add cache reader

test=develop

paddle/fluid/API.spec

@@ -511,6 +511,7 @@ paddle.fluid.unique_name.guard ArgSpec(args=['new_generator'], varargs=None, key
 paddle.fluid.recordio_writer.convert_reader_to_recordio_file ArgSpec(args=['filename', 'reader_creator', 'feeder', 'compressor', 'max_num_records', 'feed_order'], varargs=None, keywords=None, defaults=(Compressor.Snappy, 1000, None))
 paddle.fluid.recordio_writer.convert_reader_to_recordio_files ArgSpec(args=['filename', 'batch_per_file', 'reader_creator', 'feeder', 'compressor', 'max_num_records', 'feed_order'], varargs=None, keywords=None, defaults=(Compressor.Snappy, 1000, None))
 paddle.fluid.Scope Scope() -> paddle.fluid.core._Scope
+paddle.reader.cache ArgSpec(args=['reader'], varargs=None, keywords=None, defaults=None)
 paddle.reader.map_readers ArgSpec(args=['func'], varargs='readers', keywords=None, defaults=None)
 paddle.reader.buffered ArgSpec(args=['reader', 'size'], varargs=None, keywords=None, defaults=None)
 paddle.reader.compose ArgSpec(args=[], varargs='readers', keywords='kwargs', defaults=None)

python/paddle/fluid/reader.py

@@ -123,7 +123,6 @@ class PyReader(object):
         self._use_double_buffer = use_double_buffer
         self._capacity = capacity
         self._feed_list = feed_list
-        self._scope = global_scope()
         if not self._iterable:
             self._init_non_iterable()
 
@@ -153,7 +152,7 @@ class PyReader(object):
         reader_name = PyReader.unique_name_generator('create_py_reader')
         double_buffer_name = PyReader.unique_name_generator('double_buffer')
 
-        var = self._scope.var(queue_name)
+        var = global_scope().var(queue_name)
         self._queue = core.init_lod_tensor_blocking_queue(var, self._capacity)
 
         startup_blk = default_startup_program().current_block()
@@ -215,6 +214,9 @@ class PyReader(object):
             def __iter__(self):
                 return self
 
+            def __next__(self):
+                return self.next()
+
             def next(self):
                 ret = self._reader.read_next()
                 if ret:

python/paddle/reader/decorator.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 __all__ = [
-    'map_readers', 'buffered', 'compose', 'chain', 'shuffle',
+    'cache', 'map_readers', 'buffered', 'compose', 'chain', 'shuffle',
     'ComposeNotAligned', 'firstn', 'xmap_readers', 'PipeReader',
     'multiprocess_reader', 'Fake'
 ]
@@ -33,6 +33,31 @@ import zlib
 import paddle.compat as cpt
 
 
+def cache(reader):
+    """
+    Cache the reader data into memory. 
+
+    Be careful that this method may take long time to process, 
+    and consume lots of memory. :code:`reader()` would only 
+    call once. 
+
+    Args:
+        reader (generator): a reader object which yields 
+            data each time.
+
+    Returns:
+        reader (generator): a decorated reader object 
+            which yields data from cached memory.
+    """
+    all_data = tuple(reader())
+
+    def __impl__():
+        for item in all_data:
+            yield item
+
+    return __impl__
+
+
 def map_readers(func, *readers):
     """
     Creates a data reader that outputs return value of function using