Allow Cylc Config to output a metadata JSON.

cylc/flow/parsec/config.py

@@ -15,7 +15,9 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 from copy import deepcopy
+import json
 import re
+import sys
 from textwrap import dedent
 from typing import TYPE_CHECKING, Callable, Iterable, List, Optional
 
@@ -33,10 +35,12 @@
 
 if TYPE_CHECKING:
     from optparse import Values
+    from typing_extensions import Literal
 
 
 class ParsecConfig:
     """Object wrapper for parsec functions."""
+    META: "Literal['meta']" = 'meta'
 
     def __init__(
         self,
@@ -162,7 +166,7 @@ def get(self, keys: Optional[Iterable[str]] = None, sparse: bool = False):
         return cfg
 
     def idump(self, items=None, sparse=False, prefix='',
-              oneline=False, none_str='', handle=None):
+              oneline=False, none_str='', handle=None, json=False):
         """
         items is a list of --item style inputs:
            '[runtime][foo]script'.
@@ -178,7 +182,38 @@ def idump(self, items=None, sparse=False, prefix='',
                 mkeys.append(j)
         if null:
             mkeys = [[]]
-        self.mdump(mkeys, sparse, prefix, oneline, none_str, handle=handle)
+        if json:
+            self.jdump(mkeys, sparse, prefix, oneline, none_str, handle=handle)
+        else:
+            self.mdump(mkeys, sparse, prefix, oneline, none_str, handle=handle)
+
+    def jdump(
+        self,
+        mkeys=None,
+        sparse=False,
+        prefix='',
+        oneline=False,
+        none_str='',
+        handle=None
+    ):
+        """Dump a config to JSON format.
+        """
+        # When we call json.dumps we use indent to
+        # control whether output is multi-line:
+        indent = None
+        if not oneline:
+            indent = 4
+
+        for keys in mkeys:
+            if not keys:
+                keys = []
+            cfg = self.get(keys, sparse)
+            data = json.dumps(cfg, indent=indent)
+
+            # We can replace null values with our none_str:
+            data = data.replace('null', f'"{none_str}"')
+
+        print(data, file=handle or sys.stdout)
 
     def mdump(self, mkeys=None, sparse=False, prefix='',
               oneline=False, none_str='', handle=None):

cylc/flow/scripts/config.py

@@ -110,6 +110,16 @@ def get_option_parser() -> COP:
             "overrides any settings it shares with those higher up."),
         action="store_true", default=False, dest="print_hierarchy")
 
+    parser.add_option(
+        '--json',
+        help=(
+            'Print metadata from a Cylc configuration.'
+        ),
+        default=False,
+        action='store_true',
+        dest='json'
+    )
+
     parser.add_option(icp_option)
 
     platform_listing_options_group = parser.add_option_group(
@@ -188,7 +198,8 @@ async def _main(
             options.item,
             not options.defaults,
             oneline=options.oneline,
-            none_str=options.none_str
+            none_str=options.none_str,
+            json=options.json,
         )
         return
 
@@ -213,5 +224,6 @@ async def _main(
         options.item,
         not options.defaults,
         oneline=options.oneline,
-        none_str=options.none_str
+        none_str=options.none_str,
+        json=options.json
     )

tests/functional/cylc-config/11-json-dump.t

@@ -0,0 +1,96 @@
+#!/usr/bin/env bash
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#-------------------------------------------------------------------------------
+# Test cylc config can dump json files.
+# n.b. not heavily tested because most of this functionality
+# is from Standard library json.
+. "$(dirname "$0")/test_header"
+#-------------------------------------------------------------------------------
+set_test_number 6
+#-------------------------------------------------------------------------------
+
+# Test the global.cylc
+TEST_NAME="${TEST_NAME_BASE}-global"
+
+cat > "global.cylc" <<__HEREDOC__
+[platforms]
+    [[golders_green]]
+        [[[meta]]]
+            can = "Test lots of things"
+            because = metadata, is, not, fussy
+            number = 99
+__HEREDOC__
+
+export CYLC_CONF_PATH="${PWD}"
+run_ok "${TEST_NAME}" cylc config --json --one-line
+cmp_ok "${TEST_NAME}.stdout" <<__HERE__
+{"platforms": {"golders_green": {"meta": {"can": "Test lots of things", "because": "metadata, is, not, fussy", "number": "99"}}}}
+__HERE__
+
+# Test a flow.cylc
+TEST_NAME="${TEST_NAME_BASE}-workflow"
+
+cat > "flow.cylc" <<__HERE__
+[scheduling]
+    [[graph]]
+        P1D = foo
+
+[runtime]
+    [[foo]]
+__HERE__
+
+run_ok "${TEST_NAME}" cylc config . --json --icp 1000
+cmp_ok "${TEST_NAME}.stdout" <<__HERE__
+{
+    "scheduling": {
+        "graph": {
+            "P1D": "foo"
+        },
+        "initial cycle point": "1000"
+    },
+    "runtime": {
+        "root": {},
+        "foo": {
+            "completion": "succeeded"
+        }
+    }
+}
+__HERE__
+
+# Test an empty global.cylc to check:
+# * item selection
+# * null value setting
+# * showing defaults
+TEST_NAME="${TEST_NAME_BASE}-defaults-item-null-value"
+echo "" > global.cylc
+export CYLC_CONF_PATH="${PWD}"
+
+run_ok "${TEST_NAME}" cylc config \
+    -i '[scheduler][mail]' \
+    --json \
+    --defaults \
+    --null-value='zilch'
+
+cmp_ok "${TEST_NAME}.stdout" <<__HERE__
+{
+    "from": "zilch",
+    "smtp": "zilch",
+    "to": "zilch",
+    "footer": "zilch",
+    "task event batch interval": 300.0
+}
+__HERE__