Pass an optional parameter based on value in Python3

Question

I have a parameter that can be None or a String. In case it is None, I cannot pass it as parameter, as the library does not support None values nor empty strings. The library does not accept dictionaries as input neither. On the other side, I don't really want to write such a horrible alternative!

if lifecycle_policy_name:
    response = client.create_notebook_instance(
        NotebookInstanceName=NotebookInstanceName,
        InstanceType=InstanceType,
        SubnetId=SubnetId,
        SecurityGroupIds=SecurityGroupIds,
        RoleArn=RoleArn,
        Tags=Tags,
        DirectInternetAccess='Disabled',
        VolumeSizeInGB=10,
        RootAccess='Disabled',
        KmsKeyId=kms_key.get('KeyId'),
        LifecycleConfigName=lifecycle_policy_name
    )
else:
    response = client.create_notebook_instance(
        NotebookInstanceName=NotebookInstanceName,
        InstanceType=InstanceType,
        SubnetId=SubnetId,
        SecurityGroupIds=SecurityGroupIds,
        RoleArn=RoleArn,
        Tags=Tags,
        DirectInternetAccess='Disabled',
        VolumeSizeInGB=10,
        RootAccess='Disabled',
        KmsKeyId=kms_key.get('KeyId'),
    )

So as you can guess, this is calling a Boto3 api.

L3viathan · Accepted Answer · 2020-06-08 19:51:52Z

3

You could try using keyword argument expansion:

kwargs = dict(
    NotebookInstanceName=NotebookInstanceName,
    InstanceType=InstanceType,
    SubnetId=SubnetId,
    SecurityGroupIds=SecurityGroupIds,
    RoleArn=RoleArn,
    Tags=Tags,
    DirectInternetAccess='Disabled',
    VolumeSizeInGB=10,
    RootAccess='Disabled',
    KmsKeyId=kms_key.get('KeyId'),
)

if lifecycle_policy_name:
    kwargs["LifecycleConfigName"] = lifecycle_policy_name
response = client.create_notebook_instance(
    **kwargs
)

answered Jun 8, 2020 at 19:51

L3viathan

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6 Comments

Curcuma_ Over a year ago

I saw this and thought this would be read as a regular dictionary, so it is not the case right ? if yes, then this is fantastic !

L3viathan Over a year ago

kwargs is a regular dictionary, I was just to lazy to use a normal dictionary literal, since I could just copy-paste most of your code with the dict() "constructor" :D

L3viathan Over a year ago

Ah, and in case you mean how it's used: That's just **-unpacking, see e.g. this PEP.

Curcuma_ Over a year ago

probably the two stars means deconstruction or something. because I will try and if it is a regular dictionary it won't work :o

L3viathan Over a year ago

@Curcuma_ Exactly, you can use single asterisks to unpack iterables like lists into positional arguments, and double asterisks to unpack mappings like dictionaries into keyword arguments.

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