Set up pre commit hook for black (#1048)

.pre-commit-config.yaml

@@ -1,4 +1,8 @@
--   repo: https://github.com/pre-commit/mirrors-yapf
-    sha: v0.22.0
+-   repo: "https://github.com/ambv/black"
+    rev: "18.9b0"
     hooks:
-    -   id: yapf
+    - id: black
+      args: ["--fast", "-v", "-S", "-N", "."]
+      language_version: "python3.6"
+      pass_filenames: false
+      always_run: true

pyproject.toml

@@ -1,2 +1,20 @@
 [tool.black]
 line-length = 100
+exclude = '''
+/(
+    build
+  | buck-out
+  | dist
+  | _build
+  | \.eggs
+  | \.git
+  | \.hg
+  | \.mypy_cache
+  | \.nox
+  | \.tox
+  | \.venv
+  | python_modules/dagster/dagster/tutorials
+  | snapshots
+  | python_modules/dagster-airflow/dagster_airflow_tests/test_project
+)/
+'''

python_modules/dagster/docs/contributing.rst

@@ -43,6 +43,16 @@ take a few minutes.
 
 Have fun coding!
 
+6. Set up pre-commit hooks
+
+We use black to enforce a consistent code style. To set up a pre-commit hook, just run:
+
+.. code-block:: console
+
+    $ pre-commit install
+
+(The `pre-commit` package is installed in dagster's dev-requirements.)
+
 Running dagit webapp in development
 -------------------------------------
 For development, run the dagit GraphQL server on a different port than the

python_modules/dagster/docs/snapshots/snap_test_doc_build.py

@@ -345,6 +345,14 @@
 </li>
 </ol>
 <p>Have fun coding!</p>
+<ol class="arabic simple" start="6">
+<li>Set up pre-commit hooks</li>
+</ol>
+<p>We use black to enforce a consistent code style. To set up a pre-commit hook, just run:</p>
+<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> pre-commit install
+</pre></div>
+</div>
+<p>(The <cite>pre-commit</cite> package is installed in dagster’s dev-requirements.)</p>
 <div class="section" id="running-dagit-webapp-in-development">
 <h3>Running dagit webapp in development<a class="headerlink" href="#running-dagit-webapp-in-development" title="Permalink to this headline">¶</a></h3>
 <p>For development, run the dagit GraphQL server on a different port than the
@@ -1938,6 +1946,16 @@
 
 Have fun coding!
 
+6. Set up pre-commit hooks
+
+We use black to enforce a consistent code style. To set up a pre-commit hook, just run:
+
+.. code-block:: console
+
+    $ pre-commit install
+
+(The `pre-commit` package is installed in dagster's dev-requirements.)
+
 Running dagit webapp in development
 -------------------------------------
 For development, run the dagit GraphQL server on a different port than the