GroupPath: a utility to work with virtual Group hierarchies

Groups can be used to store nodes in AiiDA, but do not have any builtin
hierarchy themselves. However, often it may be useful to think of groups
as folders on a filesystem and the nodes within them as the files.

Building this functionality directly on the database would require
significant changes, but a virtual hierarchy based on the group labels
can be readily provided. This is what the new utility class `GroupPath`
facilitates. It allows group labels to be interpreted as the hierarchy
of groups. Example: consider one has groups with the following labels

    group/sub/a
    group/sub/b
    group/other/c

One could see this as the group `group` containing the sub groups `sub`
and `other`, with `sub` containing `a` and `b` itself. The `GroupPath`
class allows one to exploit this hierarchical naming::

    path = GroupPath('group')
    path.sub.a.get_group()  # will return group with label `group/sub/a`

It can also be used to create groups that do not yet exist:

    path = GroupPath()
    path.some.group.get_or_create_group()

This will create a `Group` with the label `some/group`. The `GroupPath`
class implements many other useful methods to make the traversing and
manipulating of groups a lot easier.

.ci/workchains.py

@@ -7,6 +7,7 @@
 # For further information on the license, see the LICENSE.txt file        #
 # For further information please visit http://www.aiida.net               #
 ###########################################################################
+# pylint: disable=invalid-name
 from aiida.common import AttributeDict
 from aiida.engine import calcfunction, workfunction, WorkChain, ToContext, append_, while_, ExitCode
 from aiida.engine import BaseRestartWorkChain, process_handler, ProcessHandlerReport

.pylintrc

@@ -50,7 +50,7 @@ confidence=
 # --enable=similarities". If you want to run only the classes checker, but have
 # no Warning level messages displayed, use"--disable=all --enable=classes
 # --disable=W"
-disable=bad-continuation,locally-disabled,useless-suppression,django-not-available,bad-option-value,logging-format-interpolation,no-else-raise,import-outside-toplevel
+disable=bad-continuation,locally-disabled,useless-suppression,django-not-available,bad-option-value,logging-format-interpolation,no-else-raise,import-outside-toplevel,cyclic-import
 
 # Enable the message, report, category or checker with the given id(s). You can
 # either give multiple identifier separated by comma (,) or put this option

aiida/cmdline/commands/cmd_group.py

@@ -361,3 +361,72 @@ def group_copy(source_group, destination_group):
     # Copy nodes
     dest_group.add_nodes(list(source_group.nodes))
     echo.echo_success('Nodes copied from group<{}> to group<{}>'.format(source_group.label, dest_group.label))
+
+
+@verdi_group.group('path')
+def verdi_group_path():
+    """Inspect groups of nodes, with delimited label paths."""
+
+
+@verdi_group_path.command('ls')
+@click.argument('path', type=click.STRING, required=False)
+@click.option('-R', '--recursive', is_flag=True, default=False, help='Recursively list sub-paths encountered')
+@click.option('-l', '--long', 'as_table', is_flag=True, default=False, help='List as a table, with sub-group count')
+@click.option(
+    '-d', '--with-description', 'with_description', is_flag=True, default=False, help='Show also the group description'
+)
+@click.option(
+    '--no-virtual',
+    'no_virtual',
+    is_flag=True,
+    default=False,
+    help='Only show paths that fully correspond to an existing group'
+)
+@click.option(
+    '-t',
+    '--type',
+    'group_type',
+    type=types.LazyChoice(valid_group_type_strings),
+    default=user_defined_group,
+    help='Show groups of a specific type, instead of user-defined groups. Start with semicolumn if you want to '
+    'specify aiida-internal type'
+)
+@click.option('--no-warn', is_flag=True, default=False, help='Do not issue a warning if any paths are invalid.')
+@with_dbenv()
+def group_path_ls(path, recursive, as_table, no_virtual, group_type, with_description, no_warn):
+    # pylint: disable=too-many-arguments
+    """Show a list of existing group paths."""
+    from aiida.tools.groups.paths import GroupPath, InvalidPath
+
+    try:
+        path = GroupPath(path or '', type_string=group_type, warn_invalid_child=not no_warn)
+    except InvalidPath as err:
+        echo.echo_critical(str(err))
+
+    if recursive:
+        children = path.walk()
+    else:
+        children = path.children
+
+    if as_table or with_description:
+        from tabulate import tabulate
+        headers = ['Path', 'Sub-Groups']
+        if with_description:
+            headers.append('Description')
+        rows = []
+        for child in sorted(children):
+            if no_virtual and child.is_virtual:
+                continue
+            row = [
+                child.path if child.is_virtual else click.style(child.path, bold=True),
+                len([c for c in child.walk() if not c.is_virtual])
+            ]
+            if with_description:
+                row.append('-' if child.is_virtual else child.get_group().description)
+            rows.append(row)
+        echo.echo(tabulate(rows, headers=headers))
+    else:
+        for child in sorted(children):
+            if no_virtual and child.is_virtual:
+                continue
+            echo.echo(child.path, bold=not child.is_virtual)

aiida/tools/groups/__init__.py

@@ -0,0 +1,11 @@
+# This file is part of the AiiDA code.                                    #
+#                                                                         #
+# The code is hosted on GitHub at https://github.com/aiidateam/aiida-core #
+# For further information on the license, see the LICENSE.txt file        #
+# For further information please visit http://www.aiida.net               #
+###########################################################################
+# pylint: disable=wildcard-import,undefined-variable
+"""Provides tools for interacting with AiiDA Groups."""
+from .paths import *
+
+__all__ = paths.__all__