iterable-subprocess

Python context manager to communicate with a subprocess using iterables: for when data is too big to fit in memory and has to be streamed.

Data is sent to a subprocess's standard input via an iterable, and extracted from its standard output via another iterable. This allows an external subprocess to be naturally placed in a chain of iterables for streaming processing.

Installation

pip install iterable-subprocess

Usage

A single context manager iterable_subprocess is exposed. The first parameter is the args argument passed to the Popen Constructor, and the second is an iterable whose items must be bytes instances and are sent to the subprocess's standard input.

Returned from the function is an iterable whose items are bytes instances of the process's standard output.

from iterable_subprocess import iterable_subprocess

# In a real case could be a generator function that reads from the filesystem or the network
iterable_of_bytes = (
    b'first\n',
    b'second\n',
    b'third\n',
)

with iterable_subprocess(['cat'], iterable_of_bytes) as output:
    for chunk in output:
        print(chunk)

Exceptions

Python's subprocess.Popen is used to start the process, and any exceptions it raises are propagated without transformation. For example, if the subprocess can't be found, then a FileNotFoundError is raised.

If the process starts, but exits with a non-zero return code, then an iterable_subprocess.IterableSubprocessError exception will be raised with two members:

• returncode - the return code of the process
• stderr - the final 65536 bytes of the standard error of the process

However, if the process starts, but an exception is raised from inside the context, then this exception is propagated, even if the process subsequently exits with a non-zero return code.

Example: unzip the first file of a ZIP archive while downloading

It's possible to download the bytes of a ZIP file in Python, and unzip by passing the bytes to funzip, as in the following example.

import httpx
from iterable_subprocess import iterable_subprocess

with \
        httpx.stream('GET', 'https://www.example.com/my.zip') as r, \
        iterable_subprocess(['funzip'], r.iter_bytes()) as unzipped_chunks:

    for chunk in unzipped_chunks:
        print(chunk)

Note that it's also possible to stream unzip files without resorting to another process using stream-unzip.

Example: download file using curl and process in Python

You would usually download directly from Python, but as an example, you can download using the curl executable and process its output in Python.

from iterable_subprocess import iterable_subprocess

url = 'https://data.api.trade.gov.uk/v1/datasets/uk-tariff-2021-01-01/versions/v3.0.212/tables/measures-on-declarable-commodities/data?format=csv'
with iterable_subprocess(['curl', '--no-progress-meter', '--fail-with-body', url], ()) as output:
    for chunk in output:
        print(chunk)