@@ -15,8 +15,7 @@ poetry bundle venv .build/.venv --without dev
 package-python-function .build/.venv --output-dir .build/lambda
 ```
 
-The output will be a .zip file with the same name as your project from your `pyproject.toml` file (with dashes replaced
-with underscores).
+The output will be a .zip file named after your project, as described in [Output file name](#output-file-name).
 
 ## Installation
 Use [pipx](https://github.com/pypa/pipx) to install:
@@ -27,16 +26,57 @@ pipx install package-python-function
 
 ## Usage / Arguments
 ```shell
-package-python-function venv_dir [--project PROJECT] [--output-dir OUTPUT_DIR] [--output OUTPUT]
+package-python-function venv_dir [--project PROJECT] [--output-dir OUTPUT_DIR] [--output OUTPUT] [--report REPORT]
 ```
 
 - `venv_dir` [Required]: The path to the virtual environment to package.
 - `--project` [Optional]: Path to the `pyproject.toml` file. Omit to use the `pyproject.toml` file in the current working directory.
+- `--report` [Optional]: Path to write a JSON [report file](#report-file) to. Omit to write no report.
 
-One of the following must be specified:
+`--output` and `--output-dir` cannot be used together. If neither is given, the zip is written to the current working
+directory.
 - `--output`: The full output path of the final zip file.
-- `--output-dir`: The output directory for the final zip file. The name of the zip file will be based on the project's
-name in the `pyproject.toml` file (with dashes replaced with underscores).
+- `--output-dir`: The output directory for the final zip file. The name of the zip file is described in
+[Output file name](#output-file-name).
+
+## Output file name
+
+Unless `--output` gives an exact path, the file written is `<output-dir>/<distribution_name>.zip`.
+
+`distribution_name` is the project's name — `[project].name`, or `[tool.poetry].name` if that is absent — with each run
+of characters outside `A-Z a-z 0-9 _ .` replaced by a single underscore, following the
+[PyPA escaping rules](https://peps.python.org/pep-0427/#escaping-and-unicode).
+
+**Case is preserved.** A project named `My-App` produces `My_App.zip`, not `my_app.zip`. Note that this differs from the
+wheel your build tool produces for the same project, whose filename is lowercased — so a wheel's name is not a safe way
+to predict the name of this file.
+
+## Report file
+
+Pass `--report <path>` to have the tool write a JSON description of what it produced, so a calling script does not have
+to re-derive the output path or re-measure the package.
+
+```json
+{
+  "output_file": "/abs/path/lambda/my_app.zip",
+  "distribution_name": "my_app",
+  "output_bytes": 3460000,
+  "uncompressed_bytes": 412000000,
+  "compressed_bytes": 3456789,
+  "nested_zip": false
+}
+```
+
+| Field | Meaning |
+| --- | --- |
+| `output_file` | Absolute path of the zip that was written. |
+| `distribution_name` | The normalized project name, as described in [Output file name](#output-file-name). |
+| `output_bytes` | Size of the file at `output_file`. This is the artifact you deploy. |
+| `uncompressed_bytes` | Total size of the packaged files before compression. This is the figure compared against the AWS Lambda 250 MiB unzipped limit. |
+| `compressed_bytes` | Size of the dependencies zip. Equal to `output_bytes` unless the nested-zip strategy was used, in which case the outer zip also holds the loader. |
+| `nested_zip` | Whether the nested-zip strategy was used. |
+
+The report is written only when packaging succeeds, so its presence is a reliable signal that the zip is really there.
 
 ## Notes on Reproducibility
 
 
@@ -4,24 +4,33 @@
 import sys
 
 from .packager import Packager
+from .reproducible_zipfile import date_time
 
 
 def main() -> None:
     logging.basicConfig(level=logging.INFO, stream=sys.stdout, format="%(message)s")
 
     args = parse_args()
+
+    # Validate $SOURCE_DATE_EPOCH here, so that a bad value fails before any packaging work rather than partway
+    # through writing the zip.
+    date_time()
+
     project_path = Path(args.project).resolve()
     venv_path = Path(args.venv_dir).resolve()
     output_dir_path = Path(args.output_dir).resolve()
     output_file_path = Path(args.output).resolve() if args.output else None
-    packager = Packager(venv_path, project_path, output_dir_path, output_file_path)
+    report_file_path = Path(args.report).resolve() if args.report else None
+    packager = Packager(venv_path, project_path, output_dir_path, output_file_path, report_file_path)
     packager.package()
 
 
 def parse_args() -> argparse.Namespace:
     arg_parser = argparse.ArgumentParser()
     arg_parser.add_argument("venv_dir", type=str, help="The directory path to the virtual environment to package into a zip file")
     arg_parser.add_argument("--project", type=str, default='pyproject.toml', help="The path to the project's pyproject.toml file. Omit to use pyproject.toml in the current working directory.")
-    arg_parser.add_argument("--output-dir", type=str, default='.', help="The directory path to save the output zip file. Default is the current working directory.")
-    arg_parser.add_argument("--output", type=str, default='', help="The full file path for the output file. Use this instead of --output-dir if you want total control of the output file path.")
+    output_group = arg_parser.add_mutually_exclusive_group()
+    output_group.add_argument("--output-dir", type=str, default='.', help="The directory path to save the output zip file. Default is the current working directory.")
+    output_group.add_argument("--output", type=str, default='', help="The full file path for the output file. Use this instead of --output-dir if you want total control of the output file path.")
+    arg_parser.add_argument("--report", type=str, default='', help="The file path to write a JSON report of the packaging result to. Omit to write no report.")
     return arg_parser.parse_args()
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import json
 import logging
 import shutil
 from pathlib import Path
@@ -11,26 +12,52 @@
 
 logger = logging.getLogger(__name__)
 
+class PackageTooLargeError(Exception):
+    """Raise when the content is too large for AWS Lambda, both uncompressed and compressed"""
+
+    def __init__(self, uncompressed_bytes: int, compressed_bytes: int, limit_bytes: int):
+        self.uncompressed_bytes = uncompressed_bytes
+        self.compressed_bytes = compressed_bytes
+        self.limit_bytes = limit_bytes
+        super().__init__(
+            f"The uncompressed size ({uncompressed_bytes:,} bytes) is too large for AWS Lambda, and the compressed "
+            f"size ({compressed_bytes:,} bytes) also exceeds the limit of {limit_bytes:,} bytes, so the nested-zip "
+            "strategy cannot be used either. No package was written."
+        )
+
 class Packager:
     AWS_LAMBDA_MAX_UNZIP_SIZE = 262_144_000
     DIRS_TO_EXCLUDE = ["__pycache__"]
     DIST_INFO_FILES_TO_EXCLUDE = ["RECORD", "direct_url.json"]
     EXTENSIONS_TO_EXCLUDE = [".pyc", ".pyo"]
 
-    def __init__(self, venv_path: Path, project_path: Path, output_dir: Path, output_file: Path | None):
+    def __init__(
+        self,
+        venv_path: Path,
+        project_path: Path,
+        output_dir: Path,
+        output_file: Path | None,
+        report_file: Path | None = None,
+    ):
         self.project = PythonProject(project_path)
         self.venv_path = venv_path
 
         self.output_dir = output_file.parent if output_file else output_dir
         self.output_file = output_file if output_file else output_dir / f'{self.project.distribution_name}.zip'
+        self.report_file = report_file
 
         self._uncompressed_bytes = 0
+        self._compressed_bytes = 0
+        self._nested_zip = False
 
     @property
     def input_path(self) -> Path:
         python_paths = list((self.venv_path / 'lib').glob('python*'))
         if not python_paths:
-            raise Exception("input_path")
+            raise FileNotFoundError(
+                f"No 'lib/python*' directory was found in '{self.venv_path}'. Check that this path points at a "
+                "virtual environment with the function's dependencies installed into it."
+            )
         return python_paths[0] / 'site-packages'
 
     def package(self) -> None:
@@ -41,6 +68,27 @@ def package(self) -> None:
         with NamedTemporaryFile(suffix=".zip") as dependencies_zip:
             self.zip_all_dependencies(Path(dependencies_zip.name))
 
+        if self.report_file:
+            self.write_report()
+
+    def write_report(self) -> None:
+        """
+        Write a JSON report describing the package that was just produced, so that a calling script does not have to
+        re-derive the output path or re-measure the sizes.
+        """
+        report = {
+            "output_file": str(self.output_file),
+            "distribution_name": self.project.distribution_name,
+            "output_bytes": self.output_file.stat().st_size,
+            "uncompressed_bytes": self._uncompressed_bytes,
+            "compressed_bytes": self._compressed_bytes,
+            "nested_zip": self._nested_zip,
+        }
+
+        logger.info(f"Writing report to '{self.report_file}'...")
+        self.report_file.parent.mkdir(parents=True, exist_ok=True)
+        self.report_file.write_text(json.dumps(report, indent=2) + "\n")
+
     def zip_all_dependencies(self, target_path: Path) -> None:
         logger.info(f"Zipping to {target_path}...")
 
@@ -64,17 +112,20 @@ def zip_dir(path: Path) -> None:
 
             zip_dir(self.input_path)
 
-        compressed_bytes = target_path.stat().st_size
+        compressed_bytes = self._compressed_bytes = target_path.stat().st_size
 
         logger.info(f"Uncompressed size: {self._uncompressed_bytes:,} bytes. Compressed size: {compressed_bytes:,} bytes.")
 
         if self._uncompressed_bytes > self.AWS_LAMBDA_MAX_UNZIP_SIZE:
             logger.info(f"The uncompressed size of the ZIP file is greater than the AWS Lambda limit of {self.AWS_LAMBDA_MAX_UNZIP_SIZE:,} bytes.")
             if(compressed_bytes < self.AWS_LAMBDA_MAX_UNZIP_SIZE):
                 logger.info(f"The compressed size ({compressed_bytes:,}) is less than the AWS limit, so the nested-zip strategy will be used.")
+                self._nested_zip = True
                 self.generate_nested_zip(target_path)
             else:
-                print("TODO Error.  The unzipped size it too large for AWS Lambda.")
+                raise PackageTooLargeError(
+                    self._uncompressed_bytes, compressed_bytes, self.AWS_LAMBDA_MAX_UNZIP_SIZE
+                )
         else:
             logger.info(f"Copying '{target_path}' to '{self.output_file}'")
             shutil.copy(str(target_path), str(self.output_file))
 
@@ -25,7 +25,7 @@ def name(self) -> str:
     """
     @cached_property
     def distribution_name(self) -> str:
-        return re.sub("[^\w\d.]+", "_", self.name, re.UNICODE)
+        return re.sub(r"[^\w\d.]+", "_", self.name)
 
     @cached_property
     def entrypoint_package_name(self) -> str:
@@ -41,7 +41,8 @@ def find_value(self, paths: tuple[tuple[str]]) -> str:
             value = self.get_value(path)
             if value is not None:
                 return value
-        raise Exception("TODO Exception find_value")
+        searched = ", ".join(".".join(path) for path in paths)
+        raise ValueError(f"None of the following were found in '{self.path}': {searched}.")
 
     def get_value(self, path: tuple[str]) -> Optional[str]:
         node = self.toml
 
@@ -25,7 +25,14 @@ def date_time() -> Tuple[int, int, int, int, int, int]:
     """
     source_date_epoch = os.environ.get("SOURCE_DATE_EPOCH", None)
     if source_date_epoch is not None:
-        dt = time.gmtime(int(source_date_epoch))[:6]
+        try:
+            seconds_since_epoch = int(source_date_epoch)
+        except ValueError as error:
+            raise SourceDateEpochError(
+                f"$SOURCE_DATE_EPOCH must be an integer number of seconds since the Epoch, but was "
+                f"'{source_date_epoch}'."
+            ) from error
+        dt = time.gmtime(seconds_since_epoch)[:6]
         if dt[0] < 1980:
             raise SourceDateEpochError(
                 "$SOURCE_DATE_EPOCH must be >= 315532800, since ZIP files need MS-DOS date/time format, which can be 1/1/1980, at minimum."