Prefer builtin open() over io.open() and codecs.open()

In Python3, the functions io.open() is an alias of the builtin open()
and codecs.open() is functionally equivalent. To reduce indirection,
number of imports, and number of patterns, always prefer the builtin.

https://docs.python.org/3/library/io.html#high-level-module-interface

> io.open()
>
> This is an alias for the builtin open() function.

18 files changed, 43 insertions, 53 deletions

diff --git a/sphinx/builders/applehelp.py b/sphinx/builders/applehelp.py
index 79d57210c..56e9bd6c0 100644
--- a/sphinx/builders/applehelp.py
+++ b/sphinx/builders/applehelp.py
@@ -10,7 +10,6 @@
 """
 from __future__ import print_function
 
-import codecs
 import pipes
 import plistlib
 import shlex
@@ -193,7 +192,7 @@ class AppleHelpBuilder(StandaloneHTMLBuilder):
 
         # Build the access page
         logger.info(bold(__('building access page...')), nonl=True)
-        with codecs.open(path.join(language_dir, '_access.html'), 'w') as f:  # type: ignore
+        with open(path.join(language_dir, '_access.html'), 'w') as f:
             f.write(access_page_template % {
                 'toc': htmlescape(toc, quote=True),
                 'title': htmlescape(self.config.applehelp_title)
diff --git a/sphinx/builders/changes.py b/sphinx/builders/changes.py
index 3f9bffa0d..d5244747d 100644
--- a/sphinx/builders/changes.py
+++ b/sphinx/builders/changes.py
@@ -9,7 +9,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 from os import path
 from typing import cast
 
@@ -115,9 +114,11 @@ class ChangesBuilder(Builder):
             'show_copyright': self.config.html_show_copyright,
             'show_sphinx': self.config.html_show_sphinx,
         }
-        with codecs.open(path.join(self.outdir, 'index.html'), 'w', 'utf8') as f:  # type: ignore  # NOQA
+        with open(path.join(self.outdir, 'index.html'), 'w',  # type: ignore
+                  encoding='utf8') as f:
             f.write(self.templates.render('changes/frameset.html', ctx))
-        with codecs.open(path.join(self.outdir, 'changes.html'), 'w', 'utf8') as f:  # type: ignore  # NOQA
+        with open(path.join(self.outdir, 'changes.html'), 'w',  # type: ignore
+                  encoding='utf8') as f:
             f.write(self.templates.render('changes/versionchanges.html', ctx))
 
         hltext = ['.. versionadded:: %s' % version,
@@ -135,8 +136,8 @@ class ChangesBuilder(Builder):
 
         logger.info(bold(__('copying source files...')))
         for docname in self.env.all_docs:
-            with codecs.open(self.env.doc2path(docname), 'r',  # type: ignore
-                             self.env.config.source_encoding) as f:
+            with open(self.env.doc2path(docname), 'r',  # type: ignore
+                      encoding=self.env.config.source_encoding) as f:
                 try:
                     lines = f.readlines()
                 except UnicodeDecodeError:
@@ -144,7 +145,7 @@ class ChangesBuilder(Builder):
                     continue
             targetfn = path.join(self.outdir, 'rst', os_path(docname)) + '.html'
             ensuredir(path.dirname(targetfn))
-            with codecs.open(targetfn, 'w', 'utf-8') as f:  # type: ignore
+            with open(targetfn, 'w', encoding='utf-8') as f:  # type: ignore
                 text = ''.join(hl(i + 1, line) for (i, line) in enumerate(lines))
                 ctx = {
                     'filename': self.env.doc2path(docname, None),
diff --git a/sphinx/builders/html.py b/sphinx/builders/html.py
index 3c08cc818..b4b570bdc 100644
--- a/sphinx/builders/html.py
+++ b/sphinx/builders/html.py
@@ -9,7 +9,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 import posixpath
 import re
 import sys
@@ -959,9 +958,9 @@ class StandaloneHTMLBuilder(Builder):
         try:
             searchindexfn = path.join(self.outdir, self.searchindex_filename)
             if self.indexer_dumps_unicode:
-                f = codecs.open(searchindexfn, 'r', encoding='utf-8')  # type: ignore
+                f = open(searchindexfn, 'r', encoding='utf-8')  # type: ignore
             else:
-                f = open(searchindexfn, 'rb')  # type: ignore
+                f = open(searchindexfn, 'rb')
             with f:
                 self.indexer.load(f, self.indexer_format)
         except (IOError, OSError, ValueError):
@@ -1140,7 +1139,8 @@ class StandaloneHTMLBuilder(Builder):
         # outfilename's path is in general different from self.outdir
         ensuredir(path.dirname(outfilename))
         try:
-            with codecs.open(outfilename, 'w', ctx['encoding'], 'xmlcharrefreplace') as f:  # type: ignore  # NOQA
+            with open(outfilename, 'w',  # type: ignore
+                      encoding=ctx['encoding'], errors='xmlcharrefreplace') as f:
                 f.write(output)
         except (IOError, OSError) as err:
             logger.warning(__("error writing file %s: %s"), outfilename, err)
@@ -1177,9 +1177,9 @@ class StandaloneHTMLBuilder(Builder):
         # first write to a temporary file, so that if dumping fails,
         # the existing index won't be overwritten
         if self.indexer_dumps_unicode:
-            f = codecs.open(searchindexfn + '.tmp', 'w', encoding='utf-8')  # type: ignore
+            f = open(searchindexfn + '.tmp', 'w', encoding='utf-8')  # type: ignore
         else:
-            f = open(searchindexfn + '.tmp', 'wb')  # type: ignore
+            f = open(searchindexfn + '.tmp', 'wb')
         with f:
             self.indexer.dump(f, self.indexer_format)
         movefile(searchindexfn + '.tmp', searchindexfn)
@@ -1436,9 +1436,9 @@ class SerializingHTMLBuilder(StandaloneHTMLBuilder):
     def dump_context(self, context, filename):
         # type: (Dict, unicode) -> None
         if self.implementation_dumps_unicode:
-            f = codecs.open(filename, 'w', encoding='utf-8')  # type: ignore
+            f = open(filename, 'w', encoding='utf-8')  # type: ignore
         else:
-            f = open(filename, 'wb')  # type: ignore
+            f = open(filename, 'wb')
         with f:
             self.implementation.dump(context, f, *self.additional_dump_args)
 
diff --git a/sphinx/builders/htmlhelp.py b/sphinx/builders/htmlhelp.py
index 52600ca21..4d5172a8f 100644
--- a/sphinx/builders/htmlhelp.py
+++ b/sphinx/builders/htmlhelp.py
@@ -11,7 +11,6 @@
 """
 from __future__ import print_function
 
-import codecs
 import os
 from os import path
 
@@ -208,8 +207,8 @@ class HTMLHelpBuilder(StandaloneHTMLBuilder):
     def open_file(self, outdir, basename, mode='w'):
         # type: (unicode, unicode, unicode) -> IO
         # open a file with the correct encoding for the selected language
-        return codecs.open(path.join(outdir, basename), mode,  # type: ignore
-                           self.encoding, 'xmlcharrefreplace')
+        return open(path.join(outdir, basename), mode,  # type: ignore
+                    encoding=self.encoding, errors='xmlcharrefreplace')
 
     def update_page_context(self, pagename, templatename, ctx, event_arg):
         # type: (unicode, unicode, Dict, unicode) -> None
diff --git a/sphinx/builders/linkcheck.py b/sphinx/builders/linkcheck.py
index c3fd1e88a..2189411ee 100644
--- a/sphinx/builders/linkcheck.py
+++ b/sphinx/builders/linkcheck.py
@@ -9,7 +9,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 import re
 import socket
 import threading
@@ -308,7 +307,8 @@ class CheckExternalLinksBuilder(Builder):
 
     def write_entry(self, what, docname, line, uri):
         # type: (unicode, unicode, int, unicode) -> None
-        with codecs.open(path.join(self.outdir, 'output.txt'), 'a', 'utf-8') as output:  # type: ignore  # NOQA
+        with open(path.join(self.outdir, 'output.txt'), 'a',  # type: ignore
+                  encoding='utf-8') as output:
             output.write("%s:%s: [%s] %s\n" % (self.env.doc2path(docname, None),
                                                line, what, uri))
 
diff --git a/sphinx/builders/qthelp.py b/sphinx/builders/qthelp.py
index 1b666cb4d..22ccae5f0 100644
--- a/sphinx/builders/qthelp.py
+++ b/sphinx/builders/qthelp.py
@@ -9,7 +9,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 import os
 import posixpath
 import re
@@ -145,7 +144,8 @@ class QtHelpBuilder(StandaloneHTMLBuilder):
         nspace = nspace.lower()
 
         # write the project file
-        with codecs.open(path.join(outdir, outname + '.qhp'), 'w', 'utf-8') as f:  # type: ignore  # NOQA
+        with open(path.join(outdir, outname + '.qhp'), 'w',  # type: ignore
+                  encoding='utf-8') as f:
             body = render_file('project.qhp', outname=outname,
                                title=self.config.html_title, version=self.config.version,
                                project=self.config.project, namespace=nspace,
@@ -159,7 +159,8 @@ class QtHelpBuilder(StandaloneHTMLBuilder):
         startpage = 'qthelp://' + posixpath.join(nspace, 'doc', 'index.html')
 
         logger.info(__('writing collection project file...'))
-        with codecs.open(path.join(outdir, outname + '.qhcp'), 'w', 'utf-8') as f:  # type: ignore  # NOQA
+        with open(path.join(outdir, outname + '.qhcp'), 'w',  # type: ignore
+                  encoding='utf-8') as f:
             body = render_file('project.qhcp', outname=outname,
                                title=self.config.html_short_title,
                                homepage=homepage, startpage=startpage)
diff --git a/sphinx/builders/text.py b/sphinx/builders/text.py
index 81209d165..ef8c4a7ac 100644
--- a/sphinx/builders/text.py
+++ b/sphinx/builders/text.py
@@ -9,7 +9,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 from os import path
 
 from docutils.io import StringOutput
@@ -82,7 +81,7 @@ class TextBuilder(Builder):
         outfilename = path.join(self.outdir, os_path(docname) + self.out_suffix)
         ensuredir(path.dirname(outfilename))
         try:
-            with codecs.open(outfilename, 'w', 'utf-8') as f:  # type: ignore
+            with open(outfilename, 'w', encoding='utf-8') as f:  # type: ignore
                 f.write(self.writer.output)
         except (IOError, OSError) as err:
             logger.warning(__("error writing file %s: %s"), outfilename, err)
diff --git a/sphinx/builders/xml.py b/sphinx/builders/xml.py
index 6198532c9..3e0d9224e 100644
--- a/sphinx/builders/xml.py
+++ b/sphinx/builders/xml.py
@@ -9,7 +9,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 from os import path
 
 from docutils import nodes
@@ -95,7 +94,7 @@ class XMLBuilder(Builder):
         outfilename = path.join(self.outdir, os_path(docname) + self.out_suffix)
         ensuredir(path.dirname(outfilename))
         try:
-            with codecs.open(outfilename, 'w', 'utf-8') as f:  # type: ignore
+            with open(outfilename, 'w', encoding='utf-8') as f:  # type: ignore
                 f.write(self.writer.output)
         except (IOError, OSError) as err:
             logger.warning(__("error writing file %s: %s"), outfilename, err)
diff --git a/sphinx/cmd/quickstart.py b/sphinx/cmd/quickstart.py
index 174298579..51285efa5 100644
--- a/sphinx/cmd/quickstart.py
+++ b/sphinx/cmd/quickstart.py
@@ -18,7 +18,6 @@ import re
 import sys
 import time
 from collections import OrderedDict
-from io import open
 from os import path
 
 # try to import readline, unix specific enhancement
@@ -445,7 +444,7 @@ def generate(d, overwrite=True, silent=False, templatedir=None):
         if overwrite or not path.isfile(fpath):
             if 'quiet' not in d:
                 print(__('Creating file %s.') % fpath)
-            with open(fpath, 'wt', encoding='utf-8', newline=newline) as f:
+            with open(fpath, 'wt', encoding='utf-8', newline=newline) as f:  # type: ignore
                 f.write(content)
         else:
             if 'quiet' not in d:
diff --git a/sphinx/directives/code.py b/sphinx/directives/code.py
index a98ab5883..ec389e708 100644
--- a/sphinx/directives/code.py
+++ b/sphinx/directives/code.py
@@ -7,7 +7,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 import sys
 import warnings
 from difflib import unified_diff
@@ -213,7 +212,8 @@ class LiteralIncludeReader(object):
     def read_file(self, filename, location=None):
         # type: (unicode, Any) -> List[unicode]
         try:
-            with codecs.open(filename, 'r', self.encoding, errors='strict') as f:  # type: ignore  # NOQA
+            with open(filename, 'r',  # type: ignore
+                      encoding=self.encoding, errors='strict') as f:
                 text = f.read()  # type: unicode
                 if 'tab-width' in self.options:
                     text = text.expandtabs(self.options['tab-width'])
diff --git a/sphinx/ext/autosummary/generate.py b/sphinx/ext/autosummary/generate.py
index dbadfe8d5..b4b8bcc42 100644
--- a/sphinx/ext/autosummary/generate.py
+++ b/sphinx/ext/autosummary/generate.py
@@ -20,7 +20,6 @@
 from __future__ import print_function
 
 import argparse
-import codecs
 import locale
 import os
 import pydoc
@@ -249,8 +248,8 @@ def find_autosummary_in_files(filenames):
     """
     documented = []  # type: List[Tuple[unicode, unicode, unicode]]
     for filename in filenames:
-        with codecs.open(filename, 'r', encoding='utf-8',  # type: ignore
-                         errors='ignore') as f:
+        with open(filename, 'r', encoding='utf-8',  # type: ignore
+                  errors='ignore') as f:
             lines = f.read().splitlines()
             documented.extend(find_autosummary_in_lines(lines, filename=filename))
     return documented
diff --git a/sphinx/ext/doctest.py b/sphinx/ext/doctest.py
index fa6cbed6d..c4898d583 100644
--- a/sphinx/ext/doctest.py
+++ b/sphinx/ext/doctest.py
@@ -318,8 +318,8 @@ class DocTestBuilder(Builder):
         date = time.strftime('%Y-%m-%d %H:%M:%S')
 
         self.outfile = None  # type: IO
-        self.outfile = codecs.open(path.join(self.outdir, 'output.txt'),  # type: ignore
-                                   'w', encoding='utf-8')
+        self.outfile = open(path.join(self.outdir, 'output.txt'),  # type: ignore
+                            'w', encoding='utf-8')
         self.outfile.write(('Results of doctest builder run on %s\n'
                             '==================================%s\n') %
                            (date, '=' * len(date)))
diff --git a/sphinx/ext/graphviz.py b/sphinx/ext/graphviz.py
index cf606f3b7..63f7f3dc8 100644
--- a/sphinx/ext/graphviz.py
+++ b/sphinx/ext/graphviz.py
@@ -10,7 +10,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 import posixpath
 import re
 from hashlib import sha1
@@ -142,7 +141,7 @@ class Graphviz(SphinxDirective):
             rel_filename, filename = self.env.relfn2path(argument)
             self.env.note_dependency(rel_filename)
             try:
-                with codecs.open(filename, 'r', 'utf-8') as fp:  # type: ignore
+                with open(filename, 'r', encoding='utf-8') as fp:  # type: ignore
                     dotcode = fp.read()
             except (IOError, OSError):
                 return [document.reporter.warning(
@@ -309,7 +308,7 @@ def render_dot_html(self, node, code, options, prefix='graphviz',
             self.body.append('<p class="warning">%s</p>' % alt)
             self.body.append('</object></div>\n')
         else:
-            with codecs.open(outfn + '.map', 'r', encoding='utf-8') as mapfile:  # type: ignore
+            with open(outfn + '.map', 'r', encoding='utf-8') as mapfile:  # type: ignore
                 imgmap = ClickableMapDefinition(outfn + '.map', mapfile.read(), dot=code)
                 if imgmap.clickable:
                     # has a map
diff --git a/sphinx/ext/imgmath.py b/sphinx/ext/imgmath.py
index 593835554..5be272ab1 100644
--- a/sphinx/ext/imgmath.py
+++ b/sphinx/ext/imgmath.py
@@ -9,7 +9,6 @@
     :license: BSD, see LICENSE for details.
 """
 
-import codecs
 import posixpath
 import re
 import shutil
@@ -123,7 +122,7 @@ def compile_math(latex, builder):
     """Compile LaTeX macros for math to DVI."""
     tempdir = ensure_tempdir(builder)
     filename = path.join(tempdir, 'math.tex')
-    with codecs.open(filename, 'w', 'utf-8') as f:  # type: ignore
+    with open(filename, 'w', encoding='utf-8') as f:  # type: ignore
         f.write(latex)
 
     # build latex command; old versions of latex don't have the
diff --git a/sphinx/testing/path.py b/sphinx/testing/path.py
index 585933499..6deda345f 100644
--- a/sphinx/testing/path.py
+++ b/sphinx/testing/path.py
@@ -9,7 +9,6 @@
 import os
 import shutil
 import sys
-from io import open
 
 from six import PY2, text_type
 
@@ -161,7 +160,7 @@ class path(text_type):
         """
         if isinstance(text, bytes):
             text = text.decode(encoding)
-        with open(self, 'w', encoding=encoding, **kwargs) as f:
+        with open(self, 'w', encoding=encoding, **kwargs) as f:  # type: ignore
             f.write(text)
 
     def text(self, encoding='utf-8', **kwargs):
@@ -170,7 +169,7 @@ class path(text_type):
         Returns the text in the file.
         """
         mode = 'rU' if PY2 else 'r'
-        with open(self, mode=mode, encoding=encoding, **kwargs) as f:
+        with open(self, mode=mode, encoding=encoding, **kwargs) as f:  # type: ignore
             return f.read()
 
     def bytes(self):
diff --git a/sphinx/util/docutils.py b/sphinx/util/docutils.py
index 547d74c17..96944bd6f 100644
--- a/sphinx/util/docutils.py
+++ b/sphinx/util/docutils.py
@@ -10,7 +10,6 @@
 """
 from __future__ import absolute_import
 
-import codecs
 import os
 import re
 import types
@@ -314,7 +313,7 @@ class SphinxFileOutput(FileOutput):
         # type: (unicode) -> unicode
         if (self.destination_path and self.autoclose and 'b' not in self.mode and
                 self.overwrite_if_changed and os.path.exists(self.destination_path)):
-            with codecs.open(self.destination_path, encoding=self.encoding) as f:
+            with open(self.destination_path, encoding=self.encoding) as f:  # type: ignore
                 # skip writing: content not changed
                 if f.read() == data:
                     return data
diff --git a/sphinx/util/fileutil.py b/sphinx/util/fileutil.py
index fcbc8abe6..85b6d4f47 100644
--- a/sphinx/util/fileutil.py
+++ b/sphinx/util/fileutil.py
@@ -10,7 +10,6 @@
 """
 from __future__ import absolute_import
 
-import codecs
 import os
 import posixpath
 
@@ -49,10 +48,10 @@ def copy_asset_file(source, destination, context=None, renderer=None):
             from sphinx.util.template import SphinxRenderer
             renderer = SphinxRenderer()
 
-        with codecs.open(source, 'r', encoding='utf-8') as fsrc:  # type: ignore
+        with open(source, 'r', encoding='utf-8') as fsrc:  # type: ignore
             if destination.lower().endswith('_t'):
                 destination = destination[:-2]
-            with codecs.open(destination, 'w', encoding='utf-8') as fdst:  # type: ignore
+            with open(destination, 'w', encoding='utf-8') as fdst:  # type: ignore
                 fdst.write(renderer.render_string(fsrc.read(), context))
     else:
         copyfile(source, destination)
diff --git a/sphinx/util/i18n.py b/sphinx/util/i18n.py
index edaebebda..2b557ec93 100644
--- a/sphinx/util/i18n.py
+++ b/sphinx/util/i18n.py
@@ -9,7 +9,6 @@
     :license: BSD, see LICENSE for details.
 """
 import gettext
-import io
 import os
 import re
 import warnings
@@ -69,14 +68,14 @@ class CatalogInfo(LocaleFileInfoBase):
 
     def write_mo(self, locale):
         # type: (unicode) -> None
-        with io.open(self.po_path, 'rt', encoding=self.charset) as file_po:
+        with open(self.po_path, 'rt', encoding=self.charset) as file_po:  # type: ignore
             try:
                 po = read_po(file_po, locale)
             except Exception as exc:
                 logger.warning(__('reading error: %s, %s'), self.po_path, exc)
                 return
 
-        with io.open(self.mo_path, 'wb') as file_mo:
+        with open(self.mo_path, 'wb') as file_mo:
             try:
                 write_mo(file_mo, po)
             except Exception as exc: