519 lines
20 KiB
Plaintext
519 lines
20 KiB
Plaintext
PEP: 3127
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Title: Integer Literal Support and Syntax
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Version: $Revision$
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Last-Modified: $Date$
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Author: Patrick Maupin <pmaupin@gmail.com>
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Discussions-To: Python-3000@python.org
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Status: Accepted
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Type: Standards Track
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Python-Version: 3.0
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Content-Type: text/x-rst
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Created: 14-Mar-2007
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Post-History: 18-Mar-2007
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Abstract
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========
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This PEP proposes changes to the Python core to rationalize
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the treatment of string literal representations of integers
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in different radices (bases). These changes are targeted at
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Python 3.0, but the backward-compatible parts of the changes
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should be added to Python 2.6, so that all valid 3.0 integer
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literals will also be valid in 2.6.
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The proposal is that:
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a) octal literals must now be specified
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with a leading "0o" or "0O" instead of "0";
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b) binary literals are now supported via a
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leading "0b" or "0B"; and
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c) provision will be made for binary numbers in
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string formatting.
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Motivation
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==========
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This PEP was motivated by two different issues:
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- The default octal representation of integers is silently confusing
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to people unfamiliar with C-like languages. It is extremely easy
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to inadvertently create an integer object with the wrong value,
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because '013' means 'decimal 11', not 'decimal 13', to the Python
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language itself, which is not the meaning that most humans would
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assign to this literal.
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- Some Python users have a strong desire for binary support in
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the language.
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Specification
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=============
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Grammar specification
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---------------------
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The grammar will be changed. For Python 2.6, the changed and
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new token definitions will be::
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integer ::= decimalinteger | octinteger | hexinteger |
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bininteger | oldoctinteger
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octinteger ::= "0" ("o" | "O") octdigit+
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bininteger ::= "0" ("b" | "B") bindigit+
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oldoctinteger ::= "0" octdigit+
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bindigit ::= "0" | "1"
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For Python 3.0, "oldoctinteger" will not be supported, and
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an exception will be raised if a literal has a leading "0" and
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a second character which is a digit.
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For both versions, this will require changes to PyLong_FromString
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as well as the grammar.
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The documentation will have to be changed as well: grammar.txt,
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as well as the integer literal section of the reference manual.
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PEP 306 should be checked for other issues, and that PEP should
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be updated if the procedure described therein is insufficient.
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int() specification
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--------------------
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int(s, 0) will also match the new grammar definition.
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This should happen automatically with the changes to
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PyLong_FromString required for the grammar change.
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Also the documentation for int() should be changed to explain
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that int(s) operates identically to int(s, 10), and the word
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"guess" should be removed from the description of int(s, 0).
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long() specification
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--------------------
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For Python 2.6, the long() implementation and documentation
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should be changed to reflect the new grammar.
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Tokenizer exception handling
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----------------------------
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If an invalid token contains a leading "0", the exception
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error message should be more informative than the current
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"SyntaxError: invalid token". It should explain that decimal
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numbers may not have a leading zero, and that octal numbers
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require an "o" after the leading zero.
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int() exception handling
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------------------------
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The ValueError raised for any call to int() with a string
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should at least explicitly contain the base in the error
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message, e.g.::
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ValueError: invalid literal for base 8 int(): 09
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oct() function
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---------------
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oct() should be updated to output '0o' in front of
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the octal digits (for 3.0, and 2.6 compatibility mode).
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Output formatting
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-----------------
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The string (and unicode in 2.6) % operator will have
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'b' format specifier added for binary, and the alternate
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syntax of the 'o' option will need to be updated to
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add '0o' in front, instead of '0'.
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PEP 3101 already supports 'b' for binary output.
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Transition from 2.6 to 3.0
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---------------------------
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The 2to3 translator will have to insert 'o' into any
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octal string literal.
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The Py3K compatible option to Python 2.6 should cause
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attempts to use oldoctinteger literals to raise an
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exception.
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Rationale
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=========
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Most of the discussion on these issues occurred on the Python-3000
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mailing list starting 14-Mar-2007, prompted by an observation that
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the average human being would be completely mystified upon finding
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that prepending a "0" to a string of digits changes the meaning of
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that digit string entirely.
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It was pointed out during this discussion that a similar, but shorter,
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discussion on the subject occurred in January of 2006, prompted by a
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discovery of the same issue.
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Background
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----------
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For historical reasons, Python's string representation of integers
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in different bases (radices), for string formatting and token
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literals, borrows heavily from C. [1]_ [2]_ Usage has shown that
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the historical method of specifying an octal number is confusing,
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and also that it would be nice to have additional support for binary
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literals.
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Throughout this document, unless otherwise noted, discussions about
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the string representation of integers relate to these features:
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- Literal integer tokens, as used by normal module compilation,
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by eval(), and by int(token, 0). (int(token) and int(token, 2-36)
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are not modified by this proposal.)
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* Under 2.6, long() is treated the same as int()
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- Formatting of integers into strings, either via the % string
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operator or the new PEP 3101 advanced string formatting method.
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It is presumed that:
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- All of these features should have an identical set
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of supported radices, for consistency.
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- Python source code syntax and int(mystring, 0) should
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continue to share identical behavior.
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Removal of old octal syntax
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----------------------------
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This PEP proposes that the ability to specify an octal number by
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using a leading zero will be removed from the language in Python 3.0
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(and the Python 3.0 preview mode of 2.6), and that a SyntaxError will
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be raised whenever a leading "0" is immediately followed by another
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digit.
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During the present discussion, it was almost universally agreed that::
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eval('010') == 8
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should no longer be true, because that is confusing to new users.
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It was also proposed that::
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eval('0010') == 10
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should become true, but that is much more contentious, because it is so
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inconsistent with usage in other computer languages that mistakes are
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likely to be made.
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Almost all currently popular computer languages, including C/C++,
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Java, Perl, and JavaScript, treat a sequence of digits with a
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leading zero as an octal number. Proponents of treating these
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numbers as decimal instead have a very valid point -- as discussed
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in `Supported radices`_, below, the entire non-computer world uses
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decimal numbers almost exclusively. There is ample anecdotal
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evidence that many people are dismayed and confused if they
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are confronted with non-decimal radices.
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However, in most situations, most people do not write gratuitous
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zeros in front of their decimal numbers. The primary exception is
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when an attempt is being made to line up columns of numbers. But
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since PEP 8 specifically discourages the use of spaces to try to
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align Python code, one would suspect the same argument should apply
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to the use of leading zeros for the same purpose.
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Finally, although the email discussion often focused on whether anybody
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actually *uses* octal any more, and whether we should cater to those
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old-timers in any case, that is almost entirely besides the point.
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Assume the rare complete newcomer to computing who *does*, either
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occasionally or as a matter of habit, use leading zeros for decimal
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numbers. Python could either:
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a) silently do the wrong thing with his numbers, as it does now;
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b) immediately disabuse him of the notion that this is viable syntax
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(and yes, the SyntaxWarning should be more gentle than it
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currently is, but that is a subject for a different PEP); or
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c) let him continue to think that computers are happy with
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multi-digit decimal integers which start with "0".
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Some people passionately believe that (c) is the correct answer,
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and they would be absolutely right if we could be sure that new
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users will never blossom and grow and start writing AJAX applications.
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So while a new Python user may (currently) be mystified at the
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delayed discovery that his numbers don't work properly, we can
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fix it by explaining to him immediately that Python doesn't like
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leading zeros (hopefully with a reasonable message!), or we can
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delegate this teaching experience to the JavaScript interpreter
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in the Internet Explorer browser, and let him try to debug his
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issue there.
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Supported radices
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-----------------
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This PEP proposes that the supported radices for the Python
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language will be 2, 8, 10, and 16.
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Once it is agreed that the old syntax for octal (radix 8) representation
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of integers must be removed from the language, the next obvious
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question is "Do we actually need a way to specify (and display)
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numbers in octal?"
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This question is quickly followed by "What radices does the language
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need to support?" Because computers are so adept at doing what you
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tell them to, a tempting answer in the discussion was "all of them."
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This answer has obviously been given before -- the int() constructor
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will accept an explicit radix with a value between 2 and 36, inclusive,
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with the latter number bearing a suspicious arithmetic similarity to
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the sum of the number of numeric digits and the number of same-case
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letters in the ASCII alphabet.
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But the best argument for inclusion will have a use-case to back
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it up, so the idea of supporting all radices was quickly rejected,
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and the only radices left with any real support were decimal,
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hexadecimal, octal, and binary.
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Just because a particular radix has a vocal supporter on the
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mailing list does not mean that it really should be in the
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language, so the rest of this section is a treatise on the
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utility of these particular radices, vs. other possible choices.
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Humans use other numeric bases constantly. If I tell you that
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it is 12:30 PM, I have communicated quantitative information
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arguably composed of *three* separate bases (12, 60, and 2),
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only one of which is in the "agreed" list above. But the
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*communication* of that information used two decimal digits
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each for the base 12 and base 60 information, and, perversely,
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two letters for information which could have fit in a single
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decimal digit.
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So, in general, humans communicate "normal" (non-computer)
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numerical information either via names (AM, PM, January, ...)
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or via use of decimal notation. Obviously, names are
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seldom used for large sets of items, so decimal is used for
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everything else. There are studies which attempt to explain
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why this is so, typically reaching the expected conclusion
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that the Arabic numeral system is well-suited to human
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cognition. [3]_
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There is even support in the history of the design of
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computers to indicate that decimal notation is the correct
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way for computers to communicate with humans. One of
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the first modern computers, ENIAC [4]_ computed in decimal,
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even though there were already existing computers which
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operated in binary.
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Decimal computer operation was important enough
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that many computers, including the ubiquitous PC, have
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instructions designed to operate on "binary coded decimal"
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(BCD) [5]_ , a representation which devotes 4 bits to each
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decimal digit. These instructions date from a time when the
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most strenuous calculations ever performed on many numbers
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were the calculations actually required to perform textual
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I/O with them. It is possible to display BCD without having
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to perform a divide/remainder operation on every displayed
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digit, and this was a huge computational win when most
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hardware didn't have fast divide capability. Another factor
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contributing to the use of BCD is that, with BCD calculations,
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rounding will happen exactly the same way that a human would
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do it, so BCD is still sometimes used in fields like finance,
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despite the computational and storage superiority of binary.
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So, if it weren't for the fact that computers themselves
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normally use binary for efficient computation and data
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storage, string representations of integers would probably
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always be in decimal.
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Unfortunately, computer hardware doesn't think like humans,
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so programmers and hardware engineers must often resort to
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thinking like the computer, which means that it is important
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for Python to have the ability to communicate binary data
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in a form that is understandable to humans.
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The requirement that the binary data notation must be cognitively
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easy for humans to process means that it should contain an integral
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number of binary digits (bits) per symbol, while otherwise
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conforming quite closely to the standard tried-and-true decimal
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notation (position indicates power, larger magnitude on the left,
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not too many symbols in the alphabet, etc.).
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The obvious "sweet spot" for this binary data notation is
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thus octal, which packs the largest integral number of bits
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possible into a single symbol chosen from the Arabic numeral
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alphabet.
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In fact, some computer architectures, such as the PDP8 and the
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8080/Z80, were defined in terms of octal, in the sense of arranging
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the bitfields of instructions in groups of three, and using
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octal representations to describe the instruction set.
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Even today, octal is important because of bit-packed structures
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which consist of 3 bits per field, such as Unix file permission
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masks.
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But octal has a drawback when used for larger numbers. The
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number of bits per symbol, while integral, is not itself
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a power of two. This limitation (given that the word size
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of most computers these days is a power of two) has resulted
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in hexadecimal, which is more popular than octal despite the
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fact that it requires a 60% larger alphabet than decimal,
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because each symbol contains 4 bits.
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Some numbers, such as Unix file permission masks, are easily
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decoded by humans when represented in octal, but difficult to
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decode in hexadecimal, while other numbers are much easier for
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humans to handle in hexadecimal.
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Unfortunately, there are also binary numbers used in computers
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which are not very well communicated in either hexadecimal or
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octal. Thankfully, fewer people have to deal with these on a
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regular basis, but on the other hand, this means that several
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people on the discussion list questioned the wisdom of adding
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a straight binary representation to Python.
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One example of where these numbers is very useful is in
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reading and writing hardware registers. Sometimes hardware
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designers will eschew human readability and opt for address
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space efficiency, by packing multiple bit fields into a single
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hardware register at unaligned bit locations, and it is tedious
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and error-prone for a human to reconstruct a 5 bit field which
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consists of the upper 3 bits of one hex digit, and the lower 2
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bits of the next hex digit.
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Even if the ability of Python to communicate binary information
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to humans is only useful for a small technical subset of the
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population, it is exactly that population subset which contains
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most, if not all, members of the Python core team, so even straight
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binary, the least useful of these notations, has several enthusiastic
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supporters and few, if any, staunch opponents, among the Python community.
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Syntax for supported radices
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-----------------------------
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This proposal is to to use a "0o" prefix with either uppercase
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or lowercase "o" for octal, and a "0b" prefix with either
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uppercase or lowercase "b" for binary.
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There was strong support for not supporting uppercase, but
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this is a separate subject for a different PEP, as 'j' for
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complex numbers, 'e' for exponent, and 'r' for raw string
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(to name a few) already support uppercase.
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The syntax for delimiting the different radices received a lot of
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attention in the discussion on Python-3000. There are several
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(sometimes conflicting) requirements and "nice-to-haves" for
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this syntax:
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- It should be as compatible with other languages and
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previous versions of Python as is reasonable, both
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for the input syntax and for the output (e.g. string
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% operator) syntax.
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- It should be as obvious to the casual observer as
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possible.
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- It should be easy to visually distinguish integers
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formatted in the different bases.
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Proposed syntaxes included things like arbitrary radix prefixes,
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such as 16r100 (256 in hexadecimal), and radix suffixes, similar
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to the 100h assembler-style suffix. The debate on whether the
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letter "O" could be used for octal was intense -- an uppercase
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"O" looks suspiciously similar to a zero in some fonts. Suggestions
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were made to use a "c" (the second letter of "oCtal"), or even
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to use a "t" for "ocTal" and an "n" for "biNary" to go along
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with the "x" for "heXadecimal".
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For the string % operator, "o" was already being used to denote
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octal, and "b" was not used for anything, so this works out
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much better than, for example, using "c" (which means "character"
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for the % operator).
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At the end of the day, since uppercase "O" can look like a zero
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and uppercase "B" can look like an 8, it was decided that these
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prefixes should be lowercase only, but, like 'r' for raw string,
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that can be a preference or style-guide issue.
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Open Issues
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===========
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It was suggested in the discussion that lowercase should be used
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for all numeric and string special modifiers, such as 'x' for
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hexadecimal, 'r' for raw strings, 'e' for exponentiation, and
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'j' for complex numbers. This is an issue for a separate PEP.
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This PEP takes no position on uppercase or lowercase for input,
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just noting that, for consistency, if uppercase is not to be
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removed from input parsing for other letters, it should be
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added for octal and binary, and documenting the changes under
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this assumption, as there is not yet a PEP about the case issue.
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Output formatting may be a different story -- there is already
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ample precedence for case sensitivity in the output format string,
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and there would need to be a consensus that there is a valid
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use-case for the "alternate form" of the string % operator
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to support uppercase 'B' or 'O' characters for binary or
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octal output. Currently, PEP3101 does not even support this
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alternate capability, and the hex() function does not allow
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the programmer to specify the case of the 'x' character.
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There are still some strong feelings that '0123' should be
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allowed as a literal decimal in Python 3.0. If this is the
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right thing to do, this can easily be covered in an additional
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PEP. This proposal only takes the first step of making '0123'
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not be a valid octal number, for reasons covered in the rationale.
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Is there (or should there be) an option for the 2to3 translator
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which only makes the 2.6 compatible changes? Should this be
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run on 2.6 library code before the 2.6 release?
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Should a bin() function which matches hex() and oct() be added?
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Is hex() really that useful once we have advanced string formatting?
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References
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==========
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.. [1] GNU libc manual printf integer format conversions
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(http://www.gnu.org/software/libc/manual/html_node/Integer-Conversions.html)
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.. [2] Python string formatting operations
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(http://docs.python.org/lib/typesseq-strings.html)
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.. [3] The Representation of Numbers, Jiajie Zhang and Donald A. Norman
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(http://acad88.sahs.uth.tmc.edu/research/publications/Number-Representation.pdf)
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.. [4] ENIAC page at wikipedia
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(http://en.wikipedia.org/wiki/ENIAC)
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.. [5] BCD page at wikipedia
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(http://en.wikipedia.org/wiki/Binary-coded_decimal)
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
|
||
End:
|