Updated PEP 335, posted 25-Oct-2011.
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Guido van Rossum
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pep-0335.txt
388
pep-0335.txt
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@ -2,13 +2,13 @@ PEP: 335
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Title: Overloadable Boolean Operators
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Version: $Revision$
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Last-Modified: $Date$
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Author: Gregory Ewing <greg.ewing@canterbury.ac.nz>
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Author: Gregory Ewing <greg@cosc.canterbury.ac.nz>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 29-Aug-2004
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Python-Version: 3.3
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Post-History: 05-Sep-2004, 30-Sep-2011
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Post-History: 05-Sep-2004, 30-Sep-2011, 25-Oct-2011
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Abstract
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@ -66,13 +66,23 @@ inconvenient. Examples include:
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A workaround often suggested is to use the bitwise operators '&', '|'
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and '~' in place of 'and', 'or' and 'not', but this has some
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drawbacks. The precedence of these is different in relation to the
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other operators, and they may already be in use for other purposes (as
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in example 1). There is also the aesthetic consideration of forcing
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users to use something other than the most obvious syntax for what
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they are trying to express. This would be particularly acute in the
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case of example 3, considering that boolean operations are a staple of
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SQL queries.
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drawbacks:
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* The precedence of these is different in relation to the other operators,
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and they may already be in use for other purposes (as in example 1).
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* It is aesthetically displeasing to force users to use something other
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than the most obvious syntax for what they are trying to express. This
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would be particularly acute in the case of example 3, considering that
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boolean operations are a staple of SQL queries.
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* Bitwise operators do not provide a solution to the problem of
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chained comparisons such as 'a < b < c' which involve an implicit
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'and' operation. Such expressions currently cannot be used at all
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on data types such as NumPy arrays where the result of a comparison
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cannot be treated as having normal boolean semantics; they must be
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expanded into something like (a < b) & (b < c), losing a considerable
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amount of clarity.
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Rationale
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@ -208,7 +218,7 @@ new operations::
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PyObject *PyObject_LogicalAnd1(PyObject *);
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PyObject *PyObject_LogicalOr1(PyObject *);
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PyObject *PyObject_LogicalAnd2(PyObject *, PyObject *);
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PyObject *PyObject_LogicalOr2(PyObject *, PyObject *);
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Alternatives and Optimisations
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@ -242,40 +252,40 @@ it currently is. For example, in Python 2.7,
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::
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if a and b:
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statement1
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else:
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statement2
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if a and b:
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statement1
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else:
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statement2
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generates
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::
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LOAD_GLOBAL a
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POP_JUMP_IF_FALSE false_branch
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LOAD_GLOBAL b
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POP_JUMP_IF_FALSE false_branch
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<code for statement1>
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JUMP_FORWARD end_branch
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false_branch:
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<code for statement2>
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end_branch:
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LOAD_GLOBAL a
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POP_JUMP_IF_FALSE false_branch
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LOAD_GLOBAL b
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POP_JUMP_IF_FALSE false_branch
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<code for statement1>
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JUMP_FORWARD end_branch
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false_branch:
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<code for statement2>
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end_branch:
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Under this proposal as described so far, it would become something like
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::
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LOAD_GLOBAL a
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LOGICAL_AND_1 test
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LOAD_GLOBAL b
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LOGICAL_AND_2
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test:
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POP_JUMP_IF_FALSE false_branch
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<code for statement1>
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JUMP_FORWARD end_branch
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false_branch:
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<code for statement2>
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end_branch:
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LOAD_GLOBAL a
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LOGICAL_AND_1 test
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LOAD_GLOBAL b
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LOGICAL_AND_2
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test:
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POP_JUMP_IF_FALSE false_branch
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<code for statement1>
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JUMP_FORWARD end_branch
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false_branch:
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<code for statement2>
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end_branch:
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This involves executing one extra bytecode in the short-circuiting
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case and two extra bytecodes in the non-short-circuiting case.
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@ -286,16 +296,16 @@ reduced to the same number of bytecodes as the original:
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::
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LOAD_GLOBAL a
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AND1_JUMP true_branch, false_branch
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LOAD_GLOBAL b
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AND2_JUMP_IF_FALSE false_branch
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true_branch:
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<code for statement1>
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JUMP_FORWARD end_branch
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false_branch:
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<code for statement2>
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end_branch:
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LOAD_GLOBAL a
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AND1_JUMP true_branch, false_branch
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LOAD_GLOBAL b
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AND2_JUMP_IF_FALSE false_branch
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true_branch:
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<code for statement1>
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JUMP_FORWARD end_branch
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false_branch:
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<code for statement2>
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end_branch:
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Here, AND1_JUMP performs phase 1 processing as above,
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and then examines the result. If there is a result, it is popped
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@ -350,58 +360,58 @@ Example 1: NumPy Arrays
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::
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#-----------------------------------------------------------------
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#
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# This example creates a subclass of numpy array to which
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# 'and', 'or' and 'not' can be applied, producing an array
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# of booleans.
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#
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#-----------------------------------------------------------------
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from numpy import array, ndarray
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class BArray(ndarray):
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def __str__(self):
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return "barray(%s)" % ndarray.__str__(self)
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def __and2__(self, other):
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return self & other
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def __or2__(self, other):
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return self & other
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def __not__(self):
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return self == 0
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def barray(*args, **kwds):
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return array(*args, **kwds).view(type=BArray)
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a0 = barray([0, 1, 2, 4])
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a1 = barray([1, 2, 3, 4])
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a2 = barray([5, 6, 3, 4])
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a3 = barray([5, 1, 2, 4])
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print("a0:", a0)
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print("a1:", a1)
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print("a2:", a2)
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print("a3:", a3)
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print("not a0:", not a0)
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print("a0 == a1 and a2 == a3:", a0 == a1 and a2 == a3)
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print("a0 == a1 or a2 == a3:", a0 == a1 or a2 == a3)
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#-----------------------------------------------------------------
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#
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# This example creates a subclass of numpy array to which
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# 'and', 'or' and 'not' can be applied, producing an array
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# of booleans.
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#
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#-----------------------------------------------------------------
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from numpy import array, ndarray
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class BArray(ndarray):
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def __str__(self):
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return "barray(%s)" % ndarray.__str__(self)
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def __and2__(self, other):
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return (self & other)
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def __or2__(self, other):
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return (self & other)
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def __not__(self):
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return (self == 0)
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def barray(*args, **kwds):
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return array(*args, **kwds).view(type = BArray)
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a0 = barray([0, 1, 2, 4])
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a1 = barray([1, 2, 3, 4])
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a2 = barray([5, 6, 3, 4])
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a3 = barray([5, 1, 2, 4])
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print "a0:", a0
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print "a1:", a1
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print "a2:", a2
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print "a3:", a3
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print "not a0:", not a0
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print "a0 == a1 and a2 == a3:", a0 == a1 and a2 == a3
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print "a0 == a1 or a2 == a3:", a0 == a1 or a2 == a3
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Example 1 Output
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----------------
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::
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a0: barray([0 1 2 4])
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a1: barray([1 2 3 4])
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a2: barray([5 6 3 4])
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a3: barray([5 1 2 4])
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not a0: barray([ True False False False])
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a0 == a1 and a2 == a3: barray([False False False True])
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a0 == a1 or a2 == a3: barray([False False False True])
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a0: barray([0 1 2 4])
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a1: barray([1 2 3 4])
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a2: barray([5 6 3 4])
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a3: barray([5 1 2 4])
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not a0: barray([ True False False False])
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a0 == a1 and a2 == a3: barray([False False False True])
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a0 == a1 or a2 == a3: barray([False False False True])
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Example 2: Database Queries
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@ -409,112 +419,110 @@ Example 2: Database Queries
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::
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#-----------------------------------------------------------------
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#
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# This example demonstrates the creation of a DSL for database
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# queries allowing 'and' and 'or' operators to be used to
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# formulate the query.
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#
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#-----------------------------------------------------------------
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class SQLNode:
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def __and2__(self, other):
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return SQLBinop("and", self, other)
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def __rand2__(self, other):
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return SQLBinop("and", other, self)
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def __eq__(self, other):
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return SQLBinop("=", self, other)
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class Table(SQLNode):
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def __init__(self, name):
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self.__tablename__ = name
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def __getattr__(self, name):
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return SQLAttr(self, name)
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def __sql__(self):
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return self.__tablename__
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class SQLBinop(SQLNode):
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def __init__(self, op, opnd1, opnd2):
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self.op = op.upper()
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self.opnd1 = opnd1
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self.opnd2 = opnd2
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def __sql__(self):
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return "(%s %s %s)" % (sql(self.opnd1), self.op, sql(self.opnd2))
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class SQLAttr(SQLNode):
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def __init__(self, table, name):
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self.table = table
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self.name = name
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def __sql__(self):
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return "%s.%s" % (sql(self.table), self.name)
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class SQLSelect(SQLNode):
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def __init__(self, targets):
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self.targets = targets
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self.where_clause = None
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def where(self, expr):
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self.where_clause = expr
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return self
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def __sql__(self):
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result = "SELECT %s" % ", ".join(sql(target) for target in self.targets)
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if self.where_clause:
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result = "%s WHERE %s" % (result, sql(self.where_clause))
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return result
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def sql(expr):
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if isinstance(expr, SQLNode):
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return expr.__sql__()
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elif isinstance(expr, str):
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return "'%s'" % expr.replace("'", "''")
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else:
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return str(expr)
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def select(*targets):
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return SQLSelect(targets)
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#--------------------------------------------------------------------------------
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::
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dishes = Table("dishes")
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customers = Table("customers")
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orders = Table("orders")
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query = select(customers.name, dishes.price, orders.amount).where(
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customers.cust_id == orders.cust_id and orders.dish_id == dishes.dish_id
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and dishes.name == "Spam, Eggs, Sausages and Spam")
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print(repr(query))
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print(sql(query))
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#-----------------------------------------------------------------
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#
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# This example demonstrates the creation of a DSL for database
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# queries allowing 'and' and 'or' operators to be used to
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# formulate the query.
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#
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#-----------------------------------------------------------------
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class SQLNode(object):
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def __and2__(self, other):
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return SQLBinop("and", self, other)
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def __rand2__(self, other):
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return SQLBinop("and", other, self)
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def __eq__(self, other):
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return SQLBinop("=", self, other)
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class Table(SQLNode):
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def __init__(self, name):
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self.__tablename__ = name
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def __getattr__(self, name):
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return SQLAttr(self, name)
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def __sql__(self):
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return self.__tablename__
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class SQLBinop(SQLNode):
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def __init__(self, op, opnd1, opnd2):
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self.op = op.upper()
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self.opnd1 = opnd1
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self.opnd2 = opnd2
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def __sql__(self):
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return "(%s %s %s)" % (sql(self.opnd1), self.op, sql(self.opnd2))
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class SQLAttr(SQLNode):
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def __init__(self, table, name):
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self.table = table
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self.name = name
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def __sql__(self):
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return "%s.%s" % (sql(self.table), self.name)
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class SQLSelect(SQLNode):
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def __init__(self, targets):
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self.targets = targets
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self.where_clause = None
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def where(self, expr):
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self.where_clause = expr
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return self
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def __sql__(self):
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result = "SELECT %s" % ", ".join([sql(target) for target in self.targets])
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if self.where_clause:
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result = "%s WHERE %s" % (result, sql(self.where_clause))
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return result
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def sql(expr):
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if isinstance(expr, SQLNode):
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return expr.__sql__()
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elif isinstance(expr, str):
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return "'%s'" % expr.replace("'", "''")
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else:
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return str(expr)
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def select(*targets):
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return SQLSelect(targets)
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#-----------------------------------------------------------------
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dishes = Table("dishes")
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customers = Table("customers")
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orders = Table("orders")
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query = select(customers.name, dishes.price, orders.amount).where(
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customers.cust_id == orders.cust_id and orders.dish_id == dishes.dish_id
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and dishes.name == "Spam, Eggs, Sausages and Spam")
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print repr(query)
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print sql(query)
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Example 2 Output
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----------------
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::
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<__main__.SQLSelect object at 0x1cc830>
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SELECT customers.name, dishes.price, orders.amount WHERE
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(((customers.cust_id = orders.cust_id) AND (orders.dish_id =
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dishes.dish_id)) AND (dishes.name = 'Spam, Eggs, Sausages and Spam'))
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<__main__.SQLSelect object at 0x1cc830>
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SELECT customers.name, dishes.price, orders.amount WHERE
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(((customers.cust_id = orders.cust_id) AND (orders.dish_id =
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dishes.dish_id)) AND (dishes.name = 'Spam, Eggs, Sausages and Spam'))
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Copyright
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