- configuration: default values for serialization now constants rather than "magic" strings

DHParser/configuration.py

@@ -32,7 +32,12 @@ program and before any DHParser-function is invoked.
 
 from typing import Dict, Hashable, Any
 
-__all__ = ('CONFIG_PRESET',)
+__all__ = ('CONFIG_PRESET',
+           'XML_SERIALIZATION',
+           'SXPRESSION_SERIALIZATION',
+           'COMPACT_SERIALIZATION',
+           'JSON_SERIALIZATION',
+           'SERIALIZATIONS')
 
 CONFIG_PRESET = dict()  # type: Dict[Hashable, Any]
 
@@ -77,15 +82,25 @@ CONFIG_PRESET['max_parser_dropouts'] = 3
 # Possible values are:
 # 'XML'          - output as XML
 # 'S-expression' - output as S-expression, i.e. a list-like format
-# 'compact'      - compact tree output, i.e. children a represented
-#                  on indented lines with no opening or closing tags,
-#                  brackets etc.
-# Default values: "compact" for concrete syntax trees and "XML" for
-#                 abstract syntax trees and "S-expression" for any
-#                 other kind of tree.
-CONFIG_PRESET['cst_serialization'] = "compact"
-CONFIG_PRESET['ast_serialization'] = "XML"
-CONFIG_PRESET['default_serialization'] = "S-expression"
+# 'compact'      - compact tree output, i.e. children a represented on
+#                  indented lines with no opening or closing tags, brackets
+#                  etc.
+# 'json'         - output in JSON-format. This is probably the least
+#                  readable representation, but useful for serialization, for
+#                  example, to return syntax trees from remote procedure calls.
+# Default values: "compact" for concrete syntax trees and "XML" for abstract
+#                 syntax trees and "S-expression" for any other kind of tree.
+XML_SERIALIZATION = "XML"
+SXPRESSION_SERIALIZATION = "S-expression"
+COMPACT_SERIALIZATION = "compact"
+JSON_SERIALIZATION = "json"
+
+SERIALIZATIONS = frozenset({XML_SERIALIZATION, SXPRESSION_SERIALIZATION,
+                            COMPACT_SERIALIZATION, JSON_SERIALIZATION})
+
+CONFIG_PRESET['cst_serialization'] = COMPACT_SERIALIZATION
+CONFIG_PRESET['ast_serialization'] = XML_SERIALIZATION
+CONFIG_PRESET['default_serialization'] = SXPRESSION_SERIALIZATION
 
 # Defines the maximum line length for flattened S-expressions.
 # Below this threshold S-expressions will be returned in flattened

DHParser/syntaxtree.py

@@ -28,6 +28,8 @@ import copy
 import json
 from typing import Callable, cast, Iterator, List, AbstractSet, Set, Union, Tuple, Optional, Dict
 
+from DHParser.configuration import SERIALIZATIONS, XML_SERIALIZATION, SXPRESSION_SERIALIZATION, \
+    COMPACT_SERIALIZATION, JSON_SERIALIZATION
 from DHParser.error import Error, ErrorCode, linebreaks, line_col
 from DHParser.stringview import StringView
 from DHParser.toolkit import get_config_value, re
@@ -837,16 +839,17 @@ def serialize(node: Node, how: str = 'default') -> str:
     elif switch == 'default':
         switch = get_config_value('default_serialization').lower()
 
-    if switch == 's-expression':
+    if switch == SXPRESSION_SERIALIZATION.lower():
         return node.as_sxpr(flatten_threshold=get_config_value('flatten_sxpr_threshold'))
-    elif switch == 'xml':
+    elif switch == XML_SERIALIZATION.lower():
         return node.as_xml()
-    elif switch == 'json':
+    elif switch == JSON_SERIALIZATION.lower():
         return node.as_json()
-    elif switch == 'compact':
+    elif switch == COMPACT_SERIALIZATION.lower():
         return node.as_sxpr(compact=True)
     else:
-        raise ValueError('Unknown serialization %s, %s' % (how, switch))
+        raise ValueError('Unknown serialization %s. Allowed values are either: %s or : %s'
+                         % (how, "'ast', 'cst', 'default'", ", ".join(list(SERIALIZATIONS))))
 
 
 class FrozenNode(Node):

DHParser/toolkit.py

@@ -105,7 +105,7 @@ def set_config_value(key: Hashable, value: Any):
     Changes a configuration value thread-safely. The configuration
     value will be set only for the current thread. In order to
     set configuration values for any new thread, add the key and value
-    to CONFIG_PRESET, before the thread is started.
+    to CONFIG_PRESET, before any thread accessing config values is started.
     :param key:    the key (an immutable, usually a string)
     :param value:  the value
     """