Getting Started with Python (the useful parts)

Python is the glue language of modern AI, but you do not need every language feature to be productive. You need a clean environment, a handful of data structures, and the habit of writing small scripts that touch real files and take real arguments. This post gets you there with one tool you build, run, and extend yourself — not a wall of syntax you’ll forget by Friday.

Install and isolate

python3 --version          # aim for 3.10+
python3 -m venv .venv
source .venv/bin/activate  # Windows (PowerShell): .venv\Scripts\Activate.ps1
python -c "import sys; print(sys.executable)"   # confirm it points inside .venv
pip install --upgrade pip

Two things trip up almost everyone new to Python:

• On Ubuntu/Debian the command is python3, not python, until you activate a venv (which usually symlinks python for you). If python is “not found,” check that first.
• python -c "import sys; print(sys.executable)" is your sanity check. If it prints a system path instead of something ending in .venv/bin/python, your venv isn’t active — and any pip install you run next pollutes the wrong environment.

One project → one virtualenv. Never pip install into the system Python for learning work; it turns into unremovable clutter within a month.

Core ideas in one screen

from pathlib import Path

# list comprehension: transform + filter in one line
lengths = [len(w) for w in "a longer sentence here".split() if len(w) > 3]

# dict comprehension: build lookups fast
by_len = {w: len(w) for w in ["cat", "elephant", "ox"]}

# `with` guarantees the file handle closes even if read() raises
text = Path("notes.txt").read_text(encoding="utf-8") if Path("notes.txt").exists() else ""

Mini project: a real CLI tool, not a toy snippet

Most “getting started” posts show you a script you run once. Build a small command-line tool instead — it forces you to touch arguments, files, and error paths, which is where Python actually gets used.

#!/usr/bin/env python3
"""wordcount.py - rank word frequency across one or more text files."""
import argparse
import re
from collections import Counter
from pathlib import Path

STOPWORDS = {
    "the", "a", "an", "and", "or", "but", "is", "are", "was", "were",
    "to", "of", "in", "on", "for", "it", "this", "that",
}


def read_words(path: Path) -> list[str]:
    text = path.read_text(encoding="utf-8").lower()
    return re.findall(r"[a-z']+", text)


def main() -> None:
    parser = argparse.ArgumentParser(description="Count word frequency in text files.")
    parser.add_argument("files", nargs="+", type=Path, help="one or more text files")
    parser.add_argument("-n", "--top", type=int, default=10, help="how many words to show")
    parser.add_argument("--no-stopwords", action="store_true", help="drop common stopwords")
    args = parser.parse_args()

    counts: Counter[str] = Counter()
    for path in args.files:
        if not path.exists():
            raise SystemExit(f"error: {path} does not exist")
        counts.update(read_words(path))

    if args.no_stopwords:
        for word in STOPWORDS:
            counts.pop(word, None)

    for word, n in counts.most_common(args.top):
        print(f"{n:5d}  {word}")


if __name__ == "__main__":
    main()

Run it end to end:

printf "the quick brown fox jumps over the lazy dog. the dog barks at the fox.\n" > notes.txt
python wordcount.py notes.txt
python wordcount.py notes.txt --no-stopwords -n 5
python wordcount.py missing.txt      # exercises the error path — should exit cleanly, not traceback

The first run shows the at the top with count 4. The second, with stopwords stripped, promotes dog and fox (count 2 each) to the top. The third should print your SystemExit message and exit with a non-zero code — try echo $? right after to confirm.

Extend it yourself: add a --min-length flag, or read from stdin when no files are given (if not sys.stdin.isatty(): ...).

Common footguns

• Mutable default arguments — def f(x=[]) reuses the same list across every call. Use def f(x=None): x = x or [].
• Bare except: — swallows KeyboardInterrupt and typos alike. Catch the specific exception you can actually handle.
• range(len(x)) instead of enumerate(x) — works, but hides intent and invites off-by-one bugs.
• Comparing floats with == — 0.1 + 0.2 == 0.3 is False. Use math.isclose().
• Forgetting .lower() before comparing strings — silent case-mismatch bugs that “work on my test data.”
• Mixing tabs and spaces — Python 3 raises TabError; configure your editor to show whitespace and insert spaces only.

You know you’re done when…

• python -c "import sys; print(sys.executable)" prints a path inside .venv, not your system Python
• python wordcount.py notes.txt runs with no traceback and prints ranked counts
• --no-stopwords visibly changes which words rank highest
• python wordcount.py missing.txt fails with your error message, not a raw FileNotFoundError traceback
• You can explain, without looking it up, why def f(x=[]) is a bug

Next

• Python in 10 Days — structured sprint
• Getting Started with PyTorch — tensors next
• Getting Started with FastAPI — wrap scripts like this one behind HTTP
• LLM Mastery — when you want language models from first principles