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Best practice for dealing with Optional Parameters in Script Tools?

ZacharyHart

Here's a crude example to at least put forth some effort in asking this question. I tried searching and got some results but I fear my inquiry is too simplistic to find viable results.

Let's say we have just two parameters defined in the code; and in the script tool we have configured parameter 1 to be 'required' and parameter 2 to be 'optional'.

RequiredParameter = arcpy.GetParameterAsText(0)
OptionalParameter = arcpy.GetParameterAsText(1)

So if your code has a line expecting OptionalParameter, the script will fail. So how best to evaluate it being supplied by the user or not?

A few options I have found:

Evaluate for presence of the optional parameter:

if OptionalParameter and OptionalParameter != "#":
    #Do something with optional parameter

Try-Except block:

try:
    #arcpy tool using the supplied optional parameter
except:
    pass #move on with life never knowing if anything went wrong or why it did

I suppose something could be done within the script tool validation, but the code itself would have to be modified as well.

Pitfalls of either? Better options exist?

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Accepted answers

blake.terhune

I'll just toss out that you could also simply check for "truthiness"

if isOptional:
    # isOptional has a value.
    # do a thing.
else:
    # isOptional has no value.
    # handle it.

Anonymous User

Ok, I use this style for testing if the parameters from the tool's UI has been set, or set it to a default- (very useful for maintaining the scripts that are meant for standalone execution and as a script tool). Though, you may not even need it since it will either be a value or None and the conditional later on in your code would determine the course of action, but it is explicitly setting the variable.

isOptional = arcpy.GetParameterAsText(1)
if isOptional == '' or isOptional is None:
    isOptional = None

All comments

ZacharyHart

Thanks everyone I really learned a lot from this!

blake.terhune

I've seen some of the Esri GP tools do this. When you run the tool and then copy the Python snippet, it'll put "#" for the optional parameters that were left empty. I have never done this explicitly with my Python toolboxes and I don't see a need for it.

Anonymous User

Maybe it was a thing back in '13 but I haven't seen any parameter come through as '#' so could be outdated?

ZacharyHart

Can anyone attest to the veracity of this statement here?

"The hash sign ("#") string is what the geoprocessing framework passes in for unspecified values so you have to look for that in addition to the empty/null string case."

stack exchange link

I ask because I noticed no one suggesting testing for != "#"

danretired

I prefer `named` parameters rather than positional parameters, particularly for those that are optional.

More of a notation, than a solution, but assume the `funk` is a script associated with a tool. I have omitted error checking for the first two parameters to ensure that they have values (type hints can be used but that is another story.

So assume, that I pass the tool the tool 4 parameters (use GetParameter or GetParameterAsText if you want to check for None as 'None')

def funk(req1, req2, **kwargs):
    """kwargs are optional named parameters.  I prefer `named` parameters."""
    opt1 = kwargs.get('opt1', None)
    opt2 = kwargs.get('opt2', None)
    args = [req1, req2, opt1, opt2]
    return  args
    

out = funk(1, 2, opt2=3)

print(out)
[1, 2, None, 3]

note that the optional parameter (opt1) is checked for its existence (aka, it was named in the call to the script).

In `custom` python toolboxes you can set default values for your parameters like "None", "nothing to see here", "zilch".... whatever, then test for it in your scripts. Avoid blanks or "" or '' because they all look the same, put in a default "null/ignore me" value. You will thanks yourself in a year or two when you have to figure out what you did

blake.terhune

I'll just toss out that you could also simply check for "truthiness"

if isOptional:
    # isOptional has a value.
    # do a thing.
else:
    # isOptional has no value.
    # handle it.

Anonymous User

isOptional = arcpy.GetParameterAsText(1)
if isOptional == '' or isOptional is None:
    isOptional = None

ZacharyHart

Ok I see where you are going!

I've never set a default to none. Is none a valid input for default or something like "" needs to be set?

Anonymous User

So keep it assigned to none and in your code where you need that parameter, split your process to execute either option.

if optionalparameter is not none:

Use it

else:

Don’t use it.

ZacharyHart

I was thinking something similar but don't know how to implement it exactly. But the trick here is that if a user supplies no value, that is actually valid because they don't want to employ this parameter and this is the most likely scenario. Follow me?

blake.terhune

I would say you should find a way to use a default value if OptionalParameter is not defined and write your code logic in a way that can handle any valid value (including the default).

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