capafmt: use yaml parser that supports comments to reformat

capa/rules.py

@@ -1,9 +1,12 @@
-import yaml
 import uuid
 import codecs
 import logging
 import binascii
 
+import six
+import yaml
+import ruamel.yaml
+
 import capa.engine
 from capa.engine import *
 import capa.features
@@ -509,13 +512,27 @@ class Rule(object):
                 raise InvalidRuleWithPath(path, str(e))
 
     def to_yaml(self):
-        import six
-        from ruamel.yaml import YAML
+        # reformat the yaml document with a common style.
+        # this includes:
+        #  - ordering the meta elements
+        #  - indenting the nested items with two spaces
+        #
+        # we use the ruamel.yaml parser for this, because it supports roundtripping of documents with comments.
 
+        # order the meta elements in the following preferred order.
+        # any custom keys will come after this.
         COMMON_KEYS = ("name", "namespace", "rule-category", "author", "att&ck", "mbc", "examples", "scope")
 
-        yaml = YAML(typ='rt')
+        yaml = ruamel.yaml.YAML(typ='rt')
+        # use block mode, not inline json-like mode
         yaml.default_flow_style = False
+        # indent lists by two spaces below their parent
+        #
+        #     features:
+        #       - or:
+        #         - mnemonic: aesdec
+        #         - mnemonic: vaesdec
+        yaml.indent(sequence=2, offset=2)
 
         definition = yaml.load(self.definition)
         # definition retains a reference to `meta`,
@@ -541,79 +558,7 @@ class Rule(object):
 
         ostream = six.BytesIO()
         yaml.dump(definition, ostream)
-        print(ostream.getvalue().decode('utf-8'))
-        return ''
-
-
-        definition = yaml.safe_load(self.definition)
-
-        formatted = DefaultOrderedDict(default_factory=lambda: DefaultOrderedDict(default_factory=DefaultOrderedDict))
-        meta = definition["rule"]["meta"]
-
-
-        for key in COMMON_KEYS:
-            if key in meta:
-                formatted["rule"]["meta"][key] = meta[key]
-
-        for key in sorted(meta.keys()):
-            if key in COMMON_KEYS:
-                continue
-            formatted["rule"]["meta"][key] = meta[key]
-
-        formatted["rule"]["features"] = definition["rule"]["features"]
-
-        return yaml.dump(formatted, Dumper=CapaDumper, default_flow_style=False)
-
-
-class DefaultOrderedDict(collections.OrderedDict):
-    # Source: http://stackoverflow.com/a/6190500/562769
-    def __init__(self, default_factory=None, *a, **kw):
-        if (default_factory is not None and not isinstance(default_factory, collections.Callable)):
-            raise TypeError('first argument must be callable')
-        super(DefaultOrderedDict, self).__init__(*a, **kw)
-        self.default_factory = default_factory
-
-    def __getitem__(self, key):
-        try:
-            return super(DefaultOrderedDict, self).__getitem__(key)
-        except KeyError:
-            return self.__missing__(key)
-
-    def __missing__(self, key):
-        if self.default_factory is None:
-            raise KeyError(key)
-        value = self.default_factory()
-        self[key] = value
-        return value
-
-
-class CapaDumper(yaml.Dumper):
-    '''
-    Tweak the yaml serializer to emit sequences/lists with additional indentation.
-    ref: https://stackoverflow.com/a/39681672/87207
-
-    before:
-
-        rule:
-          features:
-          - or:
-            - count(mnemonic(rdtsc)): 2 or more
-            - mnemonic: icebp
-
-    after:
-
-        rule:
-          features:
-            - or:
-                - count(mnemonic(rdtsc)): 2 or more
-                - mnemonic: icebp
-    '''
-    def __init__(self, *args, **kwargs):
-        super(CapaDumper, self).__init__(*args, **kwargs)
-        self.add_representer(DefaultOrderedDict, lambda dumper, data: dumper.represent_dict(data.iteritems()))
-
-    def increase_indent(self, flow=False, indentless=False):
-        return super(CapaDumper, self).increase_indent(flow, False)
+        return ostream.getvalue().decode('utf-8').rstrip("\n")
 
 
 def get_rules_with_scope(rules, scope):

setup.py

@@ -9,6 +9,7 @@ requirements = [
     "tqdm",
     "pyyaml",
     "tabulate",
+    "ruamel.yaml"
 ]
 
 if sys.version_info >= (3, 0):