os & sys Modules

os: Environment and Filesystem

import os

port = int(os.getenv("PORT", "8000"))        # default when unset
os.environ["MODE"] = "prod"                  # visible to child processes
root = Path(os.environ["HOME"])              # os.environ is a mutable mapping

for dirpath, dirnames, filenames in os.walk("src"):   # top-down by default
    ...

os.makedirs("out/logs", exist_ok=True)
os.remove("tmp.bin")            # unlink alias; raises FileNotFoundError
print(os.sep, os.path.join("a", "b"), os.name)   # '/', 'a/b', 'posix'

Prefer os.getenv(key, default) over os.environ.get for readability. Spawn external tools with subprocess.run(["git", "status"], check=True) — not os.system, which routes through the shell, returns only an exit status, and invites quoting bugs.

sys: Arguments, Paths, and Streams

import sys

if len(sys.argv) != 2:                      # argv[0] is the script path
    print("usage: tool <file>", file=sys.stderr)
    sys.exit(2)                             # raises SystemExit with code 2

sys.path.insert(0, "vendor")                # search path is a plain list
print(sys.version_info[:2], sys.platform)   # (3, 12), 'win32' / 'linux'

data = sys.stdin.read()
sys.stdout.reconfigure(encoding="utf-8")    # fix mojibake on Windows consoles
sys.stdout.write("ok\n")

sys.exit never "returns" — it raises SystemExit, which finally blocks still see, so cleanup code runs normally. sys.modules and sys.path are inspectable at runtime; sys.setrecursionlimit(10000) trades headroom for stack risk — pushing far beyond available C-stack can segfault the interpreter, not raise a Python error.

Pitfalls and Interview Angles

Platform checks: os.name gives coarse families (posix/nt); sys.platform distinguishes linux from darwin from win32. Use sys.platform when behavior differs per OS.

Encoding. Piping UTF-8 through sys.stdout under a legacy Windows code page raises or mangles text; PYTHONIOENCODING=utf-8 or reconfigure fixes it.

Environment snapshots. Children inherit os.environ at spawn time — later edits do not retroactively change a running process.

Interview angle: design a small CLI — parse sys.argv (or argparse), return nonzero exit codes on failure, write diagnostics to stderr, and describe how you would make the same script locate its data directory portably using Path(__file__) rather than the cwd.