Refactoring to use contexts managers, list comprehensions when more i…

…diomatic, and minor renaming to help reader clarity (#2335)

* Refactoring to use contexts managers, list comprehensions when more idiomatic, and minor renaming to help reader clarity.

* Fix flake8 warning in video_utils.py

torchvision/datasets/utils.py

@@ -95,16 +95,9 @@ def list_dir(root, prefix=False):
             only returns the name of the directories found
     """
     root = os.path.expanduser(root)
-    directories = list(
-        filter(
-            lambda p: os.path.isdir(os.path.join(root, p)),
-            os.listdir(root)
-        )
-    )
-
+    directories = [p for p in os.listdir(root) if os.path.isdir(os.path.join(root, p))]
     if prefix is True:
         directories = [os.path.join(root, d) for d in directories]
-
     return directories
 
 
@@ -119,16 +112,9 @@ def list_files(root, suffix, prefix=False):
             only returns the name of the files found
     """
     root = os.path.expanduser(root)
-    files = list(
-        filter(
-            lambda p: os.path.isfile(os.path.join(root, p)) and p.endswith(suffix),
-            os.listdir(root)
-        )
-    )
-
+    files = [p for p in os.listdir(root) if os.path.isfile(os.path.join(root, p)) and p.endswith(suffix)]
     if prefix is True:
         files = [os.path.join(root, d) for d in files]
-
     return files
 
 

torchvision/datasets/video_utils.py

@@ -1,6 +1,7 @@
 import bisect
 import math
 from fractions import Fraction
+from typing import List
 
 import torch
 from torchvision.io import (
@@ -45,20 +46,23 @@ def unfold(tensor, size, step, dilation=1):
     return torch.as_strided(tensor, new_size, new_stride)
 
 
-class _DummyDataset(object):
+class _VideoTimestampsDataset(object):
     """
-    Dummy dataset used for DataLoader in VideoClips.
-    Defined at top level so it can be pickled when forking.
+    Dataset used to parallelize the reading of the timestamps
+    of a list of videos, given their paths in the filesystem.
+
+    Used in VideoClips and defined at top level so it can be
+    pickled when forking.
     """
 
-    def __init__(self, x):
-        self.x = x
+    def __init__(self, video_paths: List[str]):
+        self.video_paths = video_paths
 
     def __len__(self):
-        return len(self.x)
+        return len(self.video_paths)
 
     def __getitem__(self, idx):
-        return read_video_timestamps(self.x[idx])
+        return read_video_timestamps(self.video_paths[idx])
 
 
 class VideoClips(object):
@@ -132,7 +136,7 @@ def _compute_frame_pts(self):
         import torch.utils.data
 
         dl = torch.utils.data.DataLoader(
-            _DummyDataset(self.video_paths),
+            _VideoTimestampsDataset(self.video_paths),
             batch_size=16,
             num_workers=self.num_workers,
             collate_fn=self._collate_fn,

torchvision/io/video.py

@@ -70,27 +70,23 @@ def write_video(filename, video_array, fps: Union[int, float], video_codec="libx
     if isinstance(fps, float):
         fps = np.round(fps)
 
-    container = av.open(filename, mode="w")
-
-    stream = container.add_stream(video_codec, rate=fps)
-    stream.width = video_array.shape[2]
-    stream.height = video_array.shape[1]
-    stream.pix_fmt = "yuv420p" if video_codec != "libx264rgb" else "rgb24"
-    stream.options = options or {}
-
-    for img in video_array:
-        frame = av.VideoFrame.from_ndarray(img, format="rgb24")
-        frame.pict_type = "NONE"
-        for packet in stream.encode(frame):
+    with av.open(filename, mode="w") as container:
+        stream = container.add_stream(video_codec, rate=fps)
+        stream.width = video_array.shape[2]
+        stream.height = video_array.shape[1]
+        stream.pix_fmt = "yuv420p" if video_codec != "libx264rgb" else "rgb24"
+        stream.options = options or {}
+
+        for img in video_array:
+            frame = av.VideoFrame.from_ndarray(img, format="rgb24")
+            frame.pict_type = "NONE"
+            for packet in stream.encode(frame):
+                container.mux(packet)
+
+        # Flush stream
+        for packet in stream.encode():
             container.mux(packet)
 
-    # Flush stream
-    for packet in stream.encode():
-        container.mux(packet)
-
-    # Close the file
-    container.close()
-
 
 def _read_from_stream(
     container, start_offset, end_offset, pts_unit, stream, stream_name
@@ -234,37 +230,35 @@ def read_video(filename, start_pts=0, end_pts=None, pts_unit="pts"):
     audio_frames = []
 
     try:
-        container = av.open(filename, metadata_errors="ignore")
+        with av.open(filename, metadata_errors="ignore") as container:
+            if container.streams.video:
+                video_frames = _read_from_stream(
+                    container,
+                    start_pts,
+                    end_pts,
+                    pts_unit,
+                    container.streams.video[0],
+                    {"video": 0},
+                )
+                video_fps = container.streams.video[0].average_rate
+                # guard against potentially corrupted files
+                if video_fps is not None:
+                    info["video_fps"] = float(video_fps)
+
+            if container.streams.audio:
+                audio_frames = _read_from_stream(
+                    container,
+                    start_pts,
+                    end_pts,
+                    pts_unit,
+                    container.streams.audio[0],
+                    {"audio": 0},
+                )
+                info["audio_fps"] = container.streams.audio[0].rate
+
     except av.AVError:
         # TODO raise a warning?
         pass
-    else:
-        if container.streams.video:
-            video_frames = _read_from_stream(
-                container,
-                start_pts,
-                end_pts,
-                pts_unit,
-                container.streams.video[0],
-                {"video": 0},
-            )
-            video_fps = container.streams.video[0].average_rate
-            # guard against potentially corrupted files
-            if video_fps is not None:
-                info["video_fps"] = float(video_fps)
-
-        if container.streams.audio:
-            audio_frames = _read_from_stream(
-                container,
-                start_pts,
-                end_pts,
-                pts_unit,
-                container.streams.audio[0],
-                {"audio": 0},
-            )
-            info["audio_fps"] = container.streams.audio[0].rate
-
-        container.close()
 
     vframes = [frame.to_rgb().to_ndarray() for frame in video_frames]
     aframes = [frame.to_ndarray() for frame in audio_frames]
@@ -293,6 +287,14 @@ def _can_read_timestamps_from_packets(container):
     return False
 
 
+def _decode_video_timestamps(container):
+    if _can_read_timestamps_from_packets(container):
+        # fast path
+        return [x.pts for x in container.demux(video=0) if x.pts is not None]
+    else:
+        return [x.pts for x in container.decode(video=0) if x.pts is not None]
+
+
 def read_video_timestamps(filename, pts_unit="pts"):
     """
     List the video frames timestamps.
@@ -326,26 +328,18 @@ def read_video_timestamps(filename, pts_unit="pts"):
     pts = []
 
     try:
-        container = av.open(filename, metadata_errors="ignore")
+        with av.open(filename, metadata_errors="ignore") as container:
+            if container.streams.video:
+                video_stream = container.streams.video[0]
+                video_time_base = video_stream.time_base
+                try:
+                    pts = _decode_video_timestamps(container)
+                except av.AVError:
+                    warnings.warn(f"Failed decoding frames for file {filename}")
+                video_fps = float(video_stream.average_rate)
     except av.AVError:
         # TODO add a warning
         pass
-    else:
-        if container.streams.video:
-            video_stream = container.streams.video[0]
-            video_time_base = video_stream.time_base
-            try:
-                if _can_read_timestamps_from_packets(container):
-                    # fast path
-                    pts = [x.pts for x in container.demux(video=0) if x.pts is not None]
-                else:
-                    pts = [
-                        x.pts for x in container.decode(video=0) if x.pts is not None
-                    ]
-            except av.AVError:
-                warnings.warn(f"Failed decoding frames for file {filename}")
-            video_fps = float(video_stream.average_rate)
-        container.close()
 
     pts.sort()