Expressions¶

Several features need a condition over telemetry or argument values: container identification, conditional entries, context alarms, verifier checks and transmission constraints all take an expression. Expressions are built with six comparison helpers and two combinators:

Helper	Meaning
`eq(ref, value)`	equal to
`ne(ref, value)`	not equal to
`lt(ref, value)` / `lte(ref, value)`	less than (or equal)
`gt(ref, value)` / `gte(ref, value)`	greater than (or equal)
`all_of(expr1, expr2, ...)`	true when all sub-expressions hold (AND)
`any_of(expr1, expr2, ...)`	true when at least one holds (OR)

Y.all_of(
    Y.eq(mode, "NOMINAL"),
    Y.any_of(
        Y.gt(battery_voltage, 11.0),
        Y.eq(external_power, True),
    ),
)

Combinators nest to any depth. Each helper is also available as a class (EqExpression, AndExpression, …) with identical meaning; the functions are simply shorter.

What can be referenced¶

The first operand, ref, accepts:

a Parameter — the usual case;
an Argument — inside command definitions, to make entries or constraints depend on another argument of the same command;
a ParameterMember or ArgumentMember — a member inside an aggregate, built from the parameter/argument and the member object(s) forming the path (see Parameters);
a plain string — the qualified name of a parameter not managed with PyMDB.

# Compare against a member of an aggregate parameter
apid = Y.ParameterMember(ccsds_packet_id, path=apid_member)
my_packet = Y.Container(
    system=spacecraft,
    name="my_packet",
    base=ccsds_space_packet,
    condition=Y.eq(apid, 101),
)

The comparison value is a plain Python value matching the engineering type: a number, a string (for enumeration states), a boolean.

Calibrated or raw¶

By default comparisons use the calibrated (engineering) value. Pass calibrated=False to compare against the raw value instead:

Y.eq(status_field, 0x2A, calibrated=False)

This matters chiefly in container conditions on parameters that have a calibrator or enumeration: comparing raw values spares Yamcs the calibration during packet identification, and lets you match wire-level constants directly.