Add groupby_as_dict filter (#2323)

* Add groupby_as_dict filter.

* Test all error cases.

plugins/filter/groupby.py

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2021, Felix Fontein <felix@fontein.de>
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+from ansible.errors import AnsibleFilterError
+from ansible.module_utils.common._collections_compat import Mapping, Sequence
+
+
+def groupby_as_dict(sequence, attribute):
+    '''
+    Given a sequence of dictionaries and an attribute name, returns a dictionary mapping
+    the value of this attribute to the dictionary.
+
+    If multiple dictionaries in the sequence have the same value for this attribute,
+    the filter will fail.
+    '''
+    if not isinstance(sequence, Sequence):
+        raise AnsibleFilterError('Input is not a sequence')
+
+    result = dict()
+    for list_index, element in enumerate(sequence):
+        if not isinstance(element, Mapping):
+            raise AnsibleFilterError('Sequence element #{0} is not a mapping'.format(list_index))
+        if attribute not in element:
+            raise AnsibleFilterError('Attribute not contained in element #{0} of sequence'.format(list_index))
+        result_index = element[attribute]
+        if result_index in result:
+            raise AnsibleFilterError('Multiple sequence entries have attribute value {0!r}'.format(result_index))
+        result[result_index] = element
+    return result
+
+
+class FilterModule(object):
+    ''' Ansible list filters '''
+
+    def filters(self):
+        return {
+            'groupby_as_dict': groupby_as_dict,
+        }