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A Python library for simple evaluation of natural language predicates

Project Description


PyPred is a package to do predicate evaluation in Python. It uses a
PLY (Lex/Yacc for Python) to parse inputs into an AST tree which it
then evaluates. The PyPred provides simple APIs to do the evaluation
is most sitations, but allows for customized evaluation techniques for
more complex situations.


The grammer that PyPred understands if fairly limited, and is restricted
to boolean logic.

It supports the following:

* Logical operators `not`, `and`, `or`
* Comparison operators >, >=, <, <=, =, !=, 'is', 'is not'
* Parenthesis to disambiguate
* The subset check operator `contains`
* The regular expression matcher `matches`
* String literals, quoted if they include spaces
* Numeric literals
* Constants true, false, undefined, null, empty

Grammer Examples

To demonstate the capabilities of the pypred grammer, the following
examples are provided.

name is 'Jack' and friend_name is 'Jill'

This predicate checks that the input document has a field name equal to
"Jack", and a field friend\_name equal to "Jill"

event is "Record Score" and ((score >= 500 and highest_score_wins) or (score < 10 and lowest_score_wins))

This is a slightly more advanced predicate. It checks that this is a "Record Score" event,
and that the score is either greater than or equal to 500 in the case that a high score is desireable,
or that the score is less than 10 if a low score is desirable.

server matches "east-web-([\d]+)" and errors contains "CPU load" and environment != test

This checks for any webserver hostname matching a numeric suffix, such as "east-web-001", with
"CPU load" being reported as an error in a non-test environment.


Using the pypred package is very simple as well. It has a single primary
interface, which is the `Predicate` class. It is instantiated with
a string predicate.

The main interface for it is:
* Predicate(Pred) : Creates a new predicate object

* Predicate.description(): Returns a human readable version of the tree if valid

*\_valid() : Returns if the predicate is valid

* Predicate.errors(): If not valid, returns a list of tokeniztion, syntax, and semantic errors

* Predicate.evaluate(document) : Evaluates the given document against the predicate

* Predicate.analyze(document) : Evaluates the given document against the predicate,
returns the results, as well as a dictionary that includes more information about
the evaluation, including the failure reasons. This is generally much slower than
evaluate in the failure cases.

One of the critical aspects of evaluating a predicate is the resolution of
literals. When the AST needs a value to substitute a variable, it calls the
`resolve_identifier` method of the Predicate. The default behavior is flexible,
and support string literals, dictionary lookups, nested dictionaries, and
call back resolution via `set_resolver`. However, if a client wants to customize
the resolution of identifier, they can simply override this method.

Human Readable Outputs

PyPred tries to make it possible to provide human readable output of
both predicates as well as any error messages that are encountered.
Here is an example of a human readable description of:

p = Predicate('server matches "east-web-([\d]+)" and errors contains "CPU load" and environment != test')
print p.description()

AND operator at line: 1, col 34
MatchOperator at line: 1, col 7
Literal server at line: 1, col 0
Regex 'east-web-([\\d]+)' at line: 1, col 15
AND operator at line: 1, col 65
ContainsOperator at line: 1, col 45
Literal errors at line: 1, col 38
Literal "CPU load" at line: 1, col 54
!= comparison at line: 1, col 81
Literal environment at line: 1, col 69
Literal test at line: 1, col 84

Here is an example of the output during a failed evaluation:

p = Predicate('server matches "east-web-([\d]+)" and errors contains "CPU load" and environment != test')
res, info = p.analyze({'server': 'east-web-001', 'errors': [], 'environment': 'prod'})
assert res == False

{'failed': ["Right side: 'CPU load' not in left side: [] for ContainsOperator at line: 1, col 45",
'Left hand side of AND operator at line: 1, col 65 failed',
'Right hand side of AND operator at line: 1, col 34 failed'],
'literals': {'"CPU load"': 'CPU load',
'errors': [],
'server': 'east-web-001'}}

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