Your first cell: Python right in the browser

The investigator's tool

SQL pulls data out of a database. Python helps with everything else: gluing tables together, computing metrics, drawing charts, hunting anomalies. SQL can do some of this too — the tools overlap, and that is fine.

Here Python runs right in the browser: the same machine that opened this page executes your code. Nothing to install, nothing to break.

"A bear has moved into my archive. A panda, to be precise. Get used to her — she will be fetching your tables. But not today." — QUERY

This lesson takes three things as its foundation: variables, arithmetic and print. Everything else — lists, loops, how functions work — waits for chapter 2, and not before these three feel ordinary.

We will read code by one scheme, worth memorizing:

line of code → what Python did → what is now in memory → what got printed

Start with a three-line cell. Press Run.

What happened, line by line

Python executes code top to bottom, one line at a time: it reads a line, does what it says, moves on.

Line 1: orders = 1284

• what Python did: put the number 1284 in memory and hung the label orders on it;
• what is now in memory: orders → 1284;
• what got printed: nothing. Assignment is silent — the first thing that surprises beginners.

The = sign here is not mathematical equality. Read it as an arrow right to left: "take the value on the right and remember it under the name on the left". A name with a value is called a variable.

Line 2: print(orders)

• what Python did: looked at what lies under the label orders and printed it;
• what is now in memory: unchanged — print only reads;
• what got printed: 1284.

Line 3: print('Orders on March 14:', orders)

• what Python did: printed two values separated by a space — first the quoted text, then the variable's value;
• what got printed: Orders on March 14: 1284.

About the quotes: quoted text is something Python does not try to understand, it prints it literally. So print(orders) prints 1284, while print('orders') would print the word orders. That is exactly the day-one mistake everyone makes.

print is the output command: everything passed inside the parentheses, comma-separated, lands on screen separated by spaces. Strictly it is a function, and what a function is comes in chapter 2; for now treat it as a "show me" button.

And last: memory lives until the end of the cell. The variable from line 1 is available on every line below — every calculation rests on that.

A variable is a label, not a box

The second assignment did not create a second variable. It moved the label orders onto a new number:

after line 1:   orders → 1284
after line 4:   orders → 1301     (the old value is gone)

The old value disappears without a trace — if you still need it, you had to keep it under a different name: orders_yesterday = 1284, orders_today = 1301.

That is why variable names matter more than they seem on day one: a, b, x1 will baffle even their author a week later, while orders_today reads itself. The rules in this course are simple: Latin letters, digits and underscores, never start with a digit, no spaces — hence avg_check, not avg check.

Now let us compute something meaningful — on an example where it is easy to lie to yourself.

Turnover is not revenue

The average check is the average amount of one order. Multiply it by the number of orders:

1284 orders × 730 ₽ = 937,320 ₽

What is that number? The temptation to call it "revenue" is enormous — and it is the course's first terminology trap.

937,320 ₽ is turnover, also known as GMV (gross merchandise value): the total value of all orders that passed through the platform. Buyers did pay that money — but they paid it to the sellers, not to Kotomarket.

Kotomarket is a marketplace: it does not own the food and the beds, it matches sellers with buyers. Its own money is the commission on each order, plus ad and paid-delivery income. At a 12% commission the platform's revenue for that day is:

937,320 ₽ × 0.12 ≈ 112,478 ₽

Nearly an eightfold gap. Mixing these numbers up in a report is not a slip of the tongue — it is a different business.

For an ordinary shop selling its own goods, turnover and revenue coincide — no confusion there. It appears exactly where an intermediary appears: a marketplace, a ride aggregator, a delivery service.

L., entry #8: "The board was shown a slide reading 'revenue 2.3M a day'. That was GMV. The commission is 280 thousand, and the whole company lives on it. Nobody corrected it. I did. I was asked not to spoil the mood."

Arithmetic and comments

Comments. Everything after # to the end of the line is skipped by Python — a note for humans. In commission_rate = 0.12 # platform commission, 12% the variable names the number and the comment names the unit. Both labels are free and both save you a month later.

Arithmetic. Four signs cover nearly everything an analyst needs:

Precedence is the ordinary school one: * and / first, then + and -, parentheses above all. (1412 - 41) * 695 and 1412 - 41 * 695 are two very different numbers.

Computation inside an assignment. The line gmv = orders * avg_check runs in two beats: Python first computes the right-hand expression (1284 * 730 → 937320) and only then hangs the label gmv on the result. What sits in memory afterwards is a number, not a formula: change orders later and gmv will not recompute itself.

round(...) rounds to a whole number: round(112478.4) → 112478.

Three patterns you already know

Five cells — and you hold three calculations that half of analytics is built from:

product:     gmv = orders * avg_check                'how much in total'
share:       conversion = buyers / visitors * 100    'what fraction'
remainder:   delivered = shipped - lost              'how many are left'

In chapter 1 the same three formulas apply to DAU, funnels and retention — only the variable names change.

Now the assignments. They work like this: you finish the code in the cell and press the button — the system runs your code and checks the result, and if it does not add up it tells you what is off. Being wrong is fine: unlimited attempts, and a hint is right there.