# Python Data Reader Design Doc

During the training and testing phases, PaddlePaddle programs need to read data. To help the users write code that performs reading input data, we define the following:

- A *reader*: A function that reads data (from file, network, random number generator, etc) and yields the data items.
- A *reader creator*: A function that returns a reader function.
- A *reader decorator*: A function, which takes in one or more readers, and returns a reader.
- A *batch reader*: A function that reads data (from *reader*, file, network, random number generator, etc) and yields a batch of data items.

and also provide a function which can convert a reader to a batch reader, frequently used reader creators and reader decorators.

## Data Reader Interface

*Data reader* doesn't have to be a function that reads and yields data items. It can just be any function without any parameters that creates an iterable (anything can be used in `for x in iterable`) as follows:

```
iterable = data_reader()
```

The item produced from the iterable should be a **single** entry of data and **not** a mini batch. The entry of data could be a single item or a tuple of items. Item should be of one of the [supported types](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 etc.)

An example implementation for single item data reader creator is as follows:

```python
def reader_creator_random_image(width, height):
    def reader():
        while True:
            yield numpy.random.uniform(-1, 1, size=width*height)
    return reader
```

An example implementation for multiple item data reader creator is as follows:
```python
def reader_creator_random_image_and_label(width, height, label):
    def reader():
        while True:
            yield numpy.random.uniform(-1, 1, size=width*height), label
    return reader
```

## Batch Reader Interface

*Batch reader* can be any function without any parameters that creates an iterable (anything can be used in `for x in iterable`). The output of the iterable should be a batch (list) of data items. Each item inside the list should be a tuple.

Here are some valid outputs:

```python
# a mini batch of three data items. Each data item consist three columns of data, each of which is 1.
[(1, 1, 1),
(2, 2, 2),
(3, 3, 3)]

# a mini batch of three data items, each data item is a list (single column).
[([1,1,1],),
([2,2,2],),
([3,3,3],)]
```

Please note that each item inside the list must be a tuple, below is an invalid output:
```python
 # wrong, [1,1,1] needs to be inside a tuple: ([1,1,1],).
 # Otherwise it is ambiguous whether [1,1,1] means a single column of data [1, 1, 1],
 # or three columns of data, each of which is 1.
[[1,1,1],
[2,2,2],
[3,3,3]]
```

It is easy to convert from a reader to a batch reader:

```python
mnist_train = paddle.dataset.mnist.train()
mnist_train_batch_reader = paddle.batch(mnist_train, 128)
```

It is also straight forward to create a custom batch reader:

```python
def custom_batch_reader():
    while True:
        batch = []
        for i in xrange(128):
            batch.append((numpy.random.uniform(-1, 1, 28*28),)) # note that it's a tuple being appended.
        yield batch

mnist_random_image_batch_reader = custom_batch_reader
```

## Usage

Following is how we can use the reader with PaddlePaddle:
The batch reader, a mapping from item(s) to data layer, the batch size and the number of total passes will be passed into `paddle.train` as follows:

```python
# two data layer is created:
image_layer = paddle.layer.data("image", ...)
label_layer = paddle.layer.data("label", ...)

# ...
batch_reader = paddle.batch(paddle.dataset.mnist.train(), 128)
paddle.train(batch_reader, {"image":0, "label":1}, 128, 10, ...)
```

## Data Reader Decorator

The *Data reader decorator* takes in a single reader or multiple data readers and returns a new data reader. It is similar to a [python decorator](https://wiki.python.org/moin/PythonDecorators), but it does not use `@` in the syntax.

Since we have a strict interface for data readers (no parameters and return a single data item), a data reader can be used in a flexible way using data reader decorators. Following are a few examples:

### Prefetch Data

Since reading data may take some time and training can not proceed without data, it is generally a good idea to prefetch the data.

Use `paddle.reader.buffered` to prefetch data:

```python
buffered_reader = paddle.reader.buffered(paddle.dataset.mnist.train(), 100)
```

`buffered_reader` will try to buffer (prefetch) `100` data entries.

### Compose Multiple Data Readers

For example, if we want to use a source of real images (say reusing mnist dataset), and a source of random images as input for [Generative Adversarial Networks](https://arxiv.org/abs/1406.2661).

We can do the following :

```python
def reader_creator_random_image(width, height):
    def reader():
        while True:
            yield numpy.random.uniform(-1, 1, size=width*height)
    return reader

def reader_creator_bool(t):
    def reader:
        while True:
            yield t
    return reader

true_reader = reader_creator_bool(True)
false_reader = reader_creator_bool(False)

reader = paddle.reader.compose(paddle.dataset.mnist.train(), data_reader_creator_random_image(20, 20), true_reader, false_reader)
# Skipped 1 because paddle.dataset.mnist.train() produces two items per data entry.
# And we don't care about the second item at this time.
paddle.train(paddle.batch(reader, 128), {"true_image":0, "fake_image": 2, "true_label": 3, "false_label": 4}, ...)
```

### Shuffle

Given the shuffle buffer size `n`, `paddle.reader.shuffle` returns a data reader that buffers `n` data entries and shuffles them before a data entry is read.

Example:
```python
reader = paddle.reader.shuffle(paddle.dataset.mnist.train(), 512)
```

## Q & A

### Why does a reader return only a single entry, and not a mini batch?

Returning a single entry makes reusing existing data readers much easier (for example, if an existing reader returns 3 entries instead if a single entry, the training code will be more complicated because it need to handle cases like a batch size 2).

We provide a function: `paddle.batch` to turn (a single entry) reader into a batch reader.

### Why do we need a batch reader, isn't is sufficient to give the reader and batch_size as arguments during training ?

In most of the cases, it would be sufficient to give the reader and batch_size as arguments to the train method. However sometimes the user wants to customize the order of data entries inside a mini batch, or even change the batch size dynamically. For these cases using a batch reader is very efficient and helpful.

### Why use a dictionary instead of a list to provide mapping?

Using a dictionary (`{"image":0, "label":1}`) instead of a list (`["image", "label"]`) gives the advantage that the user can easily reuse the items (e.g., using `{"image_a":0, "image_b":0, "label":1}`) or even skip an item (e.g., using `{"image_a":0, "label":2}`).

### How to create a custom data reader creator ?

```python
def image_reader_creator(image_path, label_path, n):
    def reader():
        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()
        l.close()
    return reader

# images_reader_creator creates a reader
reader = image_reader_creator("/path/to/image_file", "/path/to/label_file", 1024)
paddle.train(paddle.batch(reader, 128), {"image":0, "label":1}, ...)
```

### How is `paddle.train` implemented

An example implementation of paddle.train is:

```python
def train(batch_reader, mapping, batch_size, total_pass):
    for pass_idx in range(total_pass):
        for mini_batch in batch_reader(): # this loop will never end in online learning.
            do_forward_backward(mini_batch, mapping)
```