Using DataPipes

Suppose that we want to load data from CSV files with the following steps:

  • List all CSV files in a directory

  • Load CSV files

  • Parse CSV file and yield rows

There are a few built-in DataPipes that can help us with the above operations.

As an example, the source code for CSVParser looks something like this:

class CSVParserIterDataPipe(IterDataPipe):
    def __init__(self, dp, **fmtparams) -> None:
        self.dp = dp
        self.fmtparams = fmtparams

    def __iter__(self) -> Iterator[Union[Str_Or_Bytes, Tuple[str, Str_Or_Bytes]]]:
        for path, file in self.source_datapipe:
            stream = self._helper.skip_lines(file)
            stream = self._helper.strip_newline(stream)
            stream = self._helper.decode(stream)
            yield from self._helper.return_path(stream, path=path)  # Returns 1 line at a time as List[str or bytes]

As mentioned in a different section, DataPipes can be invoked using their functional forms (recommended) or their class constructors. A pipeline can be assembled as the following:

import torchdata.datapipes as dp

FOLDER = 'path/2/csv/folder'
datapipe = dp.iter.FileLister([FOLDER]).filter(filter_fn=lambda filename: filename.endswith('.csv'))
datapipe = dp.iter.FileOpener(datapipe, mode='rt')
datapipe = datapipe.parse_csv(delimiter=',')

for d in datapipe: # Iterating through the data

You can find the full list of built-in IterDataPipes here and MapDataPipes here.

Working with DataLoader

In this section, we will demonstrate how you can use DataPipe with DataLoader. For the most part, you should be able to use it just by passing dataset=datapipe as an input arugment into the DataLoader. For detailed documentation related to DataLoader, please visit this page.

For this example, we will first have a helper function that generates some CSV files with random label and data.

import csv
import random

def generate_csv(file_label, num_rows: int = 5000, num_features: int = 20) -> None:
    fieldnames = ['label'] + [f'c{i}' for i in range(num_features)]
    writer = csv.DictWriter(open(f"sample_data{file_label}.csv", "w"), fieldnames=fieldnames)
    writer.writerow({col: col for col in fieldnames})  # writing the header row
    for i in range(num_rows):
        row_data = {col: random.random() for col in fieldnames}
        row_data['label'] = random.randint(0, 9)

Next, we will build our DataPipes to read and parse through the generated CSV files:

import numpy as np
import torchdata.datapipes as dp

def build_datapipes(root_dir="."):
    datapipe = dp.iter.FileLister(root_dir)
    datapipe = datapipe.filter(filter_fn=lambda filename: "sample_data" in filename and filename.endswith(".csv"))
    datapipe = dp.iter.FileOpener(datapipe, mode='rt')
    datapipe = datapipe.parse_csv(delimiter=",", skip_lines=1)
    datapipe = row: {"label": np.array(row[0], np.int32),
                                         "data": np.array(row[1:], dtype=np.float64)})
    return datapipe

Lastly, we will put everything together in '__main__' and pass the DataPipe into the DataLoader.

from import DataLoader

if __name__ == '__main__':
    num_files_to_generate = 3
    for i in range(num_files_to_generate):
    datapipe = build_datapipes()
    dl = DataLoader(dataset=datapipe, batch_size=50, shuffle=True)
    first = next(iter(dl))
    labels, features = first['label'], first['data']
    print(f"Labels batch shape: {labels.size()}")
    print(f"Feature batch shape: {features.size()}")

The following statements will be printed to show the shapes of a single batch of labels and features.

Labels batch shape: 50
Feature batch shape: torch.Size([50, 20])

You can find more DataPipe implementation examples for various research domains on this page.

Implementing a Custom DataPipe

Currently, we already have a large number of built-in DataPipes and we expect them to cover most necessary data processing operations. If none of them supports your need, you can create your own custom DataPipe.

As a guiding example, let us implement an IterDataPipe that applies a callable to the input iterator. For MapDataPipe, take a look at the map folder for examples, and follow the steps below for the __getitem__ method instead of the __iter__ method.


The naming convention for DataPipe is “Operation”-er, followed by IterDataPipe or MapDataPipe, as each DataPipe is essentially a container to apply an operation to data yielded from a source DataPipe. For succinctness, we alias to just “Operation-er” in init files. For our IterDataPipe example, we’ll name the module MapperIterDataPipe and alias it as iter.Mapper under torchdata.datapipes.


DataSets are now generally constructed as stacks of DataPipes, so each DataPipe typically takes a source DataPipe as its first argument. Here is a simplified version of Mapper as an example:

from torchdata.datapipes.iter import IterDataPipe

class MapperIterDataPipe(IterDataPipe):
    def __init__(self, source_dp: IterDataPipe, fn) -> None:
        self.source_dp = source_dp
        self.fn = fn


  • Avoid loading data from the source DataPipe in __init__ function, in order to support lazy data loading and save memory.

  • If IterDataPipe instance holds data in memory, please be ware of the in-place modification of data. When second iterator is created from the instance, the data may have already changed. Please take IterableWrapper class as reference to deepcopy data for each iterator.

  • Avoid variables names that are taken by the functional names of existing DataPipes. For instance, .filter is is functional name that can be used to invoke FilterIterDataPipe. Having a variable named filter inside another IterDataPipe can lead to confusion.


For IterDataPipes, an __iter__ function is needed to consume data from the source IterDataPipe then apply the operation over the data before yield.

class MapperIterDataPipe(IterDataPipe):
    # ... See __init__() defined above

    def __iter__(self):
        for d in self.dp:
            yield self.fn(d)


In many cases, as in our MapperIterDataPipe example, the __len__ method of a DataPipe returns the length of the source DataPipe.

class MapperIterDataPipe(IterDataPipe):
    # ... See __iter__() defined above

    def __len__(self):
        return len(self.dp)

However, note that __len__ is optional for IterDataPipe and often inadvisable. For CSVParserIterDataPipe in the using DataPipes section below, __len__ is not implemented because the number of rows in each file is unknown before loading it. In some special cases, __len__ can be made to either return an integer or raise an Error depending on the input. In those cases, the Error must be a TypeError to support Python’s build-in functions like list(dp).

Registering DataPipes with the functional API

Each DataPipe can be registered to support functional invocation using the decorator functional_datapipe.

class MapperIterDataPipe(IterDataPipe):
   # ...

The stack of DataPipes can then be constructed using their functional forms (recommended) or class constructors:

import torchdata.datapipes as dp

# Using functional form (recommended)
datapipes1 = dp.iter.FileOpener(['a.file', 'b.file']).map(fn=decoder).shuffle().batch(2)
# Using class constructors
datapipes2 = dp.iter.FileOpener(['a.file', 'b.file'])
datapipes2 = dp.iter.Mapper(datapipes2, fn=decoder)
datapipes2 = dp.iter.Shuffler(datapipes2)
datapipes2 = dp.iter.Batcher(datapipes2, 2)

In the above example, datapipes1 and datapipes2 represent the exact same stack of IterDataPipes. We recommend using the functional form of DataPipes.


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