# Write a DataFlow¶

## Write a Source DataFlow¶

There are several existing DataFlow, e.g. ImageFromFile, DataFromList, which you can use if your data format is simple. In general, you probably need to write a source DataFlow to produce data for your task, and then compose it with existing modules (e.g. mapping, batching, prefetching, ...).

The easiest way to create a DataFlow to load custom data, is to wrap a custom generator, e.g.:

def my_data_loader():
while True:
yield [my_array, my_label]



To write more complicated DataFlow, you need to inherit the base DataFlow class. Usually, you just need to implement the get_data() method which yields a datapoint every time.

class MyDataFlow(DataFlow):
def get_data(self):
for k in range(100):
digit = np.random.rand(28, 28)
label = np.random.randint(10)
yield [digit, label]


Optionally, you can implement the following two methods:

• size(). Return the number of elements the generator can produce. Certain tensorpack features might use it.

• reset_state(). It is guaranteed that the actual process which runs a DataFlow will invoke this method before using it. So if this DataFlow needs to do something after a fork(), you should put it here. reset_state() must be called once and only once for each DataFlow instance.

A typical example is when your DataFlow uses random number generator (RNG). Then you would need to reset the RNG here. Otherwise, child processes will have the same random seed. The RNGDataFlow base class does this for you. You can subclass RNGDataFlow to access self.rng whose seed has been taken care of.

DataFlow implementations for several well-known datasets are provided in the dataflow.dataset module, you can take them as a reference.

## More Data Processing¶

You can put any data processing you need in the source DataFlow you write, or you can write a new DataFlow for data processing on top of the source DataFlow, e.g.:

class ProcessingDataFlow(DataFlow):
def __init__(self, ds):
self.ds = ds

def get_data(self):
for datapoint in self.ds.get_data():
# do something
yield new_datapoint