Add an abstract base class for connection classes

[Ryan J Ollos]

Discussed in #11580, add an abstract base class for the connection classes: SQLiteConnection, PostgreSQLConnection and MySQLConnection. With this addition, implementations of any new database backends will have an interface to adhere to.

[Ryan J Ollos]

I guess these changes should probably go on the trunk.

[Ryan J Ollos]

Initially I used the pre-Python26 pattern of defining a class with methods that raise NotImplementedErrors. However this doesn't prevent a user from instantiating the abstract base class (unless we also define an __init__ method that raises a NotImplementedError), and it doesn't perform any runtime checks when constructing the derived instance to guarantee that the derived class has implemented all of the abstract methods.

>>> import trac.db.api
>>> c = trac.db.api.Connection()
>>> c.like()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "trac/db/api.py", line 209, in like
    raise NotImplementedError
NotImplementedError

Using the Python 2.6 ​AbstractBaseClass module we can enforce that all of the abstractmethods have been implemented. If we didn't define like on the SQLiteConnection class, the result would be:

>>> import trac.db.sqlite_backend
>>> csqlite = trac.db.sqlite_backend.SQLiteConnection('/home/user/Workspace/tracdev/tracdev/db/trac.db')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class SQLiteConnection with abstract methods like

Proposed changes can be found in log:rjollos.git:t11581. In particular, please let me know if you have any better text for the Connection class method docstrings.

[Ryan J Ollos]

I decided to commit changes in #11600 and leave these changes for a few days to see if there is any feedback. Proposed changes have been rebased on the trunk in log:rjollos.git:t11581.3.

[Ryan J Ollos]

Committed to trunk in [12762].

[Ryan J Ollos]

I'm thinking that the abstract class Connection should be named ConnectionBase. ConnectionBase will provide better name differentiation from ConnectionWrapper.

Two existing base classes that follow this convention are WikiMacroBase and CachedPropertyBase. Several base classes such as Repository do not follow the convention. Most of the abstract classes can be seen in [12782].

Going forward, should we use the convention that base classes should end in Base? The detail could be added to TracDev/CodingStyle.

[Ryan J Ollos]

Change from comment:5 committed in [12868].

[Ryan J Ollos]

Replying to rjollos:

Going forward, should we use the convention that base classes should end in Base? The detail could be added to TracDev/CodingStyle.

This detail was added to the coding style page in TracDev/CodingStyle@19.