cli: when cdk.out is missing, cannot synthesize or list

I deleted my cdk.out/ directory hoping to freshly generate its contents next time I ran cdk synth, but instead I get a file not found error.

Before I deleted cdk.out, synth, ls, and deploy were working fine.

Reproduction Steps

1. rm -r cdk.out
2. cdk ls

Error Log

ENOENT: no such file or directory, open 'cdk.out/manifest.json'

Environment

• CLI Version : 1.19.0
• Framework Version: 1.16.0
• OS : MacOS Catalina
• Language : Python
• Node version: v12.14.0

Other

> cdk --verbose ls
CDK toolkit version: 1.19.0 (build 5597bbe)
Command line arguments: {
  _: [ 'ls' ],
  verbose: true,
  v: true,
  'ignore-errors': false,
  ignoreErrors: false,
  json: false,
  j: false,
  ec2creds: undefined,
  i: undefined,
  'version-reporting': undefined,
  versionReporting: undefined,
  'path-metadata': true,
  pathMetadata: true,
  'asset-metadata': true,
  assetMetadata: true,
  'role-arn': undefined,
  r: undefined,
  roleArn: undefined,
  staging: true,
  'no-color': false,
  noColor: false,
  fail: false,
  long: false,
  l: false,
  '$0': '/usr/local/bin/cdk'
}
Determining whether we're on an EC2 instance.
Does not look like EC2 instance.
cdk.json: {
  "app": "python3 private_load_balancer.py"
}
merged settings: {
  versionReporting: true,
  pathMetadata: true,
  output: 'cdk.out',
  app: 'python3 private_load_balancer.py',
  context: {},
  tags: [],
  assetMetadata: true,
  toolkitBucket: {},
  staging: true
}
Setting "CDK_DEFAULT_REGION" environment variable to us-west-2
Resolving default credentials
Retrieved account ID X from disk cache
Setting "CDK_DEFAULT_ACCOUNT" environment variable to X
context: {
  'aws:cdk:enable-path-metadata': true,
  'aws:cdk:enable-asset-metadata': true
}
outdir: cdk.out
env: {
  CDK_DEFAULT_REGION: 'us-west-2',
  CDK_DEFAULT_ACCOUNT: 'X',
  CDK_CONTEXT_JSON: '{"aws:cdk:enable-path-metadata":true,"aws:cdk:enable-asset-metadata":true}',
  CDK_OUTDIR: 'cdk.out',
  CDK_CLI_ASM_VERSION: '1.16.0',
  CDK_CLI_VERSION: '1.19.0'
}
ENOENT: no such file or directory, open 'cdk.out/manifest.json'
Error: ENOENT: no such file or directory, open 'cdk.out/manifest.json'
    at Object.openSync (fs.js:440:3)
    at Object.readFileSync (fs.js:342:35)
    at new CloudAssembly (/usr/local/lib/node_modules/aws-cdk/node_modules/@aws-cdk/cx-api/lib/cloud-assembly.ts:77:36)
    at Object.execProgram (/usr/local/lib/node_modules/aws-cdk/lib/api/cxapp/exec.ts:85:10)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at AppStacks.synthesizeStacks (/usr/local/lib/node_modules/aws-cdk/lib/api/cxapp/stacks.ts:205:24)
    at AppStacks.listStacks (/usr/local/lib/node_modules/aws-cdk/lib/api/cxapp/stacks.ts:179:22)
    at AppStacks.selectStacks (/usr/local/lib/node_modules/aws-cdk/lib/api/cxapp/stacks.ts:109:20)
    at cliList (/usr/local/lib/node_modules/aws-cdk/bin/cdk.ts:337:20)
    at main (/usr/local/lib/node_modules/aws-cdk/bin/cdk.ts:188:16)

This seems very similar to a previous issue: https://github.com/aws/aws-cdk/issues/3717 but I did not find a clear resolution in the comments on issue 3717. One comment said “went away when I updated my cdk.ts” but it is not clear to me whether I can or should do that. I installed CDK by npm install -g aws-cdk, so I would expect all TypeScript to be “inside the black box”.

---

This is 🐛 Bug Report

[peterwoodworth]

I’ve independently found that app.synth() is required in Python. No idea why personally

[allister-db]

Adding an app.synth() call to my Python script made it start working.

Before:

def main():
  app = App()
  PrivateBalancerStack(app, MyEnv.name, env=AWS_ENV, description="Private ALB resources.")

> rm -rf cdk.out cdk.context.json ; cdk ls
ENOENT: no such file or directory, open 'cdk.out/manifest.json'

After:

def main():
  app = App()
  PrivateBalancerStack(app, MyEnv.name, env=AWS_ENV, description="Private ALB resources.")
  app.synth()

> rm -rf cdk.out cdk.context.json ; cdk ls
private-alb-stack

Perhaps I misunderstood the way synthesis works (I thought app.synth() would not be necessary if I am using cdk synth from the command line). If this is simply operator error, let me know and I will close this case.