Support lazy loading in dataset

Add an option for lazy loading, which will parse the data when using it. Specifically, when calling `dataset.query`, it will start to parse the query data, and `train` and `gallery` will not be parsed in advanced. This is useful when you want to run on test dataset but train dataset is large.

See examples in market1501.py

fastreid/data/datasets/bases.py

@@ -19,9 +19,9 @@ class Dataset(object):
     This is the base class for ``ImageDataset`` and ``VideoDataset``.
 
     Args:
-        train (list): contains tuples of (img_path(s), pid, camid).
-        query (list): contains tuples of (img_path(s), pid, camid).
-        gallery (list): contains tuples of (img_path(s), pid, camid).
+        train (list or Callable): contains tuples of (img_path(s), pid, camid).
+        query (list or Callable): contains tuples of (img_path(s), pid, camid).
+        gallery (list or Callable): contains tuples of (img_path(s), pid, camid).
         transform: transform function.
         mode (str): 'train', 'query' or 'gallery'.
         combineall (bool): combines train, query and gallery in a
@@ -32,17 +32,14 @@ class Dataset(object):
 
     def __init__(self, train, query, gallery, transform=None, mode='train',
                  combineall=False, verbose=True, **kwargs):
-        self.train = train
-        self.query = query
-        self.gallery = gallery
+        self._train = train
+        self._query = query
+        self._gallery = gallery
         self.transform = transform
         self.mode = mode
         self.combineall = combineall
         self.verbose = verbose
 
-        self.num_train_pids = self.get_num_pids(self.train)
-        self.num_train_cams = self.get_num_cams(self.train)
-
         if self.combineall:
             self.combine_all()
 
@@ -56,6 +53,24 @@ class Dataset(object):
             raise ValueError('Invalid mode. Got {}, but expected to be '
                              'one of [train | query | gallery]'.format(self.mode))
 
+    @property
+    def train(self):
+        if callable(self._train):
+            self._train = self._train()
+        return self._train
+
+    @property
+    def query(self):
+        if callable(self._query):
+            self._query = self._query()
+        return self._query
+
+    @property
+    def gallery(self):
+        if callable(self._gallery):
+            self._gallery = self._gallery()
+        return self._gallery
+
     def __getitem__(self, index):
         raise NotImplementedError
 
@@ -102,15 +117,14 @@ class Dataset(object):
             for img_path, pid, camid in data:
                 if pid in self._junk_pids:
                     continue
-                pid = self.dataset_name + "_test_" + str(pid)
-                camid = self.dataset_name + "_test_" + str(camid)
+                pid = getattr(self, "dataset_name", "Unknown") + "_test_" + str(pid)
+                camid = getattr(self, "dataset_name", "Unknown") + "_test_" + str(camid)
                 combined.append((img_path, pid, camid))
 
         _combine_data(self.query)
         _combine_data(self.gallery)
 
-        self.train = combined
-        self.num_train_pids = self.get_num_pids(self.train)
+        self._train = combined
 
     def check_before_run(self, required_files):
         """Checks if required files exist before going deeper.
@@ -134,9 +148,6 @@ class ImageDataset(Dataset):
     data in each batch has shape (batch_size, channel, height, width).
     """
 
-    def __init__(self, train, query, gallery, **kwargs):
-        super(ImageDataset, self).__init__(train, query, gallery, **kwargs)
-
     def show_train(self):
         num_train_pids, num_train_cams = self.parse_data(self.train)
 

fastreid/data/datasets/market1501.py

@@ -62,11 +62,10 @@ class Market1501(ImageDataset):
             required_files.append(self.extra_gallery_dir)
         self.check_before_run(required_files)
 
-        train = self.process_dir(self.train_dir)
-        query = self.process_dir(self.query_dir, is_train=False)
-        gallery = self.process_dir(self.gallery_dir, is_train=False)
-        if self.market1501_500k:
-            gallery += self.process_dir(self.extra_gallery_dir, is_train=False)
+        train = lambda: self.process_dir(self.train_dir)
+        query = lambda: self.process_dir(self.query_dir, is_train=False)
+        gallery = lambda: self.process_dir(self.gallery_dir, is_train=False) + \
+                          (self.process_dir(self.extra_gallery_dir, is_train=False) if self.market1501_500k else [])
 
         super(Market1501, self).__init__(train, query, gallery, **kwargs)