Variables & Comments

Everything you'll ever build in Python rests on one humble idea: giving a name to a value so you can use it later. A price, a stock symbol, a quantity, the result of a calculation - each gets a name, and from then on you refer to it by that name. This is the variable, and it's the single most important concept in the whole of programming. Good news: you already understand it, because you've used it since school algebra. Let's make it concrete.

What is a variable?

A variable is a named box that holds a value. You put something in the box and write a label on it; later, you fetch it by its label. In Python you create one with a single = sign:

buy_price = 1300.00

Read that out loud as "buy_price gets 1300.00" - the name on the left, the value on the right. The = here does not mean "equals" in the maths sense; it means "put this value into this name." (The "are these equal?" question uses ==, which we'll meet in Chapter 6.)

Storing the pieces of a trade

Variables really shine when you combine them. Here's an entire stock position described with nothing but names and arithmetic - notice how readable it is:

# A single stock position, described entirely with variables.
symbol = "RELIANCE"       # which stock we are holding
quantity = 50             # how many shares
buy_price = 1300.00       # the price we paid per share
last_price = 1313.60      # the latest market price

# Variables can be combined in calculations, just like in algebra.
invested = quantity * buy_price          # money we put in
current_value = quantity * last_price    # what the holding is worth now
profit = current_value - invested        # unrealised profit (could be negative)

print("Position:", symbol)
print("Invested:", invested)
print("Current value:", current_value)
print("Profit:", profit)

Position: RELIANCE
Invested: 65000.0
Current value: 65680.0
Profit: 680.0

Read it top to bottom and it almost speaks English: we hold 50 shares of RELIANCE bought at 1300, the last price is 1313.60, so the holding is worth 65,680 and we're up 680. The computer doesn't care what we call things - but we do, and good names turned four numbers into a story.

Variables can change

The "variable" is well named - the value can vary. Point the same name at a new value any time, and the old one is simply forgotten. This is exactly how you'd track a price that keeps moving through the day:

# A variable is not fixed - you can re-assign it a new value any time.
price = 1300.00
print("Opening price:", price)

price = 1308.75          # the market moved; point the same name at a new value
print("Midday price:", price)

price = 1313.60          # and again
print("Latest price:", price)

Opening price: 1300.0
Midday price: 1308.75
Latest price: 1313.6

Each price = ... line re-labels the box with a fresh number. There's no history kept - price always holds whatever you last put in it. (When we do want to keep every value, we'll reach for a list, in Chapter 9.)

Good names save you (and bad ones cost you)

Python lets you name a variable almost anything, so the discipline is on you. Compare:

x = 50 * 1300        # what on earth is this?
invested = quantity * buy_price   # ...oh, obviously.

Both run identically. One you'll understand in six months; the other you won't understand in six minutes. A few simple rules and habits:

• Names can use letters, digits and underscores, but can't start with a digit and can't contain spaces.
• The Python style is snake_case - lowercase words joined by underscores: buy_price, last_traded_price, stop_loss.
• Spell things out. qty is fine, q is a riddle. Favour clarity over cleverness.

Comments: notes to your future self

A comment is a note in your code that Python ignores completely. Anything after a # on a line is for human eyes only:

stop_loss = buy_price * 0.98   # exit if price falls 2% below our entry

The best comments explain the why, not the what. The code already says what it does (buy_price * 0.98); the comment adds the bit the code can't - that this is a 2% stop. Don't narrate the obvious; explain the reasoning, the assumption, or the gotcha.

Try it yourself

• In 01_a_position.py, change quantity to 100 and re-run. Both the value and the profit should double - one edit, ripples everywhere. That's the power of naming.
• Rename the variable last_price to something shorter and worse, like p, everywhere it appears. It still runs - but notice how much harder the code is to read. Then change it back.
• Add a comment above the profit line explaining, in your own words, why it can be a negative number.

Recap

• A variable is a name pointing to a value, created with name = value ("name gets value").
• = means assign, not "equals"; combine variables in calculations just like algebra.
• Variables can be re-assigned freely - the name always holds whatever you last put in it.
• Good names (clear, snake_case, readable aloud) turn numbers into a story; bad names cost you later.
• Comments (# ...) are notes Python ignores - use them to explain the why, not the obvious what.

You've been writing numbers like 1300.00 and 50 without thinking about what kind of thing each one is. But Python very much cares about the difference between a whole number, a decimal, a piece of text and a yes/no answer. In the next chapter we meet the basic data types - and learn how to convert between them.