issue-103: add keep_prefix option to load for preserving comments

- Add keep_prefix flag to lkml.load (default False)
- Include raw preamble in output under _prefix when enabled
- Add functional tests for with/without prefix

Author: yaelmi3

docs/source/simple.rst

@@ -70,6 +70,14 @@ Fields that can be repeated (e.g. ``view``, ``dimension``, or ``join``) are comb
                    {'name': 'amount', 'sql': '${TABLE}.amount'},
                    {'name': 'status', 'sql': '${TABLE}.status'}]}
 
+The ``keep_prefix`` parameter preserves leading comments and whitespace in a ``_prefix`` field.
+
+.. doctest::
+
+   >>> result = lkml.load("# comment\ndimension: id {}", keep_prefix=True)
+   >>> result['_prefix']
+   '# comment\n'
+
 Here's an example of some LookML that has been parsed into a dictionary. Note that the repeated key ``join`` has been transformed into a plural key ``joins``: a list of dictionaries representing each join::
 
    {

lkml/__init__.py

@@ -30,11 +30,13 @@ def parse(text: str) -> DocumentNode:
     return tree
 
 
-def load(stream: Union[str, IO]) -> dict:
+def load(stream: Union[str, IO], keep_prefix: bool = False) -> dict:
     """Parse LookML into a Python dictionary.
 
     Args:
         stream: File object or string containing LookML to be parsed
+        keep_prefix: If True, include the raw string of comments and whitespace 
+            at the top of the document under the "_prefix" key.
 
     Raises:
         TypeError: If stream is neither a string or a file object
@@ -50,6 +52,8 @@ def load(stream: Union[str, IO]) -> dict:
     tree: DocumentNode = parse(text)
     visitor = DictVisitor()
     tree_as_dict: dict = visitor.visit(tree)
+    if keep_prefix:
+        tree_as_dict["_prefix"] = tree.prefix
     return tree_as_dict
 
 

tests/test_functional.py

@@ -120,3 +120,13 @@ def test_repeated_dump_does_not_mutate_input():
     first = lkml.dump(parsed)
     second = lkml.dump(parsed)
     assert first == second
+
+
+def test_load_with_prefix():
+    raw_text = (Path(__file__).parent / "resources" / "view_with_all_fields.view.lkml").read_text(encoding="utf-8")
+    prefix = "# This is a comment\n# and another comment\n"
+    raw_text = f"{prefix}{raw_text}"
+    parsed = lkml.load(raw_text, keep_prefix=True)
+    assert parsed["_prefix"] == prefix
+    parsed_without_prefix = lkml.load(raw_text)
+    assert "_prefix" not in parsed_without_prefix