|
5 | 5 | including deprecation warnings, while others verify that other aspects of attribute |
6 | 6 | access are not inadvertently broken by mechanisms introduced to issue the warnings. |
7 | 7 |
|
8 | | -A note on multiprocessing: Because USE_SHELL has no instance state, this module does not |
9 | | -include tests of pickling and multiprocessing. |
| 8 | +A note on multiprocessing |
| 9 | +========================= |
| 10 | +
|
| 11 | +Because USE_SHELL has no instance state, this module does not include tests of pickling |
| 12 | +and multiprocessing: |
10 | 13 |
|
11 | 14 | - Just as with a simple class attribute, when a class attribute with custom logic is |
12 | 15 | later set to a new value, it may have either its initial value or the new value when |
|
28 | 31 |
|
29 | 32 | - That USE_SHELL cannot be set on instances, and that when retrieved on instances it |
30 | 33 | always gives the same value as on the class, is covered in the tests here. |
| 34 | +
|
| 35 | +A note on metaclass conflicts |
| 36 | +============================= |
| 37 | +
|
| 38 | +The most important DeprecationWarning is for the code ``Git.USE_SHELL = True``, which is |
| 39 | +a security risk. But this warning may not be possible to implement without a custom |
| 40 | +metaclass. This is because a descriptor in a class can customize all forms of attribute |
| 41 | +access on its instances, but can only customize getting an attribute on the class. |
| 42 | +Retrieving a descriptor from a class calls its ``__get__`` method (if defined), but |
| 43 | +replacing or deleting it does not call its ``__set__`` or ``__delete__`` methods. |
| 44 | +
|
| 45 | +Adding a metaclass is a potentially breaking change. This is because derived classes |
| 46 | +that use an unrelated metaclass, whether directly or by inheriting from a class such as |
| 47 | +abc.ABC that uses one, will raise TypeError when defined. These would have to be |
| 48 | +modified to use a newly introduced metaclass that is a lower bound of both. Subclasses |
| 49 | +remain unbroken in the far more typical case that they use no custom metaclass. |
| 50 | +
|
| 51 | +The tests in this module do not establish whether the danger of setting Git.USE_SHELL to |
| 52 | +True is high enough, and applications of deriving from Git and using an unrelated custom |
| 53 | +metaclass marginal enough, to justify introducing a metaclass to issue the warnings. |
31 | 54 | """ |
32 | 55 |
|
33 | 56 | importcontextlib |
|