Only keep the requested fields of items from an iterator

[swissspidy]

Follow-up to #6420. When items given as an iterator can't be streamed, the formatter collects all of them first. That happens when:

• the format needs every item first (table, or a custom format without streaming), or
• a requested field is missing from the first item. For posts, this is the case for most meta fields.

Until now, every item was collected with all of its properties. For wp post list, that meant the content of every post was held in memory, although only a few fields are output.

What changes

As each item is read from the iterator, it's reduced to the keys of the requested fields.

How fields are resolved: the same way validate_fields() does. Each field is resolved by the first item that has it, preferring the key without the formatter's prefix (title over post_title). Before that, items keep nothing for that field; after it, items keep only the resolved key, with its value read at that moment. Objects count accessible properties, including magic __isset/__get; arrays count existing keys. No other property is read, so an unused prefixed alternative never triggers a magic getter.

What stays the same: field resolution, the "Field not found in any item" warnings and the output are the same as for the full items. They depend only on which item first has each field and on the resolved keys' values.

Values are read while the item is current. A command that clears a cache between chunks, like wp post list in wp-cli/entity-command#663, no longer loses values that are read lazily, such as post meta.

Not reduced:

• arrays (unchanged),
• the ids and count formats, which use the whole items,
• formatters with no fields,
• --field, which has its own path.

Results

wp post list on a site with ~96,600 posts, with wp-cli/entity-command#663. Both columns use current php-cli-tools, which includes wp-cli/php-cli-tools#202.

	Before	After
--format=table	7.2 s, 531 MB	6.6 s, 211 MB
--format=table --fields=ID,post_title,url	8.4 s, 788 MB	7.0 s, 204 MB
--format=csv --fields=ID,_edit_lock	6.8 s, 612 MB	6.1 s, 557 MB

STDOUT and STDERR are byte-identical before and after in all three cases. This was rechecked after resolving each field only once; the memory use didn't change.

The meta-field case only improves a little, because #663 keeps the object cache when a meta field is requested. Without that, the formatter used to read the meta only after every post was loaded, which needed one query per post. With this change the meta is read during iteration, so #663 could clear the cache in that case too: --format=csv --fields=ID,_edit_lock then takes 5.4 s and 169 MB, and the table 5.4 s and 195 MB. That can be done in entity-command once it requires a WP-CLI version with this change.

Tests

• PHPUnit:
  • test_non_streamed_iterator_items_are_read_while_they_are_current uses items whose magic property is only available until the next item is generated. It fails on main, which outputs null for those values.
  • test_non_streamed_iterator_items_only_read_the_resolved_key covers two cases:
    • items with a magic post_title get no __get() call when title resolves;
    • when the first item only has post_title, later items use that key too, for both arrays and iterators.
  • test_non_streamed_iterator_matches_array checks that a non-streaming custom format gets the same rows and resolved fields from an array and from an iterator. The items mix objects and arrays and have prefixed, unprefixed and partially missing keys.
• Behat: a new scenario compares table output and STDERR for array and generator input, including the "Field not found in any item" warning, a prefixed and an unprefixed key, a nested array and a boolean.
• Suites: formatter.feature (30 scenarios) and the PHPUnit suite pass. The only PHPUnit failures are the 2 FileCacheTest ones that also fail on main when run as root.
• Static checks: PHPCS is clean, and PHPStan reports no new errors.

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Summary by CodeRabbit

• Bug Fixes
  • Formatter output for generators and other iterators now matches array output, including requested fields and warnings for fields missing from every item.
  • Iterator items with temporary or magic properties are read while available, and only the resolved field is accessed, avoiding unrelated property access.
  • Unprefixed field values are displayed correctly, and requested fields are collected consistently across iterator items, including when fields are resolved from later items.

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📝 Walkthrough

Walkthrough

Formatter collection now reduces traversable items to requested fields as it reads them. Tests cover magic-property capture during iteration, prefixed-key resolution, and array-to-iterator formatting parity.

Changes

Formatter iterator field reduction

Layer / File(s)	Summary
Reduce traversable items during collection php/WP_CLI/Formatter.php	display_items() and read_remaining_items() reduce traversable items before collecting them. reduce_item() resolves requested fields and retains accessible keys. item_has_key() checks accessible object properties and array keys.
Verify iterator field capture and output parity tests/FormatterTest.php, features/formatter.feature	Tests cover magic-property capture, prefixed-key resolution, and array-to-iterator parity. The feature scenario compares array and generator output and checks missing-field warnings for both inputs.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: ernilambar

Merge Risk: ⚪ Minimal · up to 90f17

Iterator formatting retains requested values as items are read and reuses the resolved key across collection paths. The reported parity tests support merging with minimal risk.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 70.59% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: retaining only requested fields when collecting iterator items.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Inline comments:
Review comments at @php/WP_CLI/Formatter.php:
- Around line 674-677: Update Formatter::reduce_item to resolve each field’s key
once and reuse that selection for subsequent rows, reading only the resolved
key. Share the resolved-field state across each collection traversal, including
read_remaining_items, so an earlier row containing only the prefixed key can
select it for later rows.

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Copilot review overview

Review effort: Lite
Findings: 4

Open (4)

• reduce_item() re-implements field resolution semantics described as matching validate_fields().… · New
• Formatter::add_format() appears to mutate global/static formatter state. Reusing the same format… · New
• Formatter::add_format() appears to mutate global/static formatter state. Reusing the same format… · New
• Formatter::add_format() appears to mutate global/static formatter state. Reusing the same format… · New

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Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
php/WP_CLI/Formatter.php	96.55%	1 Missing ⚠️

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