[make] rewrite modified_py_files in python to be cross-platform

Makefile

@@ -3,14 +3,9 @@
 
 check_dirs := examples tests src utils
 
-# get modified files since the branch was made
-fork_point_sha := $(shell git merge-base --fork-point master)
-joined_dirs := $(shell echo $(check_dirs) | tr " " "|")
-modified_py_files := $(shell git diff --name-only $(fork_point_sha) | egrep '^($(joined_dirs))' | egrep '\.py$$')
-#$(info modified files are: $(modified_py_files))
-
 modified_only_fixup:
-	@if [ -n "$(modified_py_files)" ]; then \
+	$(eval modified_py_files := $(shell python utils/get_modified_files.py $(check_dirs)))
+	@if test -n "$(modified_py_files)"; then \
 		echo "Checking/fixing $(modified_py_files)"; \
 		black $(modified_py_files); \
 		isort $(modified_py_files); \

utils/get_modified_files.py

@@ -0,0 +1,34 @@
+# coding=utf-8
+# Copyright 2020 The HuggingFace Inc. team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# this script reports modified .py files under the desired list of top-level sub-dirs passed as a list of arguments, e.g.:
+#   python ./utils/get_modified_files.py utils src tests examples
+#
+# it uses git to find the forking point and which files were modified - i.e. files not under git won't be considered
+# since the output of this script is fed into Makefile commands it doesn't print a newline after the results
+
+import re
+import subprocess
+import sys
+
+
+fork_point_sha = subprocess.check_output("git merge-base --fork-point master".split()).decode("utf-8")
+modified_files = subprocess.check_output(f"git diff --name-only {fork_point_sha}".split()).decode("utf-8").split()
+
+joined_dirs = "|".join(sys.argv[1:])
+regex = re.compile(fr"^({joined_dirs}).*?\.py$")
+
+relevant_modified_files = [x for x in modified_files if regex.match(x)]
+print(" ".join(relevant_modified_files), end="")