PEP 528: Readability and style updates

pep-0528.txt

@@ -21,8 +21,7 @@ the active code page.
 This PEP proposes changing the default standard stream implementation on Windows
 to use the Unicode APIs. This will allow users to print and input the full range
 of Unicode characters at the default Windows console. This also requires a
-subtle change to how the tokenizer parses text from readline hooks, that should
-have no backwards compatibility issues.
+subtle change to how the tokenizer parses text from readline hooks.
 
 Specific Changes
 ================
@@ -46,7 +45,7 @@ utf-16-le and converted into utf-8 when returned to Python.
 
 The use of an ASCII compatible encoding is required to maintain compatibility
 with code that bypasses the ``TextIOWrapper`` and directly writes ASCII bytes to
-the standard streams (for example, [process_stdinreader.py]_). Code that assumes
+the standard streams (for example, `Twisted's process_stdinreader.py`_). Code that assumes
 a particular encoding for the standard streams other than ASCII will likely
 break.
 
@@ -78,8 +77,9 @@ behaviour.
 Alternative Approaches
 ======================
 
-The ``win_unicode_console`` package [win_unicode_console]_ is a pure-Python
-alternative to changing the default behaviour of the console.
+The `win_unicode_console package`_ is a pure-Python alternative to changing the
+default behaviour of the console. It implements essentially the same
+modifications as described here using pure Python code.
 
 Code that may break
 ===================
@@ -94,21 +94,21 @@ Assuming stdin/stdout encoding
 
 Code that assumes that the encoding required by ``sys.stdin.buffer`` or
 ``sys.stdout.buffer`` is ``'mbcs'`` or a more specific encoding may currently be
-working by chance, but could encounter issues under this change. For example::
+working by chance, but could encounter issues under this change. For example:
 
-    sys.stdout.buffer.write(text.encode('mbcs'))
-    r = sys.stdin.buffer.read(16).decode('cp437')
+    >>> sys.stdout.buffer.write(text.encode('mbcs'))
+    >>> r = sys.stdin.buffer.read(16).decode('cp437')
 
 To correct this code, the encoding specified on the ``TextIOWrapper`` should be
-used, either implicitly or explicitly::
+used, either implicitly or explicitly:
 
-    # Fix 1: Use wrapper correctly
-    sys.stdout.write(text)
-    r = sys.stdin.read(16)
+    >>> # Fix 1: Use wrapper correctly
+    >>> sys.stdout.write(text)
+    >>> r = sys.stdin.read(16)
 
-    # Fix 2: Use encoding explicitly
-    sys.stdout.buffer.write(text.encode(sys.stdout.encoding))
-    r = sys.stdin.buffer.read(16).decode(sys.stdin.encoding)
+    >>> # Fix 2: Use encoding explicitly
+    >>> sys.stdout.buffer.write(text.encode(sys.stdout.encoding))
+    >>> r = sys.stdin.buffer.read(16).decode(sys.stdin.encoding)
 
 Incorrectly using the raw object
 --------------------------------
@@ -117,32 +117,57 @@ Code that uses the raw IO object and does not correctly handle partial reads and
 writes may be affected. This is particularly important for reads, where the
 number of characters read will never exceed one-fourth of the number of bytes
 allowed, as there is no feasible way to prevent input from encoding as much
-longer utf-8 strings::
+longer utf-8 strings.
 
-    >>> stdin = open(sys.stdin.fileno(), 'rb')
-    >>> data = stdin.raw.read(15)
+    >>> raw_stdin = sys.stdin.buffer.raw
+    >>> data = raw_stdin.read(15)
     abcdefghijklm
     b'abc'
     # data contains at most 3 characters, and never more than 12 bytes
     # error, as "defghijklm\r\n" is passed to the interactive prompt
 
 To correct this code, the buffered reader/writer should be used, or the caller
-should continue reading until its buffer is full.::
+should continue reading until its buffer is full.
 
-    # Fix 1: Use the buffered reader/writer
-    >>> stdin = open(sys.stdin.fileno(), 'rb')
+    >>> # Fix 1: Use the buffered reader/writer
+    >>> stdin = sys.stdin.buffer
     >>> data = stdin.read(15)
     abcedfghijklm
     b'abcdefghijklm\r\n'
 
-    # Fix 2: Loop until enough bytes have been read
-    >>> stdin = open(sys.stdin.fileno(), 'rb')
+    >>> # Fix 2: Loop until enough bytes have been read
+    >>> raw_stdin = sys.stdin.buffer.raw
     >>> b = b''
     >>> while len(b) < 15:
-    ... b += stdin.raw.read(15)
+    ...     b += raw_stdin.read(15)
     abcedfghijklm
     b'abcdefghijklm\r\n'
 
+Using the raw object with small buffers
+---------------------------------------
+
+Code that uses the raw IO object and attempts to read less than four characters
+will now receive an error. Because it's possible that any single character may
+require up to four bytes when represented in utf-8, requests must fail.
+
+    >>> raw_stdin = sys.stdin.buffer.raw
+    >>> data = raw_stdin.read(3)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+    ValueError: must read at least 4 bytes
+
+The only workaround is to pass a larger buffer.
+
+    >>> # Fix: Request at least four bytes
+    >>> raw_stdin = sys.stdin.buffer.raw
+    >>> data = raw_stdin.read(4)
+    a
+    b'a'
+    >>> >>>
+
+(The extra ``>>>`` is due to the newline remaining in the input buffer and is
+expected in this situation.)
+
 Copyright
 =========
 
@@ -151,7 +176,5 @@ This document has been placed in the public domain.
 References
 ==========
 
-.. [process_stdinreader.py] Twisted's process_stdinreader.py
-   (https://github.com/twisted/twisted/blob/trunk/src/twisted/test/process_stdinreader.py)
-.. [win_unicode_console] win_unicode_console package
-   (https://pypi.org/project/win_unicode_console/)
+.. _Twisted's process_stdinreader.py: https://github.com/twisted/twisted/blob/trunk/src/twisted/test/process_stdinreader.py
+.. _win_unicode_console package: https://pypi.org/project/win_unicode_console/