Fields

An M42PL command's class arguments are used to configured the commands fields.

M42PL provide two base classes to interact with the pipelines fields:

• m42pl.fields.Field: read/write a single field
• m42pl.fields.FieldsMap: read multiple fields

Field

The class m42pl.field.Field takes 4 arguments:

Name	Type	Default	Description
name	str, Any	-	Field name or literal value
default	Any	None	Default value if the field is not found
type	type, tuple[type], None	None	Allowed type for the field's value
seqn	bool	False	Force the field's value to be wrapped into a sequence

The following schema shows how a field is read and processed:

FieldsMap

To do.

Example

class MyCommand(StreamingCommand):

    _aliases_ = ['my-command',]

    def __init__(self, static: str, dynamic: str = 'example.dynamic'):
        super().__init__(static, dynamic)
        self.static = Field(static)
        self.dynamic = Field(dynamic)

    async def setup(self, event, pipeline, context):
        self.static_value = await self.static.read(event, pipeline, context)

    async def target(self, event, pipeline, context):
        dynamic_value = await self.dynamic.read(event, pipeline, context)
        yield event

Let's start with the basic:

• static is a required argument
• dynamic is an optional argument with the default value example.dynamic
• MyCommand.static and MyCommand.dyanmic are both Field instances
• MyCommand.static and MyCommand.dyanmic will both read their values from an event

Now for the fields behaviour:

• MyCommand.static will read the value of the event's field name pointed by the argument static
• MyCommand.static will be read once, in MyCommand.setup
• MyCommand.dynamic will read the value of the event's field name pointed by the argument dynamic
• MyCommand.dynamic will be read for each event, in MyCommand.target

Assuming the following script and event:

| my-command example.static

{
    "example": {
        "static": "foo",
        "dynamic": 42
    }
}

We get the following table:

Kind	Name	Value	Configured by
Class argument	static	example.satic	Command parser
Class argument	dynamic	example.dynamic	Argument default value
Field	static	"foo"	Event
Field	dynamic	42	Event

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