yapf: add a new formatter

src/python/CMakeLists.txt

 add_formatter(autopep8 reformat autopep8)
 add_formatter(pep8 check pep8)
 add_formatter(flake8 check flake8)
+add_formatter(yapf reformat yapf)

src/python/format.yapf

+#!/bin/sh
+# This formatter uses the `yapf` tool to perform formatting checks of files
+# written in Python.
+#
+# It uses the `yapf` executable in `$PATH`. The configuration file is named
+# `.style.yapf` in any parent directory of the target file. They are optional,
+# but are used if they exist.
+
+set -e
+
+readonly path="$1"
+shift
+
+if ! [ -f "$path" ]; then
+    echo >&2 'error: could not find file to format: '"$path"
+    exit 4
+fi
+
+if ! which "yapf" > /dev/null; then
+    echo >&2 'error: could not find the `yapf` binary'
+    exit 3
+fi
+
+exec yapf --in-place "$path"

src/python/yapf.yaml.in

+# This script is only suitable for checking, not reformatting.
+formatter_yapf:
+    kind: "yapf"
+    script: "@CMAKE_INSTALL_PREFIX@/bin/format.yapf"
+    config_files:
+        - ".style.yapf"
+        - "**/.style.yapf"

test/yapf/CMakeLists.txt

+add_formatter_test(yapf pass format.py pass)
+add_formatter_test(yapf format format.py format)
+add_formatter_test(yapf with-config format.py pass)

test/yapf/format/format.py

+def weird_formatting ():
+      pass

test/yapf/format/format.py.clean

+def weird_formatting ():
+      pass

test/yapf/format/format.py.expect

+def weird_formatting():
+    pass

test/yapf/pass/format.py

+def ok_formatting():
+    pass

test/yapf/pass/format.py.clean

+def ok_formatting():
+    pass

test/yapf/with-config/.style.yapf

+[style]
+based_on_style = pep8
+indent_width = 2

test/yapf/with-config/format.py

+def weird_formatting():
+  pass

test/yapf/with-config/format.py.clean

+def weird_formatting():
+  pass