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+# Python Data Reader Design Doc
+
+Paddle reads data from data reader during training. It will be passed into `paddle.train` as a parameter.
+
+## Data Reader Interface
+
+Data reader is a function with no parameter that creates a iterable (anything can be used in `for x in iterable`):
+
+```
+iterable = data_reader()
+```
+
+Element produced for the iterable should be a **single** entry of data, **not** a mini batch. That entry of data could be a single item, or a tuple of items. Item should be of [supported type](http://www.paddlepaddle.org/doc/ui/data_provider/pydataprovider2.html?highlight=dense_vector#input-types) (e.g., numpy 1d array of float32, int, list of int)
+
+An example implementation for single item data reader:
+
+```python
+def data_reader_fake_image():
+	while True:
+		yield numpy.random.uniform(-1, 1, size=20*20)
+```
+
+An example implementation for multiple item data reader:
+```python
+def data_reader_fake_image_and_label():
+	while True:
+		yield numpy.random.uniform(-1, 1, size=20*20), False
+```
+
+## Usage
+
+data reader, mapping from item(s) read to data layer, batch size and number of total pass will be passed into `paddle.train`:
+
+```python
+# two data layer is created:
+image_layer = paddle.layer.data("image", ...)
+label_layer = paddle.layer.data("label", ...)
+
+# ...
+
+paddle.train(paddle.dataset.mnist, {"image":0, "label":1}, 128, 10, ...)
+```
+
+## Data Reader Decorators
+
+Data reader decorators takes a single or multiple data reader, returns a new data reader. It is similar to a [python decorator](https://wiki.python.org/moin/PythonDecorators), but it does not use `@` syntax.
+
+Since we have a strict interface for data readers (no parameter, return a single data item). Data reader can be used flexiable via data reader decorators. Following are a few examples:
+
+### Prefetch Data
+
+Since reading data may take time and training can not proceed without data. It is generally a good idea to prefetch data.
+
+Use `paddle.reader.buffered` to prefetch data:
+
+```python
+buffered_reader = paddle.reader.buffered(paddle.dataset.mnist, 100)
+```
+
+`buffered_reader` will try to buffer (prefetch) `100` data entries.
+
+### Compose Multiple Data Readers
+
+For example, we want to use a source of real images (reusing mnist dataset), and a source of fake images as input for [Generative Adversarial Networks](https://arxiv.org/abs/1406.2661).
+
+We can do:
+
+```python
+def data_reader_fake_image():
+	while True:
+		yield numpy.random.uniform(-1, 1, size=20*20)
+
+def data_reader_bool(t):
+	while True:
+		yield t
+
+true_reader = lambda : data_reader_bool(True)
+false_reader = lambda : data_reader_bool(False)
+
+reader = paddle.reader.combine(paddle.dataset.mnist, data_reader_fake_image, true_reader, false_reader)
+# Skipped 1 because paddle.dataset.mnist produces two items per data entry.
+# And we don't care second item at this time.
+paddle.train(reader, {"true_image":0, "fake_image": 2, "true_label": 3, "false_label": 4}, ...)
+```
+
+### Shuffle
+
+Given shuffle buffer size `n`, `paddle.reader.shuffle` will return a data reader that buffers `n` data entries and shuffle them before a data entry is read.
+
+Example:
+```python
+reader = paddle.reader.shuffle(paddle.dataset.mnist, 512)
+```
+
+## Q & A
+
+### Why return only a single entry, but not a mini batch?
+
+If a mini batch is returned, data reader need to take care of batch size. But batch size is a concept for training, it makes more sense for user to specify batch size as a parameter for `train`.
+
+Practically, always return a single entry make reusing existing data reader much easier (e.g., if existing data reader return not a single entry but 3 entries, training code will be more complex because it need to handle cases like batch size 2).
+
+### Why use a dictionary but not a list to provide mapping?
+
+We decided to use dictionary (`{"image":0, "label":1}`) instead of list (`["image", "label"]`) is because that user can easily resue item (e.g., using `{"image_a":0, "image_b":0, "label":1}`) or skip item (e.g., using `{"image_a":0, "label":2}`).
+
+### How to create custom data reader
+
+```python
+def image_reader(image_path, label_path):
+	f = open(image_path)
+	l = open(label_path)
+	images = numpy.fromfile(
+		f, 'ubyte', count=n * 28 * 28).reshape((n, 28 * 28)).astype('float32')
+	images = images / 255.0 * 2.0 - 1.0
+	labels = numpy.fromfile(l, 'ubyte', count=n).astype("int")
+	for i in xrange(n):
+		yield images[i, :], labels[i] # a single entry of data is created each time
+	f.close()
+
+# use python lambda to change image_reader into a function with no parameters.
+reader = lambda : image_reader("/path/to/image_file", "/path/to/label_file")
+paddle.train(reader, {"image":0, "label":1}, ...)
+```
+
+### How is `paddle.train` implemented
+
+An example implementation of paddle.train could be:
+
+```python
+def minibatch_decorater(reader, minibatch_size):
+	def ret():
+		r = reader()
+		buf = [r.next() for x in xrange(minibatch_size)]
+		while len(buf) > 0:
+			yield buf
+			buf = [r.next() for x in xrange(minibatch_size)]
+	return ret
+
+def train(reader, mapping, batch_size, total_pass):
+	for pass_idx in range(total_pass):
+		for mini_batch in minibatch_decorater(reader): # this loop will never end in online learning.
+			do_forward_backward(mini_batch, mapping)
+```