Add env flag for running tests with pycodestyle

Add optional PEP8_VALIDATE env flag for running prettier output from
tests into pycodestyle. If pycodestyle returns any errors, the test
fails.

Adds pycodestyle as a vendor dependency.

Several tests currently fail with the PEP8_VALIDATE flag enabled.

Proof of concept for prettier#1.

tests_config/pycodestyle.js

@@ -0,0 +1,60 @@
+"use strict";
+
+const spawnSync = require("child_process").spawnSync;
+const path = require("path");
+const normalizeOptions = require("prettier/src/main/options").normalize;
+
+// Only enable style-related errors, and use default ignored error list
+// See http://pycodestyle.pycqa.org/en/latest/intro.html#error-codes
+const selectedErrors = ["E1", "E2", "E3", "W1", "W2", "W3", "W5"];
+const ignoredErrors = [
+  "E121",
+  "E123",
+  "E126",
+  "E133",
+  "E226",
+  "E241",
+  "E242",
+  "E704",
+  "W503",
+  "W504"
+];
+
+function getPycodestyleOutput(string, options) {
+  const normalizedOptions = normalizeOptions(options);
+
+  // Use the same version of Python to run pycodestyle that we're using for prettier
+  const pythonExectuable = `python${
+    normalizedOptions.pythonVersion == "2" ? "" : "3"
+  }`;
+
+  const executionResult = spawnSync(
+    pythonExectuable,
+    [
+      path.join(__dirname, "../vendor/python/pycodestyle.py"),
+      `--select=${selectedErrors.join(",")}`,
+      `--ignore=${ignoredErrors.join(",")}`,
+      `--max-line-length=${normalizedOptions.printWidth}`,
+      "-"
+    ],
+    {
+      input: string
+    }
+  );
+
+  if (!executionResult.stderr) {
+    throw new Error("Failed to execute pycodestyle");
+  }
+
+  const error = executionResult.stderr.toString();
+
+  if (error) {
+    throw new Error(error);
+  }
+
+  return executionResult.stdout.toString();
+}
+
+module.exports = {
+  getPycodestyleOutput: getPycodestyleOutput
+};

tests_config/run_spec.js

@@ -5,8 +5,10 @@ const extname = require("path").extname;
 const prettier = require("prettier");
 const massageAST = require("prettier/src/common/clean-ast").massageAST;
 const normalizeOptions = require("prettier/src/main/options").normalize;
+const getPycodestyleOutput = require("./pycodestyle").getPycodestyleOutput;
 
 const AST_COMPARE = process.env["AST_COMPARE"];
+const PEP8_VALIDATE = process.env["PEP8_VALIDATE"];
 
 function run_spec(dirname, parsers, options) {
   options = Object.assign(
@@ -39,6 +41,10 @@ function run_spec(dirname, parsers, options) {
         expect(raw(source + "~".repeat(80) + "\n" + output)).toMatchSnapshot(
           filename
         );
+
+        if (PEP8_VALIDATE) {
+          expect(getPycodestyleOutput(output, mergedOptions)).toEqual("");
+        }
       });
 
       parsers.slice(1).forEach(parserName => {