Work with evaluated models

The Taurus eval: scheme creates Taurus attributes whose values are computed from Python expressions. It is useful for quick calculations, combining values from other Taurus models, and prototyping data sources without implementing a new scheme.

Use a Python expression

Create an evaluation attribute with taurus.Attribute():

import taurus

attr = taurus.Attribute('eval:Q(rand(), "V")')
print(attr.read().rvalue)

When no evaluation device is specified, Taurus uses its default evaluator, which provides a selection of NumPy functions and other safe symbols.

Use values from other Taurus models

Enclose another model name in braces to use its value in an expression. For example, the following form displays a Tango attribute and a value ten times larger:

taurus form \
    "tango:sys/tg_test/1/short_scalar" \
    "eval:{tango:sys/tg_test/1/short_scalar}*10"

Evaluation models can be used by normal Taurus widgets just like models from other schemes. For example:

taurus trend \
    "tango:sys/tg_test/1/short_scalar" \
    "eval:-{tango:sys/tg_test/1/short_scalar}"

Evaluations are also units-aware. For example:

taurus form "eval:({eval:Q('3N')}/{eval:Q('2kg')}).to_base_units()"

This evaluates to 1.5 m/s**2.

Access Python modules

The @module.* syntax creates an evaluation device exposing symbols from a Python module. For example:

taurus form 'eval:@datetime.*/date.today().isoformat()'

or:

taurus form 'eval:@os.path.*/exists("/tmp")'

The module name may contain dots, as in the os.path example above.

Use a custom evaluation device

Evaluation devices derive from taurus.core.util.safeeval.SafeEvaluator. You can create a named evaluator and add symbols programmatically:

import taurus

dev = taurus.Device('eval:@mydev')
dev.addSafe({'message': 'hello world'})

attr = taurus.Attribute('eval:@mydev/message.upper()')
print(attr.read().rvalue)

This is useful for prototyping, but applications that expose a reusable data source should normally consider implementing a dedicated Taurus scheme.

Further examples

The Taurus training material contains more advanced examples, including accessing class instances and exposing writable Python properties as Taurus attributes:

Taurus training: Evaluation Scheme