Small Python modernization of Brotli code.

PiperOrigin-RevId: 549289787

python/bro.py

@@ -1,7 +1,11 @@
 #! /usr/bin/env python
 """Compression/decompression utility using the Brotli algorithm."""
 
+# Note: Python2 has been deprecated long ago, but some projects out in
+# the wide world may still use it nevertheless. This should not
+# deprive them from being able to run Brotli.
 from __future__ import print_function
+
 import argparse
 import os
 import platform
@@ -9,8 +13,9 @@ import sys
 
 import brotli
 
+
 # default values of encoder parameters
-DEFAULT_PARAMS = {
+_DEFAULT_PARAMS = {
     'mode': brotli.MODE_GENERIC,
     'quality': 11,
     'lgwin': 22,
@@ -19,17 +24,32 @@ DEFAULT_PARAMS = {
 
 
 def get_binary_stdio(stream):
-    """ Return the specified standard input, output or errors stream as a
-    'raw' buffer object suitable for reading/writing binary data from/to it.
+    """Return the specified stdin/stdout/stderr stream.
+
+    If the stdio stream requested (i.e. sys.(stdin|stdout|stderr))
+    has been replaced with a stream object that does not have a `.buffer`
+    attribute, this will return the original stdio stream's buffer, i.e.
+    `sys.__(stdin|stdout|stderr)__.buffer`.
+
+    Args:
+      stream: One of 'stdin', 'stdout', 'stderr'.
+
+    Returns:
+      The stream, as a 'raw' buffer object (i.e. io.BufferedIOBase subclass
+      instance such as io.Bufferedreader/io.BufferedWriter), suitable for
+      reading/writing binary data from/to it.
     """
-    assert stream in ['stdin', 'stdout', 'stderr'], 'invalid stream name'
-    stdio = getattr(sys, stream)
+    if stream == 'stdin': stdio = sys.stdin
+    elif stream == 'stdout': stdio = sys.stdout
+    elif stream == 'stderr': stdio = sys.stderr
+    else:
+        raise ValueError('invalid stream name: %s' % (stream,))
     if sys.version_info[0] < 3:
         if sys.platform == 'win32':
             # set I/O stream binary flag on python2.x (Windows)
             runtime = platform.python_implementation()
             if runtime == 'PyPy':
-                # the msvcrt trick doesn't work in pypy, so I use fdopen
+                # the msvcrt trick doesn't work in pypy, so use fdopen().
                 mode = 'rb' if stream == 'stdin' else 'wb'
                 stdio = os.fdopen(stdio.fileno(), mode, 0)
             else:
@@ -38,12 +58,20 @@ def get_binary_stdio(stream):
                 msvcrt.setmode(stdio.fileno(), os.O_BINARY)
         return stdio
     else:
-        # get 'buffer' attribute to read/write binary data on python3.x
-        if hasattr(stdio, 'buffer'):
+        try:
             return stdio.buffer
-        else:
-            orig_stdio = getattr(sys, '__%s__' % stream)
-            return orig_stdio.buffer
+        except AttributeError:
+            # The Python reference explains
+            # (-> https://docs.python.org/3/library/sys.html#sys.stdin)
+            # that the `.buffer` attribute might not exist, since
+            # the standard streams might have been replaced by something else
+            # (such as an `io.StringIO()` - perhaps via
+            # `contextlib.redirect_stdout()`).
+            # We fall back to the original stdio in these cases.
+            if stream == 'stdin': return sys.__stdin__.buffer
+            if stream == 'stdout': return sys.__stdout__.buffer
+            if stream == 'stderr': return sys.__stderr__.buffer
+            assert False, 'Impossible Situation.'
 
 
 def main(args=None):
@@ -114,30 +142,36 @@ def main(args=None):
         help='Base 2 logarithm of the maximum input block size. '
         'Range is 16 to 24. If set to 0, the value will be set based '
         'on the quality. Defaults to 0.')
-    # set default values using global DEFAULT_PARAMS dictionary
-    parser.set_defaults(**DEFAULT_PARAMS)
+    # set default values using global _DEFAULT_PARAMS dictionary
+    parser.set_defaults(**_DEFAULT_PARAMS)
 
     options = parser.parse_args(args=args)
 
     if options.infile:
-        if not os.path.isfile(options.infile):
-            parser.error('file "%s" not found' % options.infile)
+        try:
             with open(options.infile, 'rb') as infile:
                 data = infile.read()
+        except OSError:
+            parser.error('Could not read --infile: %s' % (infile,))
     else:
         if sys.stdin.isatty():
             # interactive console, just quit
-            parser.error('no input')
+            parser.error('No input (called from interactive terminal).')
         infile = get_binary_stdio('stdin')
         data = infile.read()
 
     if options.outfile:
-        if os.path.isfile(options.outfile) and not options.force:
-            parser.error('output file exists')
+        # Caution! If `options.outfile` is a broken symlink, will try to
+        # redirect the write according to symlink.
+        if os.path.exists(options.outfile) and not options.force:
+            parser.error(('Target --outfile=%s already exists, '
+                          'but --force was not requested.') % (outfile,))
         outfile = open(options.outfile, 'wb')
+        did_open_outfile = True
     else:
         outfile = get_binary_stdio('stdout')
-
+        did_open_outfile = False
+    try:
         try:
             if options.decompress:
                 data = brotli.decompress(data)
@@ -148,12 +182,12 @@ def main(args=None):
                     quality=options.quality,
                     lgwin=options.lgwin,
                     lgblock=options.lgblock)
+            outfile.write(data)
+        finally:
+            if did_open_outfile: outfile.close()
     except brotli.error as e:
         parser.exit(1,
-                    'bro: error: %s: %s' % (e, options.infile or 'sys.stdin'))
-
-    outfile.write(data)
-    outfile.close()
+                    'bro: error: %s: %s' % (e, options.infile or '{stdin}'))
 
 
 if __name__ == '__main__':