Add example of how to use environment variables in Python scripts

Author: gjbex

source-code/README.md

@@ -28,6 +28,8 @@ to create it.  There is some material not covered in the presentation as well.
     configuration files.
 1. `data-formats`: illustrates how to deal with data formats such as CSV
     files, binary data and XML.
+1. `enviroment-variables`: illustrates how to use environment variables to set paths
+   in a script.
 1. `hydra`: Facebook Hydra application framework illustration.
 1. `jinja`: illustration of how to use the jinja2 template library.
 1. `logging`: illustration of Python's logging facilities.

source-code/enviroment-variables/README.md

@@ -0,0 +1,17 @@
+# Environment variables
+
+This directory shows some examples of how to use environment variables in a Python
+script.
+
+## What is it?
+
+1. `accumulate_data.py`: script that will read all `.txt` files in a directory defined
+  in an environment variable `DATA_DIR`.
+1. `data`: directory that contains two text files with data.
+
+## How to use it?
+
+```bash
+$ export DATA_DIR="$(pwd)/data"
+$ ./accumulate_data.py
+```

source-code/enviroment-variables/accumulate_data.py

@@ -0,0 +1,26 @@
+#!/usr/bin/env python
+
+from glob import glob
+import os
+from pathlib import Path
+import sys
+
+
+if __name__ == '__main__':
+    if 'DATA_DIR' in os.environ:
+        data_dir = Path(os.environ['DATA_DIR'])
+        print(f"### info: looking for data in '{data_dir}'", file=sys.stderr)
+        total = 0.0
+        for data_file in data_dir.glob('*.txt'):
+            with open(data_file, 'r') as file:
+                for line in file:
+                    line = line.strip()
+                    if not line:
+                        continue
+                    try:
+                        total += float(line)
+                    except ValueError as error:
+                        print(f'### warning: {error}', file=sys.stderr)
+        print(f'total = {total}')
+    else:
+        print('### error: DATA_DIR not set', file=sys.stderr)

source-code/enviroment-variables/data/file1.txt

@@ -0,0 +1,4 @@
+1.2
+2.3
+3.4
+

source-code/enviroment-variables/data/file2.txt

@@ -0,0 +1,5 @@
+4.5
+5.6
+# dubious values
+6.7
+7.8