from prefect import task, Parameter, Flow
from prefect.storage import S3
import prefect


@task
def main_task():
    print("foobar")
    logger = prefect.context.get("logger")
    <http://logger.info|logger.info>("foobar")


storage = S3(bucket="prefect-task-behaviour", stored_as_script=True, local_script_path="main.py")

def build_flow():
    with Flow("s3-to-prefect-flow", storage=storage) as building_flow:
        main_task()
    return building_flow


print(build_flow())

I have an S3 bucket with a copy of this code (named main.py) in it, yet I am not seeing it print out foobar, why is that?

because the flow object is not available in your script - the easiest would be to move it out from the build_flow function to the main scope. Check examples here https://github.com/anna-geller/packaging-prefect-flows/tree/master/flows e.g.

from prefect import Flow, task
from prefect.client.secrets import Secret
from prefect.storage import S3
from prefect.run_configs import LocalRun
import subprocess


AWS_ACCOUNT_ID = Secret("AWS_ACCOUNT_ID").get()
FLOW_NAME = "s3_local_run"
AGENT_LABEL = "dev"
STORAGE = S3(
    bucket="prefectdata",
    key=f"flows/{FLOW_NAME}.py",
    stored_as_script=True,
    # this will ensure to upload the Flow script to S3 during registration
    local_script_path=f"flows/{FLOW_NAME}.py",
)

RUN_CONFIG = LocalRun(labels=[AGENT_LABEL],)


@task(log_stdout=True)
def hello_world():
    print("Hello world!")


with Flow(FLOW_NAME, storage=STORAGE, run_config=RUN_CONFIG,) as flow:
    hw = hello_world()

if __name__ == "__main__":
    flow_id = flow.register("community")
    subprocess.run(f"prefect run --id {flow_id}", shell=True)
    subprocess.run(
        f"prefect agent local start --label {AGENT_LABEL} --no-hostname-label",
        shell=True,
    )