Add --pager to %whos and %history 🤖🤖 - #15408

Closes #9891. 🤖🤖

%whos and %history print straight to the console, so checking the namespace or recent input leaves a long listing in the scrollback. This adds a --pager flag to both, following @takluyver's suggestions on the issue: a new option on each existing magic, routed through IPython.core.page.page() instead of a pager subprocess.

• %history --pager writes into a StringIO instead of sys.stdout and passes the text to page.page(). Combining it with -f FILENAME raises a UsageError, since the output cannot go to both.
• %whos --pager strips the flag, runs the usual listing under redirect_stdout, and pages the captured text. The other %whos arguments (type filters) still apply.
• page.page() keeps its usual behaviour: short output is printed as normal, and it handles $PAGER and Windows.

A whatsnew entry is in docs/source/whatsnew/pr/magics-pager-option.rst, and the %whos docstring mentions the flag. Size: 5 files, 68 lines added, 2 removed.

Tests

test_history_magic_pager in tests/test_history.py and test_whos_pager in tests/test_magic.py patch page.page, check that the output reaches it rather than stdout, and check the --pager -f refusal. Both use the new flag, so neither can pass without this change. The patch was written and checked on Windows 11 with Python 3.14.3 only; this PR's CI is the check on other platforms.

Alternatives not taken

A generic decorator that adds --pager to every magic. takluyver advised doing it only in the magics where it makes sense, and a decorator would have to reach into each magic's argument parsing.

AI disclosure

This change was written with AI assistance: Claude (claude-opus-5-5) wrote the patch and a second Claude (claude-opus-5-5) instance reviewed it, running under the Mailman harness. 🤖🤖

🤖 Generated with Claude Code