📎 This article includes 2 downloadable practice files ↓
In this article
A variable is a name for a value, like naming a cell in Excel. Instead of remembering that B2 holds the quantity, you write qty = 12 and use qty from then on.
Creating variables
customer = "Sharma Traders" # text (str)
qty = 12 # whole number (int)
rate = 60.0 # decimal (float)
gst_rate = 0.18
paid = False # True/False (bool)Names: lowercase with underscores (gst_rate), no spaces, can’t start with a number. # starts a comment.
Maths
amount = qty * rate # 720.0
gst = round(amount * gst_rate, 2)
total = amount + gst| Operator | Example | Result |
|---|---|---|
| + – * / | 7 / 2 | 3.5 (always a decimal) |
| // | 7 // 2 | 3 (whole-number division) |
| % | 7 % 2 | 1 (remainder, like MOD) |
| ** | 10 ** 2 | 100 (power) |
f-strings: readable output
print(f"{customer}: {qty} x Rs {rate:,.2f} = Rs {amount:,.2f}")
# Sharma Traders: 12 x Rs 60.00 = Rs 720.00Put an f before the quotes and variables in {}. After a colon come formats: , thousands separator, .2f two decimals, .0% percentage.
Text and numbers don’t mix automatically
int("42") + 8 # 50
str(42) + "8" # "428"
"42" + 8 # TypeError!Data read from files arrives as text, so converting with int() and float() is something you’ll do constantly.
Useful text methods
customer.upper() # 'SHARMA TRADERS'
customer.lower()
" ravi kumar ".strip().title() # 'Ravi Kumar' (like TRIM + PROPER)
len(customer) # 14
customer.replace("Traders", "Trading Co")
"INV-2026-0145".split("-") # ['INV', '2026', '0145']Where beginners go wrong
| Error | Cause |
|---|---|
| NameError: name ‘Qty’ is not defined | Python is case-sensitive: qty ≠ Qty |
| TypeError: can only concatenate str | Joining text and a number; use an f-string |
| 0.1 + 0.2 = 0.30000000000000004 | Normal floating point; round() money |
Practice
Download the script (and data file, if listed) below into one folder. Open a terminal in that folder and run python lesson-NN.py. Compare with the expected output, then change something and run it again: that’s how Python is learned.
📎 Practice files for this article
- 🐍Lesson 2 scriptThe complete script from this lesson, tested.⬇ PY · 682 B
- 📄Expected outputWhat the script prints when you run it.⬇ TXT · 197 B
Free to use for learning. Files with macros (.bas) are plain text — import them with Alt+F11 → File → Import File, and always test on a copy.
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