qapi/gen.py: update write() to be more idiomatic

Make the file handling here just a tiny bit more idiomatic.
(I realize this is heavily subjective.)

Use exist_ok=True for os.makedirs and remove the exception,
use fdopen() to wrap the file descriptor in a File-like object,
and use a context manager for managing the file pointer.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201009161558.107041-31-jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/gen.py

@@ -12,7 +12,6 @@
 # See the COPYING file in the top-level directory.
 
 from contextlib import contextmanager
-import errno
 import os
 import re
 from typing import (
@@ -65,21 +64,19 @@ def write(self, output_dir: str) -> None:
             return
         pathname = os.path.join(output_dir, self.fname)
         odir = os.path.dirname(pathname)
+
         if odir:
-            try:
-                os.makedirs(odir)
-            except os.error as e:
-                if e.errno != errno.EEXIST:
-                    raise
+            os.makedirs(odir, exist_ok=True)
+
+        # use os.open for O_CREAT to create and read a non-existant file
         fd = os.open(pathname, os.O_RDWR | os.O_CREAT, 0o666)
-        f = open(fd, 'r+', encoding='utf-8')
-        text = self.get_content()
-        oldtext = f.read(len(text) + 1)
-        if text != oldtext:
-            f.seek(0)
-            f.truncate(0)
-            f.write(text)
-        f.close()
+        with os.fdopen(fd, 'r+', encoding='utf-8') as fp:
+            text = self.get_content()
+            oldtext = fp.read(len(text) + 1)
+            if text != oldtext:
+                fp.seek(0)
+                fp.truncate(0)
+                fp.write(text)
 
 
 def _wrap_ifcond(ifcond: List[str], before: str, after: str) -> str: