Enhance poetry related logging and add new flag

* Distinct logging whether the requirements.txt file is generated from
  poetry.lock vs pyproject.toml.
* New boolean flag: requirePoetryLockFile - When set to true, fail the build
  when poetry.lock is missing. This helps with creating reproducible
  builds where you want to have a fixed poetry.lock file instead of one
  generated on the fly.

Author: FinchPowers

README.md

@@ -131,6 +131,15 @@ custom:
     usePoetry: false
 ```
 
+Be aware that if no `poetry.lock` file is present, a new one will be generated on the fly. To help having predictable builds,
+you can set the `requirePoetryLockFile` flag to true to throw an error when `poetry.lock` is missing.
+
+```yaml
+custom:
+  pythonRequirements:
+    requirePoetryLockFile: false
+```
+
 ### Poetry with git dependencies
 
 Poetry by default generates the exported requirements.txt file with `-e` and that breaks pip with `-t` parameter

index.js

@@ -57,6 +57,7 @@ class ServerlessPythonRequirements {
         pipCmdExtraArgs: [],
         noDeploy: [],
         vendor: '',
+        requirePoetryLockFile: false
       },
       (this.serverless.service.custom &&
         this.serverless.service.custom.pythonRequirements) ||

lib/poetry.js

@@ -21,12 +21,30 @@ async function pyprojectTomlToRequirements(modulePath, pluginInstance) {
     generateRequirementsProgress = progress.get(
       'python-generate-requirements-toml'
     );
-    generateRequirementsProgress.update(
-      'Generating requirements.txt from "pyproject.toml"'
+  }
+
+  const emitMsg = (msg) => {
+    if (generateRequirementsProgress) {
+      generateRequirementsProgress.update(msg)
+      log.info(msg);
+    } else {
+      serverless.cli.log(msg);
+    }
+  }
+
+  try {
+    await spawn("test", ["-f", "poetry.lock"])
+    emitMsg('Generating requirements.txt from poetry.lock...');
+  } catch (e) {
+    if (options.requirePoetryLockFile) {
+      throw new Error(
+        "poetry.lock file not found - set requirePoetryLockFile to false to "
+        + "disable this error"
+      );
+    }
+    emitMsg(
+      'Generating poetry.lock and requirements.txt from pyproject.toml...'
     );
-    log.info('Generating requirements.txt from "pyproject.toml"');
-  } else {
-    serverless.cli.log('Generating requirements.txt from pyproject.toml...');
   }
 
   try {