Work at SourceForge, help us to make it a better place! We have an immediate need for a Support Technician in our San Francisco or Denver office.

Close

[c203b8]: doc / source / krb_syntax / python_premise.txt Maximize Restore History

Download this file

python_premise.txt    142 lines (119 with data), 5.2 kB

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
.. $Id$
..
.. Copyright Š 2007 Bruce Frederiksen
..
.. Permission is hereby granted, free of charge, to any person obtaining a copy
.. of this software and associated documentation files (the "Software"), to deal
.. in the Software without restriction, including without limitation the rights
.. to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
.. copies of the Software, and to permit persons to whom the Software is
.. furnished to do so, subject to the following conditions:
..
.. The above copyright notice and this permission notice shall be included in
.. all copies or substantial portions of the Software.
..
.. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
.. IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
.. FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
.. AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
.. LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
.. OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
.. THE SOFTWARE.
restindex
crumb: Python Premise
page-description:
The syntax of a python premise.
/description
format: rest
encoding: utf8
output-encoding: utf8
include: yes
initialheaderlevel: 2
/restindex
=============================================
Python Premise Syntax
=============================================
Python_premise
================
::
python_premise ::= pattern '=' python_exp
| pattern 'in' python_exp
| 'check' python_exp
| python_statements
Each of these clauses results in a python expression being executed. Their
meaning is as follows:
pattern_ '=' python_exp
*python_exp* is evaluated and the result matched
with pattern_. If the result does not match,
the clause fails and backtracking_ is initiated.
The clause always fails on backtracking_, meaning that it only produces
a single result (contrasted with ``in``).
pattern_ 'in' python_exp
*python_exp* is evaluated and the first element
from the resulting *iterable* is matched
with pattern_. On backtracking_, successive
elements from the result are matched with
pattern_. When the result is exhausted, the clause fails.
This has the effect of offering each element of the result, one at a
time, to the subsequent premise clauses. Each element is thus acted on
individually.
'check' python_exp
*python_exp* is evaluated. If the result is
python "true" the clause succeeds, otherwise
it fails. The clause always fails on backtracking_.
Python_statements
===================
::
python_statements ::= 'python' python_statement
| 'python' NL INDENT
{python_statement NL}
DEINDENT
This clause allows the inclusion of arbitrary python statements in your
rules_. This premise always succeeds; and fails on backtracking.
.. Tip::
It is important to always keep in mind the difference between `pattern
variables`_ and python variables. `Pattern variables`_ are always
indicated with a ``$`` and are only bound to a value during inferencing.
Thus, a ``python_statement`` may not set a `pattern variable`_; you must
use the special `pattern variable`_ assignment::
<pattern> = <some python expression>
Also, when a `pattern variable`_ is used within a python expression or
statement, it must be bound to a value that does not contain any unbound
`pattern variables`_.
Python variables are not visible to the inference engine. They are local
variables that are also not visible to python code in other rules_ or other
invocations of the same rule_.
Finally, python variables in the `when clause`_ of a `backward-chaining
rule`_ are not visible to the python code in the `with clause`_ of the same
rule_. (These end up in two different python functions after the `.krb
file`_ is compiled). So this won't work::
some_bc_rule
use some_goal(...)
when
...
python x_list = <some python expression>
...
with
for x in x_list: process(x)
In this case, assign the value of the python variable to a `pattern
variable`_ in the `when clause`_ and then use that `pattern variable`_ in
the `with clause`_::
some_bc_rule
use some_goal(...)
when
...
python x_list = <some python expression>
...
$x_list = tuple(x_list)
with
for x in $x_list: process(x)
.. _backtracking: ../overview/rules/backward_chaining.html#backtracking
.. _backward-chaining rule: bc_rule.html
.. _.krb file: index.html
.. _pattern: pattern.html
.. _pattern variable: pattern.html#pattern-variable
.. _pattern variables: pattern variable_
.. _rule: ../overview/rules/index.html
.. _rules: rule_
.. _when clause: bc_rule.html#when-clause
.. _with clause: bc_rule.html#with-clause