Automatically compute content_hash for uploads

[AndreyVMarkelov]

Summary

Computes the Dropbox content hash of upload payloads and sends it with the request so the server verifies upload integrity, matching the behavior of dropbox-sdk-go #140.

• dropbox/content_hash.py (new) — DropboxContentHasher / StreamHasher from dropbox-api-content-hasher, adapted for Python 3 (no six), plus a content_hash() helper.
• auto_content_hash constructor flag (default True) to opt out. Preserved across clone(), as_user()/as_admin(), and with_path_root().
• Transport fills the content_hash field on upload routes when the caller hasn't supplied one and the payload is bytes. Applies generically to all upload-style routes with a content_hash field (files_upload, files_alpha_upload, upload_session_start/append/append_v2/finish).

No public API change: content_hash was already an optional field on the upload methods; this just fills it in. The logic lives in the hand-written transport so it survives client regeneration, and the caller's argument object is never mutated (a copy is made).

Tests

• test/unit/test_content_hash.py — hasher algorithm, chunked updates, copy() independence, reuse guard, non-bytes rejection, StreamHasher passthrough.
• test/unit/test_dropbox_unit.py — auto on/off, caller-provided hash respected, flag propagation across clone/team/path-root.
• test/integration/test_dropbox.py::test_upload_auto_content_hash — asserts the server-returned content_hash matches the locally computed one (end-to-end).

Unit suite: 56 passed; flake8 clean.

Note

The end-to-end behavior (server accepting and verifying the hash) is only exercised by the integration test, which requires live credentials and runs in CI's Integration job.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.19%. Comparing base (620b8be) to head (acb5cce).

Files with missing lines	Patch %	Lines
dropbox/content_hash.py	89.15%	8 Missing and 1 partial ⚠️
dropbox/dropbox_client.py	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #538      +/-   ##
==========================================
+ Coverage   65.14%   65.19%   +0.04%     
==========================================
  Files          31       32       +1     
  Lines       54902    55001      +99     
  Branches     3901     3911      +10     
==========================================
+ Hits        35767    35858      +91     
- Misses      18981    18988       +7     
- Partials      154      155       +1     

Flag	Coverage Δ
integration	65.01% <54.54%> (-0.02%)	⬇️
unit	64.75% <86.86%> (+0.14%)	⬆️

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