_test_classes..TestResetOwner) ... ok
test_contains (tests.get_test_classes..TestResetOwner) ... ok
----------------------------------------------------------------------
Ran 4 tests in 0.001s
OK
```
The point is, it's easy to break the constraint if both elements (`Account`) and containers (`Advertiser`)
can mutate the state, or more specifically, trying to manage the ownership of an element (`Account`) at the same time.
The correct way to implement this relation is to leave all the mutation work to one of them (the `N` part in this relation,
i.e. container `Advertiser`), and link the element (`Account`) to its container only.
When considering concurrency, you need sync and transaction to make all these things work. Yes, all the mutex and locking
mechanism stuff.
If you are lucky enough and have a relational database on your hand, use `foreign keys` to add links from your
`Account` to `Advertiser` instead.