Docs: `unittest.mock.mock_open` documents the wrong default for `read_data`

[iamsharduld]

The documentation for unittest.mock.mock_open gives the signature as:

mock_open(mock=None, read_data=None)

but the actual default for read_data is the empty string, not None:

• Lib/unittest/mock.py: def mock_open(mock=None, read_data=''):
• The function's own docstring: "read_data is a string ... This is an empty string by default."
• At runtime: inspect.signature(mock_open) returns (mock=None, read_data='')

The code default was changed from None to '' in 2012 (commit 0dccf657b51, "Minor changes to the unittest.mock.mock_open helper"), but the documented signature was never updated. The surrounding prose already describes read_data as "a string", so the None in the signature is stale and slightly misleading.

Linked PRs

• gh-151443: Fix documented default of read_data in unittest.mock.mock_open #151444
• [3.15] gh-151443: Fix documented default of unittest.mock.mock_open's read_data parameter (GH-151444) #151450
• [3.14] gh-151443: Fix documented default of unittest.mock.mock_open's read_data parameter (GH-151444) #151451
• [3.13] gh-151443: Fix documented default of unittest.mock.mock_open's read_data parameter (GH-151444) #151452