Add asynchronous output handling example

Author: gjbex

source-code/subprocess/README.md

@@ -8,5 +8,8 @@ The exit code can also be retrieved.
 
 ## What is it?
 
-`wc_metrics.py`: this script computes the number of characters, words, and
-lines in a file using the shell's `wc` commaand.
+1. `wc_metrics.py`: this script computes the number of characters, words, and
+  lines in a file using the shell's `wc` commaand.
+1. `async_handling.py`: this script illustrates how to monitor the standard error
+   generated by a subprocess.
+1. `process.sh`: Bash script to execute by `async_handling.py`.

source-code/subprocess/async_handling.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python
+
+from datetime import datetime
+import subprocess
+import sys
+import time
+
+class Result:
+
+    def set(self, value):
+        self._value = value
+
+    @property
+    def value(self):
+        return self._value
+
+
+def execute(cmd, result):
+    process = subprocess.Popen(cmd,
+                               stdout=subprocess.PIPE,
+                               stderr=subprocess.PIPE,
+                               text=True)
+    for stderr_line in iter(process.stderr.readline, ''):
+        yield stderr_line.strip()
+    stdout, stderr = process.communicate()
+    return_code = process.wait()
+    if return_code:
+        raise subprocess.CalledProcessError(return_code, cmd)
+    else:
+        result.set(stdout)
+
+result = Result()
+for stderr_line in execute(['bash', 'process.sh'], result):
+    print(f'{datetime.now()}: {stderr_line}')
+print(f'result:\n{result.value}')

source-code/subprocess/process.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+for i in {1..4}
+do
+    (>&2 echo "### info: $i in progress...")
+    sleep 5
+    echo "item $i: $((i**2))"
+done