Optionally use fast YAML parser if available

Closes #340

.github/workflows/main.yml

@@ -25,6 +25,7 @@ jobs:
     strategy:
       matrix:
         python-version: ["3.8", "3.9", "3.10", "3.11"]
+        with-fastparser: [false, true]
 
     runs-on: ubuntu-20.04
 
@@ -34,6 +35,14 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
           install-just: true
+      - name: Install dev requirements
+        run: just devenv
+      - name: Install fast YAML parser
+        if: ${{ matrix.with-fastparser }}
+        run: |
+          .venv/bin/python -m pip install \
+            --no-deps --only-binary ':all:' \
+            -r requirements.fastparser.txt
       - name: Run tests
         run: just test
 

pipeline/loading.py

@@ -14,6 +14,20 @@
     YAMLWarning,
 )
 
+
+# Import and construct a fast YAML parser, if it's available
+try:
+    import ruamel.yaml.cyaml  # type: ignore
+
+    # Unlike `PARSER` below we don't use the round-trip (`rt`) option because we don't
+    # intend to use this for user-facing error reporting and so we're not interested in
+    # retaining line-numbers – we just want it to be as fast as possible.
+    FAST_PARSER: ruamel.yaml.YAML | None = ruamel.yaml.YAML(
+        typ=["safe"], pure=False
+    )  # pragma: no cover
+except ImportError:  # pragma: no cover
+    FAST_PARSER = None
+
 from . import exceptions
 
 
@@ -56,6 +70,19 @@ def make_yaml_error_more_helpful(
 
 
 def parse_yaml_file(data: str | Path, filename: str | None = None) -> dict[str, Any]:
+    # If a fast parser is availabe and can parse the input without error then we just
+    # return it. This results in a very significant speed-up for large files. If there
+    # are errors then we re-parse using the pure Python parser which gives much more
+    # helpful error messages.
+    #
+    # Note that this _is_ covered by tests in CI but, because we're not combining
+    # coverage across multiple runs, we have to mark it as uncovered.
+    if FAST_PARSER is not None:  # pragma: no cover
+        try:
+            return FAST_PARSER.load(data)  # type: ignore[no-any-return]
+        except Exception:
+            pass
+
     try:
         return PARSER.load(data)  # type: ignore[no-any-return]
     # ruyaml doesn't have a nice exception hierarchy so we have to catch these

requirements.fastparser.in

@@ -0,0 +1,4 @@
+# These are the optional packages which, if installed, we will use to parse
+# YAML but are not hard dependencies
+ruamel.yaml
+ruamel.yaml.clib

requirements.fastparser.txt

@@ -0,0 +1,12 @@
+#
+# This file is autogenerated by pip-compile with Python 3.8
+# by the following command:
+#
+#    pip-compile requirements.fastparser.in
+#
+ruamel-yaml==0.18.6
+    # via -r requirements.fastparser.in
+ruamel-yaml-clib==0.2.8
+    # via
+    #   -r requirements.fastparser.in
+    #   ruamel-yaml