PYSON

PYSON is the PYthon Statement and Object Notation. It is a lightweight domain specific language for the general representation of statements. PYSON is used to encode statements which can be evaluated in different programming languages, serving for the communication between trytond and any third party software. A PYSON parser can easily be implemented in other programming languages. So third party softwares do not need to depend on Python to be able to fully communicate with the Tryton server.

PYSON is a deterministic algorithm which will always succeed to evaluate statements. There is a default behavior for unknown values.

There is also a reference documentation of the API.

Syntax

The syntax of a PYSON statement follows this pattern:

Statement(argument1[, argument2[, ...]])

where arguments can be another statement or a value. The evaluation direction is inside out, deepest first.

PYSON Examples

Given the PYSON statement:

Eval('active_id', -1)

Eval() checks the evaluation context for the variable active_id and returns its value or -1 if not defined. A similar expression in Python looks like this:

'active_id' in locals() and active_id or -1

Given the PYSON statement:

Not(Bool(Eval('active')))

Eval() checks the evaluation context for a variable active and returns its value to Bool() or '' if not defined. Bool() returns the corresponding boolean value of the former result to Not(). Not() returns the boolean negation of the previous result. A similar expression in Python looks like this:

'active' in locals() and active == False

Given the PYSON statement:

Or(Not(Equal(Eval('state'), 'draft')), Bool(Eval('lines')))

In this example are the results of two partial expressions Not(Equal(Eval('state'), 'draft')) and Bool(Eval('lines')) evaluated by a logical OR operator. The first expression part is evaluated as follow: When the value of Eval('state') is equal to the string 'draft' then return true, else false. Not() negates the former result. A similar expression in Python looks like this:

'states' in locals() and 'lines' in locals() \
        and state != 'draft' or bool(lines)

Given the PYSON statement:

If(In('company', Eval('context', {})), '=', '!=')

In this example the result is determined by an if-then-else condition. In('company', Eval('context', {})) is evaluated like this: When the key 'company' is in the dictionary context, returns true, otherwise false. If() evaluates the former result and returns the string '=' if the result is true, otherwise returns the string '!='. A similar expression in Python looks like this:

'context' in locals() and isinstance(context, dict) \
        and 'company' in context and '=' or '!='

Given the PYSON statement:

Get(Eval('context', {}), 'company', 0))

Eval() checks the evaluation context for a variable context if defined, return the variable context, otherwise return an empty dictionary {}. Get() checks the former resulting dictionary and returns the value of the key 'company', otherwise it returns the number 0. A similar expression in Python looks like this:

'context' in locals() and context.get('company', 0) or 0