GCSFileSystem requires gcp extra at lookup time while S3FileSystem does not

[hjtran]

There's an inconsistency in how FileSystems.get_filesystem() handles missing optional dependencies between GCS and S3.

Current Behavior

S3 (without aws extra):

>>> from apache_beam.io import filesystems
>>> filesystems.FileSystems.get_filesystem("s3://blah")
<apache_beam.io.aws.s3filesystem.S3FileSystem at 0x11a0af750>

Returns the filesystem object; validation happens later when the filesystem is actually used.

GCS (without gcp extra):

>>> from apache_beam.io import filesystems
>>> filesystems.FileSystems.get_filesystem("gcs://blah")
ValueError: Unable to get filesystem from specified path, please use the correct path or ensure the required dependency is installed, e.g., pip install apache-beam[gcp]. Path specified: gcs://blah

Raises immediately because GCSFileSystem isn't registered as a subclass.

Proposed Behavior

Both should behave consistently. GCSFileSystem should be returned from get_filesystem() like S3FileSystem, allowing callers to validate dependencies when the filesystem is actually used rather than at lookup time.

Why This Matters

• Inconsistent API behavior is confusing
• Code that handles multiple filesystem types can't catch/handle GCS gracefully
• Dependency validation at usage time (not lookup time) allows for better error handling and lazy loading patterns

Environment

• Apache Beam version: 2.70.0
• Python version: 3.11

---

Generated by Claude Code, confirmed by @hjtran

[hjtran]

@damccorm do you have an opinion on this? I'd like a second opinion on the proposed behavior before someone works on it. The fix will likely just be lazy loading gcsio in gcsfilesystem.py

[damccorm]

SGTM - I agree with the proposed behavior change and with the fix you're suggesting