#19256 Allow Lambda targets to point to `python_aws_lambda_layer`s and exclude code automatically Issue created by huonw Is your feature request related to a problem? Please describe. In #19123, Pants grows support for

python_aws_lambda_layer

to package layers for AWS Lambda. These are a bunch of code that's loaded into the Lambda environment, in addition to the core package. When packaging an in-repo layer for use with in-repo functions, typically the function doesn't need to include the code in the layer. Pants could automatically manage this, without needing to specify

include_requirements=False

and/or

dependencies=["!!foo", "!!bar"]

. Describe the solution you'd like Potentially a

layers

field that takes a list of layer targets. To begin with, this can be just used for excluding the dependencies. This field can be used on both functions and other layers. (NB. a package built like this is cannot be used in isolation, only with the right layers hooked into the deployed function in AWS.) For instance, in this example: •

boto3-layer

includes the

boto3

library and its dependencies, like

requests

•

func

depends on

boto3

,

requests

and

numpy

The

layers

field means pants would notice that

boto3

(and, preferably, its transitive dependencies like

requests

, which may require #12733) will already be provided by

boto3-layer

, and thus only include

numpy

and

foo.py

in the

func

package.

# BUILD
python_requirements(name="reqs") # includes boto3, requests, numpy, etc.

python_aws_lambda_layer(
    name="boto3-layer"
    dependencies=[":reqs#boto3"],
    ...
)

python_aws_lambda_function(
    name="func",
    handler="./foo.py:handler",
    layers=[":boto3-layer"], # NEW FEATURE
    ...
)

# foo.py
import boto3
import requests
import numpy

def handler(...): ...

Describe alternatives you've considered The current approach of manually spelling out dependencies and exclusions probably works fine for many cases, e.g. a naive "requirements in a layer"/"sources in the function" split is trivial via `include_requirements`/`include_sources`. Additional context There's a long discussion in #19123 that touches on this. pantsbuild/pants