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reSTify PEP 237 (#273)

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@ -5,350 +5,335 @@ Last-Modified: $Date$
Author: Moshe Zadka, Guido van Rossum
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 11-Mar-2001
Python-Version: 2.2
Post-History: 16-Mar-2001, 14-Aug-2001, 23-Aug-2001
Abstract
========
Python currently distinguishes between two kinds of integers
(ints): regular or short ints, limited by the size of a C long
(typically 32 or 64 bits), and long ints, which are limited only
by available memory. When operations on short ints yield results
that don't fit in a C long, they raise an error. There are some
other distinctions too. This PEP proposes to do away with most of
the differences in semantics, unifying the two types from the
perspective of the Python user.
Python currently distinguishes between two kinds of integers (ints): regular
or short ints, limited by the size of a C long (typically 32 or 64 bits), and
long ints, which are limited only by available memory. When operations on
short ints yield results that don't fit in a C long, they raise an error.
There are some other distinctions too. This PEP proposes to do away with most
of the differences in semantics, unifying the two types from the perspective
of the Python user.
Rationale
=========
Many programs find a need to deal with larger numbers after the
fact, and changing the algorithms later is bothersome. It can
hinder performance in the normal case, when all arithmetic is
performed using long ints whether or not they are needed.
Many programs find a need to deal with larger numbers after the fact, and
changing the algorithms later is bothersome. It can hinder performance in the
normal case, when all arithmetic is performed using long ints whether or not
they are needed.
Having the machine word size exposed to the language hinders
portability. For examples Python source files and .pyc's are not
portable between 32-bit and 64-bit machines because of this.
Having the machine word size exposed to the language hinders portability. For
examples Python source files and .pyc's are not portable between 32-bit and
64-bit machines because of this.
There is also the general desire to hide unnecessary details from
the Python user when they are irrelevant for most applications.
An example is memory allocation, which is explicit in C but
automatic in Python, giving us the convenience of unlimited sizes
on strings, lists, etc. It makes sense to extend this convenience
to numbers.
There is also the general desire to hide unnecessary details from the Python
user when they are irrelevant for most applications. An example is memory
allocation, which is explicit in C but automatic in Python, giving us the
convenience of unlimited sizes on strings, lists, etc. It makes sense to
extend this convenience to numbers.
It will give new Python programmers (whether they are new to
programming in general or not) one less thing to learn before they
can start using the language.
It will give new Python programmers (whether they are new to programming in
general or not) one less thing to learn before they can start using the
language.
Implementation
==============
Initially, two alternative implementations were proposed (one by
each author):
Initially, two alternative implementations were proposed (one by each author):
1. The PyInt type's slot for a C long will be turned into a
1. The ``PyInt`` type's slot for a C long will be turned into a::
union {
long i;
struct {
unsigned long length;
digit digits[1];
} bignum;
};
union {
long i;
struct {
unsigned long length;
digit digits[1];
} bignum;
};
Only the n-1 lower bits of the long have any meaning; the top
bit is always set. This distinguishes the union. All PyInt
functions will check this bit before deciding which types of
operations to use.
Only the ``n-1`` lower bits of the ``long`` have any meaning; the top bit
is always set. This distinguishes the ``union``. All ``PyInt`` functions
will check this bit before deciding which types of operations to use.
2. The existing short and long int types remain, but operations
return a long int instead of raising OverflowError when a
result cannot be represented as a short int. A new type,
integer, may be introduced that is an abstract base type of
which both the int and long implementation types are
subclassed. This is useful so that programs can check
integer-ness with a single test:
2. The existing short and long int types remain, but operations return
a long int instead of raising ``OverflowError`` when a result cannot be
represented as a short int. A new type, ``integer``, may be introduced
that is an abstract base type of which both the ``int`` and ``long``
implementation types are subclassed. This is useful so that programs can
check integer-ness with a single test::
if isinstance(i, integer): ...
if isinstance(i, integer): ...
After some consideration, the second implementation plan was
selected, since it is far easier to implement, is backwards
compatible at the C API level, and in addition can be implemented
partially as a transitional measure.
After some consideration, the second implementation plan was selected, since
it is far easier to implement, is backwards compatible at the C API level, and
in addition can be implemented partially as a transitional measure.
Incompatibilities
=================
The following operations have (usually subtly) different semantics
for short and for long integers, and one or the other will have to
be changed somehow. This is intended to be an exhaustive list.
If you know of any other operation that differ in outcome
depending on whether a short or a long int with the same value is
passed, please write the second author.
The following operations have (usually subtly) different semantics for short
and for long integers, and one or the other will have to be changed somehow.
This is intended to be an exhaustive list. If you know of any other operation
that differ in outcome depending on whether a short or a long int with the same
value is passed, please write the second author.
- Currently, all arithmetic operators on short ints except <<
raise OverflowError if the result cannot be represented as a
short int. This will be changed to return a long int instead.
The following operators can currently raise OverflowError: x+y,
x-y, x*y, x**y, divmod(x, y), x/y, x%y, and -x. (The last four
can only overflow when the value -sys.maxint-1 is involved.)
- Currently, all arithmetic operators on short ints except ``<<`` raise
``OverflowError`` if the result cannot be represented as a short int. This
will be changed to return a long int instead. The following operators can
currently raise ``OverflowError``: ``x+y``, ``x-y``, ``x*y``, ``x**y``,
``divmod(x, y)``, ``x/y``, ``x%y``, and ``-x``. (The last four can only
overflow when the value ``-sys.maxint-1`` is involved.)
- Currently, x<<n can lose bits for short ints. This will be
changed to return a long int containing all the shifted-out
bits, if returning a short int would lose bits (where changing
sign is considered a special case of losing bits).
- Currently, ``x<<n`` can lose bits for short ints. This will be changed to
return a long int containing all the shifted-out bits, if returning a short
int would lose bits (where changing sign is considered a special case of
losing bits).
- Currently, hex and oct literals for short ints may specify
negative values; for example 0xffffffff == -1 on a 32-bit
machine. This will be changed to equal 0xffffffffL (2**32-1).
- Currently, hex and oct literals for short ints may specify negative values;
for example ``0xffffffff == -1`` on a 32-bit machine. This will be changed
to equal ``0xffffffffL`` (``2**32-1``).
- Currently, the '%u', '%x', '%X' and '%o' string formatting
operators and the hex() and oct() built-in functions behave
differently for negative numbers: negative short ints are
formatted as unsigned C long, while negative long ints are
formatted with a minus sign. This will be changed to use the
long int semantics in all cases (but without the trailing 'L'
that currently distinguishes the output of hex() and oct() for
long ints). Note that this means that '%u' becomes an alias for
'%d'. It will eventually be removed.
- Currently, the ``%u``, ``%x``, ``%X`` and ``%o`` string formatting operators
and the ``hex()`` and ``oct()`` built-in functions behave differently for
negative numbers: negative short ints are formatted as unsigned C long,
while negative long ints are formatted with a minus sign. This will be
changed to use the long int semantics in all cases (but without the trailing
*L* that currently distinguishes the output of ``hex()`` and ``oct()`` for
long ints). Note that this means that ``%u`` becomes an alias for ``%d``.
It will eventually be removed.
- Currently, repr() of a long int returns a string ending in 'L'
while repr() of a short int doesn't. The 'L' will be dropped;
but not before Python 3.0.
- Currently, ``repr()`` of a long int returns a string ending in *L* while
``repr()`` of a short int doesn't. The *L* will be dropped; but not before
Python 3.0.
- Currently, an operation with long operands will never return a
short int. This *may* change, since it allows some
optimization. (No changes have been made in this area yet, and
none are planned.)
- Currently, an operation with long operands will never return a short int.
This *may* change, since it allows some optimization. (No changes have been
made in this area yet, and none are planned.)
- The expression type(x).__name__ depends on whether x is a short
or a long int. Since implementation alternative 2 is chosen,
this difference will remain. (In Python 3.0, we *may* be able
to deploy a trick to hide the difference, because it *is*
annoying to reveal the difference to user code, and more so as
the difference between the two types is less visible.)
- The expression ``type(x).__name__`` depends on whether *x* is a short or a
long int. Since implementation alternative 2 is chosen, this difference
will remain. (In Python 3.0, we *may* be able to deploy a trick to hide the
difference, because it *is* annoying to reveal the difference to user code,
and more so as the difference between the two types is less visible.)
- Long and short ints are handled different by the marshal module,
and by the pickle and cPickle modules. This difference will
remain (at least until Python 3.0).
- Long and short ints are handled different by the ``marshal`` module, and by
the ``pickle`` and ``cPickle`` modules. This difference will remain (at
least until Python 3.0).
- Short ints with small values (typically between -1 and 99
inclusive) are "interned" -- whenever a result has such a value,
an existing short int with the same value is returned. This is
not done for long ints with the same values. This difference
will remain. (Since there is no guarantee of this interning, it
is debatable whether this is a semantic difference -- but code
may exist that uses 'is' for comparisons of short ints and
happens to work because of this interning. Such code may fail
if used with long ints.)
- Short ints with small values (typically between -1 and 99 inclusive) are
*interned* -- whenever a result has such a value, an existing short int with
the same value is returned. This is not done for long ints with the same
values. This difference will remain. (Since there is no guarantee of this
interning, it is debatable whether this is a semantic difference -- but code
may exist that uses ``is`` for comparisons of short ints and happens to work
because of this interning. Such code may fail if used with long ints.)
Literals
========
A trailing 'L' at the end of an integer literal will stop having
any meaning, and will be eventually become illegal. The compiler
will choose the appropriate type solely based on the value.
(Until Python 3.0, it will force the literal to be a long; but
literals without a trailing 'L' may also be long, if they are not
representable as short ints.)
A trailing *L* at the end of an integer literal will stop having any
meaning, and will be eventually become illegal. The compiler will choose the
appropriate type solely based on the value. (Until Python 3.0, it will force
the literal to be a long; but literals without a trailing *L* may also be
long, if they are not representable as short ints.)
Built-in Functions
==================
The function int() will return a short or a long int depending on
the argument value. In Python 3.0, the function long() will call
the function int(); before then, it will continue to force the
result to be a long int, but otherwise work the same way as int().
The built-in name 'long' will remain in the language to represent
the long implementation type (unless it is completely eradicated
in Python 3.0), but using the int() function is still recommended,
since it will automatically return a long when needed.
The function ``int()`` will return a short or a long int depending on the
argument value. In Python 3.0, the function ``long()`` will call the function
``int()``; before then, it will continue to force the result to be a long int,
but otherwise work the same way as ``int()``. The built-in name ``long`` will
remain in the language to represent the long implementation type (unless it is
completely eradicated in Python 3.0), but using the ``int()`` function is
still recommended, since it will automatically return a long when needed.
C API
=====
The C API remains unchanged; C code will still need to be aware of
the difference between short and long ints. (The Python 3.0 C API
will probably be completely incompatible.)
The C API remains unchanged; C code will still need to be aware of the
difference between short and long ints. (The Python 3.0 C API will probably
be completely incompatible.)
The PyArg_Parse*() APIs already accept long ints, as long as they
are within the range representable by C ints or longs, so that
functions taking C int or long argument won't have to worry about
dealing with Python longs.
The ``PyArg_Parse*()`` APIs already accept long ints, as long as they are
within the range representable by C ints or longs, so that functions taking C
int or long argument won't have to worry about dealing with Python longs.
Transition
==========
There are three major phases to the transition:
There are three major phases to the transition:
A. Short int operations that currently raise OverflowError return
a long int value instead. This is the only change in this
phase. Literals will still distinguish between short and long
ints. The other semantic differences listed above (including
the behavior of <<) will remain. Because this phase only
changes situations that currently raise OverflowError, it is
assumed that this won't break existing code. (Code that
depends on this exception would have to be too convoluted to be
concerned about it.) For those concerned about extreme
backwards compatibility, a command line option (or a call to
the warnings module) will allow a warning or an error to be
issued at this point, but this is off by default.
1. Short int operations that currently raise ``OverflowError`` return a long
int value instead. This is the only change in this phase. Literals will
still distinguish between short and long ints. The other semantic
differences listed above (including the behavior of ``<<``) will remain.
Because this phase only changes situations that currently raise
``OverflowError``, it is assumed that this won't break existing code.
(Code that depends on this exception would have to be too convoluted to be
concerned about it.) For those concerned about extreme backwards
compatibility, a command line option (or a call to the warnings module)
will allow a warning or an error to be issued at this point, but this is
off by default.
B. The remaining semantic differences are addressed. In all cases
the long int semantics will prevail. Since this will introduce
backwards incompatibilities which will break some old code,
this phase may require a future statement and/or warnings, and
a prolonged transition phase. The trailing 'L' will continue
to be used for longs as input and by repr().
2. The remaining semantic differences are addressed. In all cases the long
int semantics will prevail. Since this will introduce backwards
incompatibilities which will break some old code, this phase may require a
future statement and/or warnings, and a prolonged transition phase. The
trailing *L* will continue to be used for longs as input and by
``repr()``.
C. The trailing 'L' is dropped from repr(), and made illegal on
input. (If possible, the 'long' type completely disappears.)
The trailing 'L' is also dropped from hex() and oct().
A. Warnings are enabled about operations that will change their numeric
outcome in stage 2B, in particular ``hex()`` and ``oct()``, ``%u``,
``%x``, ``%X`` and ``%o``, ``hex`` and ``oct`` literals in the
(inclusive) range ``[sys.maxint+1, sys.maxint*2+1]``, and left shifts
losing bits.
B. The new semantic for these operations are implemented. Operations that
give different results than before will *not* issue a warning.
Phase A will be implemented in Python 2.2.
3. The trailing *L* is dropped from ``repr()``, and made illegal on input.
(If possible, the ``long`` type completely disappears.) The trailing *L*
is also dropped from ``hex()`` and ``oct()``.
Phase B will be implemented gradually in Python 2.3 and Python
2.4. Envisioned stages of phase B:
Phase 1 will be implemented in Python 2.2.
B0. Warnings are enabled about operations that will change their
numeric outcome in stage B1, in particular hex() and oct(),
'%u', '%x', '%X' and '%o', hex and oct literals in the
(inclusive) range [sys.maxint+1, sys.maxint*2+1], and left
shifts losing bits.
Phase 2 will be implemented gradually, with 2A in Python 2.3 and 2B in
Python 2.4.
B1. The new semantic for these operations are implemented.
Operations that give different results than before will *not*
issue a warning.
We propose the following timeline:
B0. Python 2.3.
B1. Python 2.4.
Phase C will be implemented in Python 3.0 (at least two years
after Python 2.4 is released).
Phase 3 will be implemented in Python 3.0 (at least two years after Python 2.4
is released).
OverflowWarning
===============
Here are the rules that guide warnings generated in situations
that currently raise OverflowError. This applies to transition
phase A. Historical note: despite that phase A was completed in
Python 2.2, and phase B0 in Python 2.3, nobody noticed that
OverflowWarning was still generated in Python 2.3. It was finally
disabled in Python 2.4. The Python builtin OverflowWarning, and
the corresponding C API PyExc_OverflowWarning, are no longer
generated or used in Python 2.4, but will remain for the (unlikely)
case of user code until Python 2.5.
Here are the rules that guide warnings generated in situations that currently
raise ``OverflowError``. This applies to transition phase 1. Historical
note: despite that phase 1 was completed in Python 2.2, and phase 2A in Python
2.3, nobody noticed that OverflowWarning was still generated in Python 2.3.
It was finally disabled in Python 2.4. The Python builtin
``OverflowWarning``, and the corresponding C API ``PyExc_OverflowWarning``,
are no longer generated or used in Python 2.4, but will remain for the
(unlikely) case of user code until Python 2.5.
- A new warning category is introduced, OverflowWarning. This is
a built-in name.
- A new warning category is introduced, ``OverflowWarning``. This is a
built-in name.
- If an int result overflows, an OverflowWarning warning is
issued, with a message argument indicating the operation,
e.g. "integer addition". This may or may not cause a warning
message to be displayed on sys.stderr, or may cause an exception
to be raised, all under control of the -W command line and the
warnings module.
- If an int result overflows, an ``OverflowWarning`` warning is issued, with a
message argument indicating the operation, e.g. "integer addition". This
may or may not cause a warning message to be displayed on ``sys.stderr``, or
may cause an exception to be raised, all under control of the ``-W`` command
line and the warnings module.
- The OverflowWarning warning is ignored by default.
- The ``OverflowWarning`` warning is ignored by default.
- The OverflowWarning warning can be controlled like all warnings,
via the -W command line option or via the
warnings.filterwarnings() call. For example:
- The ``OverflowWarning`` warning can be controlled like all warnings, via the
``-W`` command line option or via the ``warnings.filterwarnings()`` call.
For example::
python -Wdefault::OverflowWarning
python -Wdefault::OverflowWarning
cause the OverflowWarning to be displayed the first time it
occurs at a particular source line, and
cause the ``OverflowWarning`` to be displayed the first time it occurs at a
particular source line, and::
python -Werror::OverflowWarning
python -Werror::OverflowWarning
cause the OverflowWarning to be turned into an exception
whenever it happens. The following code enables the warning
from inside the program:
cause the ``OverflowWarning`` to be turned into an exception whenever it
happens. The following code enables the warning from inside the program::
import warnings
warnings.filterwarnings("default", "", OverflowWarning)
import warnings
warnings.filterwarnings("default", "", OverflowWarning)
See the python man page for the -W option and the warnings
module documentation for filterwarnings().
See the python ``man`` page for the ``-W`` option and the ``warnings``
module documentation for ``filterwarnings()``.
- If the OverflowWarning warning is turned into an error,
OverflowError is substituted. This is needed for backwards
compatibility.
- If the ``OverflowWarning`` warning is turned into an error,
``OverflowError`` is substituted. This is needed for backwards
compatibility.
- Unless the warning is turned into an exceptions, the result of
the operation (e.g., x+y) is recomputed after converting the
arguments to long ints.
- Unless the warning is turned into an exceptions, the result of the operation
(e.g., ``x+y``) is recomputed after converting the arguments to long ints.
Example
=======
If you pass a long int to a C function or built-in operation that
takes an integer, it will be treated the same as a short int as
long as the value fits (by virtue of how PyArg_ParseTuple() is
implemented). If the long value doesn't fit, it will still raise
an OverflowError. For example:
If you pass a long int to a C function or built-in operation that takes an
integer, it will be treated the same as a short int as long as the value fits
(by virtue of how ``PyArg_ParseTuple()`` is implemented). If the long value
doesn't fit, it will still raise an ``OverflowError``. For example::
def fact(n):
if n <= 1:
return 1
return n*fact(n-1)
def fact(n):
if n <= 1:
return 1
return n*fact(n-1)
A = "ABCDEFGHIJKLMNOPQ"
n = input("Gimme an int: ")
print A[fact(n)%17]
A = "ABCDEFGHIJKLMNOPQ"
n = input("Gimme an int: ")
print A[fact(n)%17]
For n >= 13, this currently raises OverflowError (unless the user
enters a trailing 'L' as part of their input), even though the
calculated index would always be in range(17). With the new
approach this code will do the right thing: the index will be
calculated as a long int, but its value will be in range.
For ``n >= 13``, this currently raises ``OverflowError`` (unless the user
enters a trailing *L* as part of their input), even though the calculated
index would always be in ``range(17)``. With the new approach this code will
do the right thing: the index will be calculated as a long int, but its value
will be in range.
Resolved Issues
===============
These issues, previously open, have been resolved.
These issues, previously open, have been resolved.
- hex() and oct() applied to longs will continue to produce a
trailing 'L' until Python 3000. The original text above wasn't
clear about this, but since it didn't happen in Python 2.4 it
was thought better to leave it alone. BDFL pronouncement here:
- ``hex()`` and ``oct()`` applied to longs will continue to produce a trailing
*L* until Python 3000. The original text above wasn't clear about this,
but since it didn't happen in Python 2.4 it was thought better to leave it
alone. BDFL pronouncement here:
http://mail.python.org/pipermail/python-dev/2006-June/065918.html
http://mail.python.org/pipermail/python-dev/2006-June/065918.html
- What to do about sys.maxint? Leave it in, since it is still
relevant whenever the distinction between short and long ints is
still relevant (e.g. when inspecting the type of a value).
- What to do about ``sys.maxint``? Leave it in, since it is still relevant
whenever the distinction between short and long ints is still relevant (e.g.
when inspecting the type of a value).
- Should we remove '%u' completely? Remove it.
- Should we remove ``%u`` completely? Remove it.
- Should we warn about << not truncating integers? Yes.
- Should we warn about ``<<`` not truncating integers? Yes.
- Should the overflow warning be on a portable maximum size? No.
- Should the overflow warning be on a portable maximum size? No.
Implementation
==============
The implementation work for the Python 2.x line is completed;
phase A was released with Python 2.2, phase B0 with Python 2.3,
and phase B1 will be released with Python 2.4 (and is already in
CVS).
The implementation work for the Python 2.x line is completed; phase 1 was
released with Python 2.2, phase 2A with Python 2.3, and phase 2B will be
released with Python 2.4 (and is already in CVS).
Copyright
=========
This document has been placed in the public domain.
This document has been placed in the public domain.
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