Processors: Post-download actions

Processors: Post-download actions#

Post-download actions sometimes need to be taken on downloaded files (unzipping, conversion to a more efficient format, etc). If these actions are time or memory consuming, it might be worth doing them only once after the file is downloaded. This is a way of trading disk space for computation time. pooch.Pooch.fetch and pooch.retrieve accept the processor argument to handle these situations.

Processors are Python callable objects (like functions or classes with a __call__ method) that are executed after a file is downloaded to perform these actions. They must have the following format:

def myprocessor(fname, action, pooch):
    '''
    Processes the downloaded file and returns a new file name.

    The function **must** take as arguments (in order):

    fname : str
        The full path of the file in the local data storage
    action : str
        Either: "download" (file doesn't exist and will be downloaded),
        "update" (file is outdated and will be downloaded), or "fetch"
        (file exists and is updated so no download is necessary).
    pooch : pooch.Pooch
        The instance of the Pooch class that is calling this function.

    The return value can be anything but is usually a full path to a file
    (or list of files). This is what will be returned by Pooch.fetch and
    pooch.retrieve in place of the original file path.
    '''
    ...
    return full_path

The processor is executed after a file downloaded attempted (whether the download actually happens or not) and before returning the path to the downloaded file. The processor lets us intercept the returned path, perform actions, and possibly return a different path.

Pooch provides built-in processors for common tasks, like decompressing files and unpacking tar and zip archives. See the List of functions and classes (API) for a full list.

Common uses cases for processors include:

Creating your own processors#

Let’s say we want to implement the pooch.Unzip processor ourselves to extract a single file from the archive. We could do that with the following function:

import os
from zipfile import ZipFile


def unpack(fname, action, pup):
    """
    Post-processing hook to unzip a file and return the unzipped file name.

    Parameters
    ----------
    fname : str
       Full path of the zipped file in local storage
    action : str
       One of "download" (file doesn't exist and will download),
       "update" (file is outdated and will download), and
       "fetch" (file exists and is updated so no download).
    pup : Pooch
       The instance of Pooch that called the processor function.

    Returns
    -------
    fname : str
       The full path to the unzipped file. (Return the same fname is your
       processor doesn't modify the file).

    """
    # Create a new name for the unzipped file. Appending something to the
    # name is a relatively safe way of making sure there are no clashes
    # with other files in the registry.
    unzipped = fname + ".unzipped"
    # Don't unzip if file already exists and is not being downloaded
    if action in ("update", "download") or not os.path.exists(unzipped):
        with ZipFile(fname, "r") as zip_file:
            # Extract the data file from within the archive
            with zip_file.open("actual-data-file.txt") as data_file:
                # Save it to our desired file name
                with open(unzipped, "wb") as output:
                    output.write(data_file.read())
    # Return the path of the unzipped file
    return unzipped


def fetch_zipped_file():
    """
    Load a large zipped sample data as a pandas.DataFrame.
    """
    # Pass in the processor to unzip the data file
    fname = GOODBOY.fetch("zipped-data-file.zip", processor=unpack)
    # fname is now the path of the unzipped file which can be loaded by
    # pandas directly
    data = pandas.read_csv(fname)
    return data

Similarly, you could build any custom processor function so long as it receives the fname, action, pup arguments. Example use cases for this would be:

  • Converting data from a download-friendly format (compressed and minimal file size) to a more user friendly format (easy to open and fast to load into memory).

  • Add missing metadata to data from public servers. You might be using public data that has known issues (poorly formated entries, missing metadata, etc) which can be fixed when the file is downloaded.

The main advantage to using a processor for these actions is that they are performed only when the file is downloaded. A modified version of the file can be kept on disk so that loading the file is easier. This is particularly convenient if the processor task takes a long time to run.

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