Python · OS, sys, argparse & subprocess

You run os.system("grep error app.log") and want the matching lines in a Python variable. Why doesn't that work, and what should you use?

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More in OS, sys, argparse & subprocess

You run an external tool with subprocess.run(["mytool", "--check"]). It fails (exit code 1) but your Python script keeps going as if nothing happened. How do you make the failure raise?

What does `r.stdout` hold after this, and why is `text=True` important? ``` r = subprocess.run( ["echo", "hi"], capture_output=True, text=True) ```

What are sys.stdin, sys.stdout, and sys.stderr, and why send error/diagnostic messages to stderr rather than stdout?

You read a config value with os.environ["API_KEY"]. It works on your machine but crashes with KeyError on a fresh deploy where the var isn't set. What's the safe way to read it?

What does this print when run as `python prog.py hello`? ``` import sys print(len(sys.argv)) print(sys.argv[0]) ```

A script is called `python add.py 2 3` with this body. What does it print? ``` import sys print(sys.argv[1] + sys.argv[2]) ```

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