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Stage 1 · Python & Data

60 min readbeginnerView source on GitHub ↗

Python Basics

Why this matters

Every lesson after this one assumes you can read and write basic Python. Not clever Python — just enough to store a value, loop over some data, make a decision, and package repeated logic into a function. That’s the whole toolkit this lesson builds, and it’s the same toolkit you’ll still be using when you write your first neural network many lessons from now.

The concept

Variables and types. A variable is a labelled box that holds a value. age = 36 creates a box named age and puts 36 in it. Python figures out the type of value from what you put in — you never declare it yourself.

Type Example Meaning
int 36 a whole number
float 1.68 a number with a decimal point
str "Ada" text, in quotes
bool True one of exactly two values: True or False

Types matter because operations mean different things for different types: "3" + "4" gives "34" (joining text), while 3 + 4 gives 7 (adding numbers). Mixing them without converting first (int("3") + 4) is one of the most common beginner errors.

Lists and dicts. A list is an ordered sequence you access by position: languages[0] is the first item. A dict (dictionary) is a lookup table you access by key instead of position: profile["name"] gets whatever value was stored under the key "name", regardless of where it sits. Reach for a list when order matters and items are similar (“all my scores”); reach for a dict when you’re labelling different pieces of one thing (“a person’s name, age, and city”).

for and if. A for loop repeats a block of code once per item in a collection: for lang in languages: runs its indented body once for each language, binding lang to the current one each time. An if statement branches: it runs its indented body only when the condition is True, and an optional else block runs otherwise. Indentation is not just style in Python — it’s how the language knows which lines belong to the loop or the branch. Four spaces per level is the standard, and mixing tabs and spaces will break your code.

Functions. A function packages a piece of logic under a name so you can reuse it without retyping it. def greet(person_name): defines a function called greet that takes one input (a parameter named person_name); the indented body runs each time you call it, like greet("Ada"). return sends a value back out to whoever called the function; print just displays something and sends nothing back — they’re easy to confuse when you’re starting out, but only return lets you use the result afterwards.

Imports. Nobody writes everything from scratch. import math loads Python’s built-in math toolkit into your program so you can use things like math.sqrt(9). Later lessons will import numpy and import pandas the same way — those aren’t part of core Python, which is what virtual environments are for.

Virtual environments. A virtual environment is an isolated, project-local folder of installed packages, so that project A can use pandas version 1 while project B uses version 2 without them colliding. You create one with python -m venv .venv, then activate it (source .venv/bin/activate on macOS/Linux, .venv\Scripts\activate on Windows) before installing anything with pip install. Every serious Python project you touch from here on should have its own virtual environment — it’s the difference between “it works on my machine” and packages fighting each other across every project you have.

Reading a traceback. When Python hits an error it can’t recover from, it prints a traceback instead of your program’s normal output. Read it from the bottom up: the last line names the error type and what went wrong (IndexError: list index out of range); the lines above it show the exact line of code that caused it, and above that, what called that code, and so on. Beginners often panic and reread from the top — train yourself to read the last line first, then walk upward only if you need more context.

In code

name = "Ada"
age = 36
height = 1.68
is_learning = True

print(type(name), type(age), type(height), type(is_learning))

languages = ["python", "sql", "python"]

profile = {"name": name, "age": age, "languages": languages}

for lang in languages:
    if lang == "python":
        print(lang, "is great for beginners")
    else:
        print(lang, "is also useful")


def greet(person_name):
    return f"Hello, {person_name}!"


print(greet(profile["name"]))

import math

print(round(math.sqrt(profile["age"]), 2))
<class 'str'> <class 'int'> <class 'float'> <class 'bool'>
python is great for beginners
sql is also useful
python is great for beginners
Hello, Ada!
6.0

Now the same list, but read past its end — this is the traceback you’ll learn to read:

prices = [10, 20, 30]
print(prices[5])
Traceback (most recent call last):
  File "script.py", line 2, in <module>
    print(prices[5])
IndexError: list index out of range

Bottom line first: IndexError: list index out of range — you asked for a position that doesn’t exist in a 3-item list. The line above shows exactly which line did it. That’s the whole skill.

Build this

Write a script that stores a list of numbers, then reports the mean, min, and max — without using any library (no import, no sum()/min()/max() built-ins either; use a for loop to compute all three yourself). Start from a list like:

numbers = [4, 8, 15, 16, 23, 42]

Your script should loop once over numbers, tracking a running total plus the smallest and largest value seen so far, and print all three results at the end.

Stretch: wrap your logic in a function summarize(numbers) that returns the three values, call it on two or three different lists, and add an if that prints "empty list" instead of crashing when you pass it []. Then try running it on [] without that check first, so you see the traceback it would otherwise produce.

Go deeper

Next: NumPy