Feature filter keep_keys (#8456)

* Add filter keep_keys. Implement feature request #8438

* Fix comment indentation.

* Fix regex reference.

* Fix indentation.

* Fix isinstance list.

* Update plugins/plugin_utils/keys_filter.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/plugin_utils/keys_filter.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/plugin_utils/keys_filter.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/plugin_utils/keys_filter.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/filter/keep_keys.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update documentation, examples, and integration tests.

* _keys_filter_target_str returns tuple of unique target strings if
target is list. Update documentation, function comments, and error
messages.

* Sort maintainers.

* Update plugins/filter/keep_keys.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update examples with explicit collection.

---------

Co-authored-by: Felix Fontein <felix@fontein.de>

plugins/filter/keep_keys.py

@@ -0,0 +1,138 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2024 Vladimir Botka <vbotka@gmail.com>
+# Copyright (c) 2024 Felix Fontein <felix@fontein.de>
+# GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or https://www.gnu.org/licenses/gpl-3.0.txt)
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+DOCUMENTATION = '''
+    name: keep_keys
+    short_description: Keep specific keys from dictionaries in a list
+    version_added: "9.1.0"
+    author:
+      - Vladimir Botka (@vbotka)
+      - Felix Fontein (@felixfontein)
+    description: This filter keeps only specified keys from a provided list of dictionaries.
+    options:
+      _input:
+        description:
+          - A list of dictionaries.
+          - Top level keys must be strings.
+        type: list
+        elements: dictionary
+        required: true
+      target:
+        description:
+          - A single key or key pattern to keep, or a list of keys or keys patterns to keep.
+          - If O(matching_parameter=regex) there must be exactly one pattern provided.
+        type: raw
+        required: true
+      matching_parameter:
+        description: Specify the matching option of target keys.
+        type: str
+        default: equal
+        choices:
+          equal: Matches keys of exactly one of the O(target) items.
+          starts_with: Matches keys that start with one of the O(target) items.
+          ends_with: Matches keys that end with one of the O(target) items.
+          regex:
+            - Matches keys that match the regular expresion provided in O(target).
+            - In this case, O(target) must be a regex string or a list with single regex string.
+'''
+
+EXAMPLES = '''
+  l:
+    - {k0_x0: A0, k1_x1: B0, k2_x2: [C0], k3_x3: foo}
+    - {k0_x0: A1, k1_x1: B1, k2_x2: [C1], k3_x3: bar}
+
+  # 1) By default match keys that equal any of the items in the target.
+  t: [k0_x0, k1_x1]
+  r: "{{ l | community.general.keep_keys(target=t) }}"
+
+  # 2) Match keys that start with any of the items in the target.
+  t: [k0, k1]
+  r: "{{ l | community.general.keep_keys(target=t, matching_parameter='starts_with') }}"
+
+  # 3) Match keys that end with any of the items in target.
+  t: [x0, x1]
+  r: "{{ l | community.general.keep_keys(target=t, matching_parameter='ends_with') }}"
+
+  # 4) Match keys by the regex.
+  t: ['^.*[01]_x.*$']
+  r: "{{ l | community.general.keep_keys(target=t, matching_parameter='regex') }}"
+
+  # 5) Match keys by the regex.
+  t: '^.*[01]_x.*$'
+  r: "{{ l | community.general.keep_keys(target=t, matching_parameter='regex') }}"
+
+  # The results of above examples 1-5 are all the same.
+  r:
+    - {k0_x0: A0, k1_x1: B0}
+    - {k0_x0: A1, k1_x1: B1}
+
+  # 6) By default match keys that equal the target.
+  t: k0_x0
+  r: "{{ l | community.general.keep_keys(target=t) }}"
+
+  # 7) Match keys that start with the target.
+  t: k0
+  r: "{{ l | community.general.keep_keys(target=t, matching_parameter='starts_with') }}"
+
+  # 8) Match keys that end with the target.
+  t: x0
+  r: "{{ l | community.general.keep_keys(target=t, matching_parameter='ends_with') }}"
+
+  # 9) Match keys by the regex.
+  t: '^.*0_x.*$'
+  r: "{{ l | community.general.keep_keys(target=t, matching_parameter='regex') }}"
+
+  # The results of above examples 6-9 are all the same.
+  r:
+    - {k0_x0: A0}
+    - {k0_x0: A1}
+'''
+
+RETURN = '''
+  _value:
+    description: The list of dictionaries with selected keys.
+    type: list
+    elements: dictionary
+'''
+
+from ansible_collections.community.general.plugins.plugin_utils.keys_filter import (
+    _keys_filter_params,
+    _keys_filter_target_str)
+
+
+def keep_keys(data, target=None, matching_parameter='equal'):
+    """keep specific keys from dictionaries in a list"""
+
+    # test parameters
+    _keys_filter_params(data, target, matching_parameter)
+    # test and transform target
+    tt = _keys_filter_target_str(target, matching_parameter)
+
+    if matching_parameter == 'equal':
+        def keep_key(key):
+            return key in tt
+    elif matching_parameter == 'starts_with':
+        def keep_key(key):
+            return key.startswith(tt)
+    elif matching_parameter == 'ends_with':
+        def keep_key(key):
+            return key.endswith(tt)
+    elif matching_parameter == 'regex':
+        def keep_key(key):
+            return tt.match(key) is not None
+
+    return [dict((k, v) for k, v in d.items() if keep_key(k)) for d in data]
+
+
+class FilterModule(object):
+
+    def filters(self):
+        return {
+            'keep_keys': keep_keys,
+        }