1065/enhancement/add ctx to __init__.py

[Spaarsh]

Which issue does this PR close?

Closes #1065

Rationale for this change

To improve ergonomics of the API by providing a pre-initialized SessionContext instance as ctx. This allows users to import and use a context directly without initialization.

What changes are included in this PR?

• Added ctx = SessionContext() in __init__.py to provide a default context instance
• Added corresponding test test_default_ctx_instance to verify functionality
• Included ctx in __all__ for explicit exports

Are there any user-facing changes?

Yes, users can now do:

from datafusion import ctx # Get pre-initialized context

Instead of:

from datafusion import SessionContext
ctx = SessionContext()  # Manual initialization

It adds a new API for the users.

[deanm0000]

To address @timsaucer's point from the issue, what about doing

ctx = SessionContext.global_ctx()

instead of just ctx=SessionContext().

I don't really know the internals so I don't know what the difference is but I did find https://github.com/apache/datafusion-python/pull/1060/files which makes that available.

[timsaucer]

My question was more about ergonomics - now that we have the global context I suspect most users don't even need to import at all. In your use case, do you find you still need it?

[deanm0000]

I see that the other PR makes functions for the file readers at the module level but AFAIK, there are still utilities only available through ctx. For example, we still have to do ctx.from_pydict rather than just from_pydict. I'm personally indifferent between having all those functions available from a module rather than an instantiated class or to have an instance of the initialized class available to import.

[timsaucer]

I recommend closing this PR or at lest changing it to:

global_ctx = SessionContext.global_ctx()

That I think will also help with making sure the naming is consistent.