Refactor file logic in demo_loader and add unit tests. (#3241)

ml-agents/mlagents/trainers/demo_loader.py

-import pathlib
 import logging
 import os
 from typing import List, Tuple
     return brain_params, demo_buffer
 
 
+def get_demo_files(path: str) -> List[str]:
+    """
+    Retrieves the demonstration file(s) from a path.
+    :param path: Path of demonstration file or directory.
+    :return: List of demonstration files
+
+    Raises errors if |path| is invalid.
+    """
+    if os.path.isfile(path):
+        if not path.endswith(".demo"):
+            raise ValueError("The path provided is not a '.demo' file.")
+        return [path]
+    elif os.path.isdir(path):
+        paths = [
+            os.path.join(path, name)
+            for name in os.listdir(path)
+            if name.endswith(".demo")
+        ]
+        if not paths:
+            raise ValueError("There are no '.demo' files in the provided directory.")
+        return paths
+    else:
+        raise FileNotFoundError(
+            f"The demonstration file or directory {path} does not exist."
+        )
+
+
 @timed
 def load_demonstration(
     file_path: str
 
     # First 32 bytes of file dedicated to meta-data.
     INITIAL_POS = 33
-    file_paths = []
-    if os.path.isdir(file_path):
-        all_files = os.listdir(file_path)
-        for _file in all_files:
-            if _file.endswith(".demo"):
-                file_paths.append(os.path.join(file_path, _file))
-        if not all_files:
-            raise ValueError("There are no '.demo' files in the provided directory.")
-    elif os.path.isfile(file_path):
-        file_paths.append(file_path)
-        file_extension = pathlib.Path(file_path).suffix
-        if file_extension != ".demo":
-            raise ValueError(
-                "The file is not a '.demo' file. Please provide a file with the "
-                "correct extension."
-            )
-    else:
-        raise FileNotFoundError(
-            "The demonstration file or directory {} does not exist.".format(file_path)
-        )
-
+    file_paths = get_demo_files(file_path)
     group_spec = None
     brain_param_proto = None
     info_action_pairs = []

ml-agents/mlagents/trainers/tests/test_demo_loader.py

 import os
 import numpy as np
+import pytest
+import tempfile
-from mlagents.trainers.demo_loader import load_demonstration, demo_to_buffer
+from mlagents.trainers.demo_loader import (
+    load_demonstration,
+    demo_to_buffer,
+    get_demo_files,
+)
 
 
 def test_load_demo():
 
     _, demo_buffer = demo_to_buffer(path_prefix + "/test_demo_dir", 1)
     assert len(demo_buffer["actions"]) == total_expected - 1
+
+
+def test_edge_cases():
+    path_prefix = os.path.dirname(os.path.abspath(__file__))
+    # nonexistent file and directory
+    with pytest.raises(FileNotFoundError):
+        get_demo_files(os.path.join(path_prefix, "nonexistent_file.demo"))
+    with pytest.raises(FileNotFoundError):
+        get_demo_files(os.path.join(path_prefix, "nonexistent_directory"))
+    with tempfile.TemporaryDirectory() as tmpdirname:
+        # empty directory
+        with pytest.raises(ValueError):
+            get_demo_files(tmpdirname)
+        # invalid file
+        invalid_fname = os.path.join(tmpdirname, "mydemo.notademo")
+        with open(invalid_fname, "w") as f:
+            f.write("I'm not a demo")
+        with pytest.raises(ValueError):
+            get_demo_files(invalid_fname)
+        # invalid directory
+        with pytest.raises(ValueError):
+            get_demo_files(tmpdirname)
+        # valid file
+        valid_fname = os.path.join(tmpdirname, "mydemo.demo")
+        with open(valid_fname, "w") as f:
+            f.write("I'm a demo file")
+        assert get_demo_files(valid_fname) == [valid_fname]
+        # valid directory
+        assert get_demo_files(tmpdirname) == [valid_fname]