Wrapping External Programs

HTMap can only map Python functions, but you might need to call an external program on the execute node. For example, you may need to use a particular Bash utility script, or run a piece of pre-compiled analysis software. In cases like this, the Python standard library’s subprocess module can be used to communicate with those programs.

For example, suppose you need to call the Dubious Barology Lyricon (dbl) program, a pre-compiled C program that you have stored in your home directory at ~/dbl. It takes a single integer argument, and “returns” a single integer by printing it to standard output. So a call to dbl on the command line looks like

$ dbl 4

To use HTMap with dbl, you could write a mapped function that looks something like

import subprocess
import htmap

def dbl(x):
    process = subprocess.run(
        ["dbl", str(x)],
        stdout=subprocess.PIPE,  # use capture_output = True in Python 3.7+

    if process.returncode != 0:
        raise Exception("call to dbl failed!")

    return_value = int(process.stdout)

    return return_value

You’ll need to be careful with functions like this - check for failures in the programs you call, because HTMap will happily return nonsense if the call fails in some strange way. If we do a map, we’ll end up with the expected result:

result = dbl.map(range(10))

print(list(result))  # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

If you want to test this yourself, here’s the Dubious Barology Lyricon (really a simple bash program):

#!/usr/bin/env bash

echo $((2 * $1))

If your external program outputs files, you may find the Output Files recipe helpful.