Working With Files#

Files are one of the most fundamental entities that users of Python work with, and they are fully supported by Flyte. In the IDL, they are known as Blob literals which are backed by the blob type.

Let’s assume our mission here is pretty simple. We download a few csv file links, read them with the python built-in csv.DictReader() function, normalize some pre-specified columns, and output the normalized columns to another csv file.

First, let’s import the libraries.

import csv
import os
from collections import defaultdict
from typing import List

import flytekit
from flytekit import task, workflow
from flytekit.types.file import FlyteFile

Next, we write a task that accepts a FlyteFile, a list of column names, and a list of column names to normalize, then outputs a csv file of only the normalized columns. For this example we’ll use z-score normalization, i.e. mean-centering and standard-deviation-scaling.


The FlyteFile literal can be scoped with a string, which gets inserted into the format of the Blob type (“jpeg” is the string in FlyteFile[typing.TypeVar("jpeg")]). The format is entirely optional, and if not specified, defaults to "".

def normalize_columns(
    csv_url: FlyteFile,
    column_names: List[str],
    columns_to_normalize: List[str],
    output_location: str,
) -> FlyteFile:
    # read the data from the raw csv file
    parsed_data = defaultdict(list)
    with open(csv_url, newline="\n") as input_file:
        reader = csv.DictReader(input_file, fieldnames=column_names)
        for row in (x for i, x in enumerate(reader) if i > 0):
            for column in columns_to_normalize:

    # normalize the data
    normalized_data = defaultdict(list)
    for colname, values in parsed_data.items():
        mean = sum(values) / len(values)
        std = (sum([(x - mean) ** 2 for x in values]) / len(values)) ** 0.5
        normalized_data[colname] = [(x - mean) / std for x in values]

    # write to local path
    out_path = os.path.join(
    with open(out_path, mode="w") as output_file:
        writer = csv.DictWriter(output_file, fieldnames=columns_to_normalize)
        for row in zip(*normalized_data.values()):
            writer.writerow({k: row[i] for i, k in enumerate(columns_to_normalize)})

    if output_location:
        return FlyteFile(path=out_path, remote_path=output_location)
        return FlyteFile(path=out_path)

When the image URL is sent to the task, the Flytekit engine translates it into a FlyteFile object on the local drive (but doesn’t download it). The act of calling download method should trigger the download, and the path attribute enables to open the file.

If the output_location argument is specified, it will be passed to the remote_path argument of FlyteFile, which will use that path as the storage location instead of a random location (Flyte’s object store).

When this task finishes, the Flytekit engine returns the FlyteFile instance, uploads the file to the location, and creates a Blob literal pointing to it.

Lastly, we define a normalize_csv_files workflow. Note that there is an output_location argument specified in the workflow. This is passed to the location input of the task. If it’s not an empty string, the task attempts to upload its file to that location.

def normalize_csv_file(
    csv_url: FlyteFile,
    column_names: List[str],
    columns_to_normalize: List[str],
    output_location: str = "",
) -> FlyteFile:
    return normalize_columns(

Finally, we can run the workflow locally.

if __name__ == "__main__":
    default_files = [
            ["Name", "Sex", "Age", "Heights (in)", "Weight (lbs)"],
            ["Index", "Eruption length (mins)", "Eruption wait (mins)"],
            ["Eruption length (mins)"],
    print(f"Running {__file__} main...")
    for index, (csv_url, column_names, columns_to_normalize) in enumerate(
        normalized_columns = normalize_csv_file(
            f"Running normalize_csv_file workflow on {default_files}: "

Total running time of the script: ( 0 minutes 0.000 seconds)

Gallery generated by Sphinx-Gallery