Extract py_run of pylint.epylint as spawn_pylint

This is the preparation for spawning pylint process with pyls specific
customization in subsequent change.

Almost all of spawn_pylint is cited from pylint/epylint.py in
6d818e3b84b853fe06f4199eb8408fe6dfe033be (the latest version on master
branch at 2019-12-04) of pylint.

BTW, this commit imports StringIO manually for compatibility between
Python 2.x and 3.x instead of using "six" like recent pylint 1.9.x
(supporting python 2.x), because requiring "six" only for StringIO
seems over-kill.

pyls/plugins/pylint_lint.py

@@ -2,19 +2,67 @@
 """Linter plugin for pylint."""
 import collections
 import logging
+import os
+import shlex
+import subprocess
 import sys
 
-from pylint.epylint import py_run
 from pyls import hookimpl, lsp
 
 try:
     import ujson as json
 except Exception:  # pylint: disable=broad-except
     import json
 
+if sys.version_info.major == 2:
+    from StringIO import StringIO
+else:
+    from io import StringIO
+
 log = logging.getLogger(__name__)
 
 
+def spawn_pylint(document, flags):
+    """Spawn pylint process as same as pylint.epylint.py_run
+    """
+    path = document.path
+    if sys.platform.startswith('win'):
+        # Now, shlex.split is not applied on path, and Windows path
+        # (including '\\') is safe enough. But turn backslashes into
+        # forward slashes in order to keep previous behavior.
+        path = path.replace('\\', '/')
+
+    env = dict(os.environ)
+    env["PYTHONPATH"] = os.pathsep.join(sys.path)
+
+    # Detect if we use Python as executable or not, else default to `python`
+    executable = sys.executable if "python" in sys.executable else "python"
+
+    # Create command line to call pylint
+    epylint_part = [executable, "-c", "from pylint import epylint;epylint.Run()"]
+    options = shlex.split(flags, posix=not sys.platform.startswith("win"))
+
+    pylint_call = [path, '-f', 'json'] + options
+    log.debug("Calling pylint with %r", pylint_call)
+
+    cli = epylint_part + pylint_call
+
+    stdout = stderr = subprocess.PIPE
+
+    # Call pylint in a subprocess
+    process = subprocess.Popen(
+        cli,
+        shell=False,
+        stdout=stdout,
+        stderr=stderr,
+        env=env,
+        universal_newlines=True,
+    )
+    proc_stdout, proc_stderr = process.communicate()
+    # Return standard output and error
+    return StringIO(proc_stdout), StringIO(proc_stderr)
+
+
 class PylintLinter(object):
     last_diags = collections.defaultdict(list)
 
@@ -56,16 +104,7 @@ def lint(cls, document, is_saved, flags=''):
             # save.
             return cls.last_diags[document.path]
 
-        # py_run will call shlex.split on its arguments, and shlex.split does
-        # not handle Windows paths (it will try to perform escaping). Turn
-        # backslashes into forward slashes first to avoid this issue.
-        path = document.path
-        if sys.platform.startswith('win'):
-            path = path.replace('\\', '/')
-
-        pylint_call = '{} -f json {}'.format(path, flags)
-        log.debug("Calling pylint with '%s'", pylint_call)
-        json_out, err = py_run(pylint_call, return_std=True)
+        json_out, err = spawn_pylint(document, flags)
 
         # Get strings
         json_out = json_out.getvalue()