Add code formatter

.pre-commit-config.yaml

@@ -0,0 +1,21 @@
+repos:
+-   repo: https://github.com/psf/black
+    rev: 20.8b1
+    hooks:
+      -   id: black
+          files: '^.*\.py'
+          types: [file]
+-   repo: https://gitlab.com/pycqa/flake8
+    rev: 3.8.4
+    hooks:
+      -   id: flake8
+          args: ['--extend-ignore=E203', '--max-line-length=88']
+          files: '^.*\.py'
+          types: [file]
+-   repo: https://github.com/pycqa/isort
+    rev: 5.6.4
+    hooks:
+      -   id: isort
+          args: ['--profile', 'black']
+          files: '^.*\.py'
+          types: [file]

contributing/README.md

@@ -18,5 +18,18 @@ $ pip install -r contributing/requirements.txt
 ### Run Formatter and Linter
 
 ```bash
-make all
+$ make all
+```
+
+### Enable pre-commit hook
+
+```bash
+$ pre-commit install
+```
+
+Basically, `pre-commit` will only run on the changed files.
+But if you execute the bellow command, you can run on all files.
+
+```bash
+$ pre-commit run --all-files
 ```

contributing/requirements.txt

@@ -1,3 +1,4 @@
 black==20.8b1
 flake8==3.8.4
-isort==5.6.4
+isort==5.6.4
+pre-commit==2.16.0