[Python-checkins] CVS: python/nondist/peps pep-0208.txt,1.6,1.7
Guido van Rossum
python-dev@python.org
Mon, 11 Dec 2000 18:00:40 -0800
Update of /cvsroot/python/python/nondist/peps
In directory slayer.i.sourceforge.net:/tmp/cvs-serv17440
Modified Files:
pep-0208.txt
Log Message:
Some stupid mechanical changes, such as remove trailing whitespace,
double spaces between sentences. Also fix a few typos, add an XXX
note, and change the URL for the implementation to be the patch
detail page instead of the download page.
I hope Neil doesn't mind.
Index: pep-0208.txt
===================================================================
RCS file: /cvsroot/python/python/nondist/peps/pep-0208.txt,v
retrieving revision 1.6
retrieving revision 1.7
diff -C2 -r1.6 -r1.7
*** pep-0208.txt 2000/12/06 21:22:52 1.6
--- pep-0208.txt 2000/12/12 02:00:37 1.7
***************
*** 21,58 ****
set the type flag are handled in a backwards compatible manner.
Allowing operations handle different types is often simpler, more
! flexible, and faster then having the interpreter do coercion.
! Rational
When implementing numeric or other related operations, it is often
desirable to provide not only operations between operands of one type
only, e.g. integer + integer, but to generalize the idea behind the
! operation to other type combinations as well, e.g. integer + float.
A common approach to this mixed type situation is to provide a method
of "lifting" the operands to a common type (coercion) and then use
! that type's operand method as execution mechanism. Yet, this strategy
! has a few drawbacks:
* the "lifting" process creates at least one new (temporary)
! operand object,
* since the coercion method is not being told about the operation
that is to follow, it is not possible to implement operation
! specific coercion of types,
* there is no elegant way to solve situations were a common type
! is not at hand, and
* the coercion method will always have to be called prior to the
! operation's method itself.
A fix for this situation is obviously needed, since these drawbacks
make implementations of types needing these features very cumbersome,
! if not impossible. As an example, have a look at the DateTime and
DateTimeDelta[1] types, the first being absolute, the second
relative. You can always add a relative value to an absolute one,
! giving a new absolute value. Yet, there is no common type which the
existing coercion mechanism could use to implement that operation.
--- 21,58 ----
set the type flag are handled in a backwards compatible manner.
Allowing operations handle different types is often simpler, more
! flexible, and faster than having the interpreter do coercion.
! Rationale
When implementing numeric or other related operations, it is often
desirable to provide not only operations between operands of one type
only, e.g. integer + integer, but to generalize the idea behind the
! operation to other type combinations as well, e.g. integer + float.
A common approach to this mixed type situation is to provide a method
of "lifting" the operands to a common type (coercion) and then use
! that type's operand method as execution mechanism. Yet, this strategy
! has a few drawbacks:
* the "lifting" process creates at least one new (temporary)
! operand object,
* since the coercion method is not being told about the operation
that is to follow, it is not possible to implement operation
! specific coercion of types,
* there is no elegant way to solve situations were a common type
! is not at hand, and
* the coercion method will always have to be called prior to the
! operation's method itself.
A fix for this situation is obviously needed, since these drawbacks
make implementations of types needing these features very cumbersome,
! if not impossible. As an example, have a look at the DateTime and
DateTimeDelta[1] types, the first being absolute, the second
relative. You can always add a relative value to an absolute one,
! giving a new absolute value. Yet, there is no common type which the
existing coercion mechanism could use to implement that operation.
***************
*** 67,73 ****
Instead of using a central coercion method, the process of handling
! different operand types is simply left to the operation. If the
operation finds that it cannot handle the given operand type
! combination, it may return a special singleton as indicator.
Note that "numbers" (anything that implements the number protocol, or
--- 67,73 ----
Instead of using a central coercion method, the process of handling
! different operand types is simply left to the operation. If the
operation finds that it cannot handle the given operand type
! combination, it may return a special singleton as indicator.
Note that "numbers" (anything that implements the number protocol, or
***************
*** 78,84 ****
careful to make numbers that don't know anything about the new
strategy (old style numbers) work just as well as those that expect
! the new scheme (new style numbers). Furthermore, binary compatibility
is a must, meaning that the interpreter may only access and use new
! style operations if the number indicates the availability of these.
A new style number is considered by the interpreter as such if and
--- 78,84 ----
careful to make numbers that don't know anything about the new
strategy (old style numbers) work just as well as those that expect
! the new scheme (new style numbers). Furthermore, binary compatibility
is a must, meaning that the interpreter may only access and use new
! style operations if the number indicates the availability of these.
A new style number is considered by the interpreter as such if and
***************
*** 86,118 ****
difference between an old style number and a new style one is that the
numeric slot functions can no longer assume to be passed arguments of
! identical type. New style slots must check all arguments for proper
! type and implement the necessary conversions themselves. This may seem
to cause more work on the behalf of the type implementor, but is in
fact no more difficult than writing the same kind of routines for an
! old style coercion slot.
If a new style slot finds that it cannot handle the passed argument
type combination, it may return a new reference of the special
! singleton Py_NotImplemented to the caller. This will cause the caller
to try the other operands operation slots until it finds a slot that
! does implement the operation for the specific type combination. If
! none of the possible slots succeed, it raises a TypeError.
To make the implementation easy to understand (the whole topic is
esoteric enough), a new layer in the handling of numeric operations is
! introduced. This layer takes care of all the different cases that need
to be taken into account when dealing with all the possible
! combinations of old and new style numbers. It is implemented by the
! two functions _PyNumber_BinaryOperation() and
! _PyNumber_TernaryOperation(), which both are internal functions that
! only the functions in Objects/abstract.c have access to. The numeric
! API (PyNumber_*) is easy to adapt to this new layer.
As a side-effect all numeric slots can be NULL-checked (this has to be
done anyway, so the added feature comes at no extra cost).
!
The scheme used by the layer to execute a binary operation is as
! follows:
v | w | Action taken
--- 86,118 ----
difference between an old style number and a new style one is that the
numeric slot functions can no longer assume to be passed arguments of
! identical type. New style slots must check all arguments for proper
! type and implement the necessary conversions themselves. This may seem
to cause more work on the behalf of the type implementor, but is in
fact no more difficult than writing the same kind of routines for an
! old style coercion slot.
If a new style slot finds that it cannot handle the passed argument
type combination, it may return a new reference of the special
! singleton Py_NotImplemented to the caller. This will cause the caller
to try the other operands operation slots until it finds a slot that
! does implement the operation for the specific type combination. If
! none of the possible slots succeed, it raises a TypeError.
To make the implementation easy to understand (the whole topic is
esoteric enough), a new layer in the handling of numeric operations is
! introduced. This layer takes care of all the different cases that need
to be taken into account when dealing with all the possible
! combinations of old and new style numbers. It is implemented by the
! two static functions binary_op() and ternary_op(), which are both
! internal functions that only the functions in Objects/abstract.c
! have access to. The numeric API (PyNumber_*) is easy to adapt to
! this new layer.
As a side-effect all numeric slots can be NULL-checked (this has to be
done anyway, so the added feature comes at no extra cost).
!
The scheme used by the layer to execute a binary operation is as
! follows:
v | w | Action taken
***************
*** 125,137 ****
The indicated action sequence is executed from left to right until
either the operation succeeds and a valid result (!=
! Py_NotImplemented) is returned or an exception is raised. Exceptions
! are returned to the calling function as-is. If a slot returns
! Py_NotImplemented, the next item in the sequence is executed.
Note that coerce(v,w) will use the old style nb_coerce slot methods
! via a call to PyNumber_Coerce().
- Ternary operations have a few more cases to handle:
-
v | w | z | Action taken
----+-----+-----+------------------------------------
--- 125,137 ----
The indicated action sequence is executed from left to right until
either the operation succeeds and a valid result (!=
! Py_NotImplemented) is returned or an exception is raised. Exceptions
! are returned to the calling function as-is. If a slot returns
! Py_NotImplemented, the next item in the sequence is executed.
Note that coerce(v,w) will use the old style nb_coerce slot methods
! via a call to PyNumber_Coerce().
!
! Ternary operations have a few more cases to handle:
v | w | z | Action taken
----+-----+-----+------------------------------------
***************
*** 144,198 ****
old | new | old | w.op(v,w,z), coerce(v,w,z), v.op(v,w,z)
old | old | old | coerce(v,w,z), v.op(v,w,z)
! The same notes as above, except that coerce(v,w,z) actually does:
- if z != Py_None:
- coerce(v,w), coerce(v,z), coerce(w,z)
- else:
- # treat z as absent variable
- coerce(v,w)
-
The current implementation uses this scheme already (there's only one
! ternary slot: nb_pow(a,b,c)).
Note that the numeric protocol is also used for some other related
tasks, e.g. sequence concatenation. These can also benefit from the
new mechanism by implementing right-hand operations for type
! combinations that would otherwise fail to work. As an example, take
! string concatenation: currently you can only do string + string. With
the new mechanism, a new string-like type could implement new_type +
string and string + new_type, even though strings don't know anything
! about new_type.
Since comparisons also rely on coercion (every time you compare an
integer to a float, the integer is first converted to float and then
! compared...), a new slot to handle numeric comparisons is needed:
! PyObject *nb_cmp(PyObject *v, PyObject *w)
This slot should compare the two objects and return an integer object
! stating the result. Currently, this result integer may only be -1, 0,
1. If the slot cannot handle the type combination, it may return a
! reference to Py_NotImplemented. Note that this slot is still in flux
! since it should take into account rich comparisons (ie. PEP 207).
! Numeric comparisons are handled by a new numeric protocol API:
! PyObject *PyNumber_Compare(PyObject *v, PyObject *w)
This function compare the two objects as "numbers" and return an
! integer object stating the result. Currently, this result integer may
! only be -1, 0, 1. In case the operation cannot be handled by the given
! objects, a TypeError is raised.
The PyObject_Compare() API needs to adjusted accordingly to make use
! of this new API.
Other changes include adapting some of the built-in functions (e.g.
! cmp()) to use this API as well. Also, PyNumber_CoerceEx() will need to
! check for new style numbers before calling the nb_coerce slot. New
style numbers don't provide a coercion slot and thus cannot be
! explicitly coerced.
--- 144,199 ----
old | new | old | w.op(v,w,z), coerce(v,w,z), v.op(v,w,z)
old | old | old | coerce(v,w,z), v.op(v,w,z)
+
+ The same notes as above, except that coerce(v,w,z) actually does:
! if z != Py_None:
! coerce(v,w), coerce(v,z), coerce(w,z)
! else:
! # treat z as absent variable
! coerce(v,w)
The current implementation uses this scheme already (there's only one
! ternary slot: nb_pow(a,b,c)).
Note that the numeric protocol is also used for some other related
tasks, e.g. sequence concatenation. These can also benefit from the
new mechanism by implementing right-hand operations for type
! combinations that would otherwise fail to work. As an example, take
! string concatenation: currently you can only do string + string. With
the new mechanism, a new string-like type could implement new_type +
string and string + new_type, even though strings don't know anything
! about new_type.
Since comparisons also rely on coercion (every time you compare an
integer to a float, the integer is first converted to float and then
! compared...), a new slot to handle numeric comparisons is needed:
! PyObject *nb_cmp(PyObject *v, PyObject *w)
This slot should compare the two objects and return an integer object
! stating the result. Currently, this result integer may only be -1, 0,
1. If the slot cannot handle the type combination, it may return a
! reference to Py_NotImplemented. [XXX Note that this slot is still
! in flux since it should take into account rich comparisons
! (i.e. PEP 207).]
! Numeric comparisons are handled by a new numeric protocol API:
! PyObject *PyNumber_Compare(PyObject *v, PyObject *w)
This function compare the two objects as "numbers" and return an
! integer object stating the result. Currently, this result integer may
! only be -1, 0, 1. In case the operation cannot be handled by the given
! objects, a TypeError is raised.
The PyObject_Compare() API needs to adjusted accordingly to make use
! of this new API.
Other changes include adapting some of the built-in functions (e.g.
! cmp()) to use this API as well. Also, PyNumber_CoerceEx() will need to
! check for new style numbers before calling the nb_coerce slot. New
style numbers don't provide a coercion slot and thus cannot be
! explicitly coerced.
***************
*** 217,221 ****
[1] http://www.lemburg.com/files/python/mxDateTime.html
! [2] http://sourceforge.net/patch/download.php?id=102652
[3] http://www.lemburg.com/files/python/CoercionProposal.html
--- 218,222 ----
[1] http://www.lemburg.com/files/python/mxDateTime.html
! [2] http://sourceforge.net/patch/?func=detailpatch&patch_id=102652&group_id=5470
[3] http://www.lemburg.com/files/python/CoercionProposal.html