app.py

#!/usr/bin/env python3

from aws_cdk import (
 core,
 aws_ec2 as ec2 
)  

app = core.App()

vpc = ec2.Vpc.from_lookup(self, 
                id="MyVPC",
                is_default=False,
                vpc_name='my-vpc')
 
vpc_subnets=vpc.select_subnets(subnet_type=ec2.SubnetType.PRIVATE)                                                                               
subnet_id=vpc_subnets.subnets[0].subnet_id
print(subnet_id)

MyStack(app,my-stack)
app.synth()