· Apr 12, 2022 2m read

Embedded Python, Using Output Parameters


In InterSystems IRIS versions >=2021.2 we can use the accompanying irispython binary to directly write python code on top of our IRIS instances. This lets us use python packages, call methods, make SQL queries, and do nearly anything in Objectscript but pythonic.

For example, below I check if a namespace is present:

import iris
# call arbitrary class methods
result = iris.cls('%SYS.Namespace').Exists('USER')
if result == 1:
    print(f"Namespace USER is present")

But, what if my method in IRIS has special parameters such as Output and ByRef parameters?
How can we use Output parameters in irispython?

For example, Ens.Director has a lot of Output parameters on it's methods, how can I use these parameters in python?


// An example method stub from Ens.Director
ClassMethod GetProductionStatus(Output pProductionName As %String, Output pState As %Integer...

Trying the normal variable methods

At first glance, you may try the following:

import os
# Set namespace the manual way

import iris

# TEST 1 with output variables
productionName, productionState = None, None
status = iris.cls('Ens.Director').GetProductionStatus(productionName, productionState) 

print("Success? -- {}".format(productionState != None))

But both tests won't return the Output Variables! You can try this yourself on any Ensemble namespace

Using iris.ref

The irispython utility iris.ref can be used to capture the Output and ByRef variables.

  1. Create an iris.ref() object
  2. Call your ObjectScript Method
  3. Use the .value variable to get the result from that parameter
import os
# Set namespace the hard way

import iris

# TEST 2 with output variables
productionName, productionState = iris.ref('productionName'), iris.ref('productionState')
status = iris.cls('Ens.Director').GetProductionStatus(productionName, productionState) 

print("Status: {}".format(status))
# see .value
print("Production: {}".format(productionName.value))
# see .value
print("Production State: {}".format(productionState.value))


Discussion (4)3
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@Nicholai Mitchko's example above is Python calling an ObjectScript method that has pass-by-ref arguments, and that works as he described. I wondered if Python methods could be written to return values in their arguments, despite the fact that Python doesn't have pass-by-ref arguments. The answer is "yes," but only if the caller passes in a data structure (like a list or a dict) that the Python method modifies. Otherwise, arguments to a Python method called from either ObjectScript or Python can't return values.