Even doing some printf debugging with

vpc_id = vpc.id.apply(lambda x: x)
print(vpc id after apply(): ", vpc_id)
create_transit_gateway_attachment(tgw_config_parameters, vpc_id, tags)

Leads to the astonishing output of:

vpc id after apply():  <pulumi.output.Output object at 0x7fa5c8475400>

error: Program failed with an unhandled exception:
error: Traceback (most recent call last):
<snip>
Exception: invoke of aws:ec2/getSubnetIds:getSubnetIds failed: Missing required argument: The argument "vpc_id" is required, but no definition was found. ()

The error still exists ..

Oh wait wait sorry misreading your code

sadly

And this did work

The codebase is split into different files and the function call to collect the subnets is in an function where I do the whole TGW setup process.

I thought about something like in the example of @hallowed-animal-47023 with

private_networks=[]

and then put these as parameter in my transitgateway() function, but the code base has a second use/intent as an example for new pulumi users in our organisation. So I put the get_subnet_ids() call in this as an example for querying resources from the api with the use of proper tagging.

This would become then something like this:

vpc_id = vpc.id.apply(lambda x: x)
create_transitgateway_attachment(tgw_config_paramenters, vpc_id, private_networks, tags)

But other resources inside this function need the vpc.id aswell, so the concurrency problem would remain.

so i was wondering if you were referencing pulumi resources not created in the same stack

the bit which caught me out is that i was trying to do the transforms outside a resource object but then use the result as input to another resource

i have since learned this is not the pulumi way

and that the transforms need to be encapsualted in a pulumi resource else transformation operations on them fail

wondered if this was similar to what you were experiencing