Type these two lines into a file and run it:

price = 19.99
print(price)

19.99

That first line made a variable. The name price now stands for the number 19.99, and your program can use that name anywhere it needs the value. Change the number, run the file again, and the output follows. Almost every program you will ever write rests on that one move: give a value a name, then work with the name.

This part is written for someone who has never stored a value in code before. If you can run a file the way Part 2 showed, you are ready for all of it. You need Python 3 and about twenty minutes, nothing else to install. Copy or retype each block and run it exactly as shown, and you should see the same output on your screen.

What a variable really is

The equals sign in Python does not mean 'is equal to' the way it does in maths. It means 'take the value on the right and attach this name to it'. Read price = 19.99 as: make the name price point at 19.99. Because it is just a name, you can point it somewhere else later.

You can repoint a name whenever you like. Watch the value change:

price = 19.99
price = 24.50
print(price)

24.50

The first value is simply forgotten. There are a few naming rules worth knowing now: a name can hold letters, digits, and underscores, it cannot start with a digit, and it cannot contain spaces. Most Python code uses lowercase words joined by underscores, such as total_spent, because it reads well and matches what you will see in real projects.

The four types you will meet first

Every value in Python has a type, and the type decides what you can do with the value. You can multiply two numbers, but multiplying two pieces of text makes no sense. Four types cover almost everything a beginner touches.

You can ask Python the type of any value with type(). This is handy when a program behaves oddly and you want to check what you are really holding.

print(type('Notebook'))
print(type(3))
print(type(19.99))

<class 'str'>
<class 'int'>
<class 'float'>

The word class here just means type. So 'Notebook' is of type str, 3 is an int, and 19.99 is a float.

Whole numbers: int

Integers are counting numbers with no decimal point. They add, subtract, and multiply the way you expect. Division is the one surprise: dividing with a single slash always gives a float, even when the answer is whole.

print(6 + 4)
print(10 / 2)

10
5.0

That 5.0 is not a mistake. A single slash is float division, so its result carries a decimal point. If you want a whole number back, use two slashes for floor division: 10 // 2 gives 5.

Decimals: float

Floats hold numbers with a fractional part, like prices and averages. They have one quirk that looks like a bug the first time you meet it.

print(0.1 + 0.2)

0.30000000000000004

You did nothing wrong. Computers store decimals in binary, and a few fractions cannot be written exactly in binary, so tiny rounding errors creep in. For money and for display, round the result: round(0.1 + 0.2, 2) gives 0.3. Keep this in the back of your mind. It comes back the moment you total up real numbers.

Text: str

A string is text inside quotes. Single or double quotes both work in Python, as long as the pair matches. You can join strings with a plus sign and repeat them with a star.

first = 'Pra'
last = 'nay'
print(first + last)
print('ha' * 3)

Pranay
hahaha

Yes or no: bool

A bool is either True or False, always capitalised. You rarely type them by hand. They fall out of comparisons, and you will lean on them in Part 5 when your program starts making decisions.

print(10 > 3)
print(5 == 4)

True
False

Turning one type into another

Types do not mix on their own, so you often convert a value from one type to another. Each type has a function named after it that does the job.

Notice the last row. Turning a float into an int does not round, it chops off everything after the decimal point, so int(3.9) is 3, not 4. If you want proper rounding, use round() instead.

Here is where the mixing rule bites. This looks reasonable and fails:

count = '10'
print(count + 5)

Traceback (most recent call last):
  File "prog.py", line 2, in <module>
    print(count + 5)
TypeError: can only concatenate str
(not "int") to str

A traceback looks alarming, but read the last line first, always. It says TypeError and then explains plainly: you tried to join a str and an int with a plus sign, and Python refuses to guess whether you meant to add numbers or stick text together. The value '10' is text because of the quotes. Convert it, and the code works:

count = '10'
print(int(count) + 5)

15

This is not a rare edge case. In Part 4 you will ask the user to type something, and whatever they type arrives as a string every single time. The int() and float() conversions you just saw are what turn that typed text into numbers you can add up.

print('My Expense Tracker')
amount = 250.0
category = 'Groceries'
print(category, amount)

My Expense Tracker
Groceries 250.0

3 Notebook at 19.99 each = 59.97

name = 'Notebook'
price = 19.99
qty = 3
total = price * qty
print(qty, name, 'at', price,
      'each =', round(total, 2))

3 Notebook at 19.99 each = 59.97

Build a one line receipt, step by step

Types click into place when you use them together on something small and real. Build a single receipt line that a shop till might print, one piece at a time, so you can see each type doing its job. Start with the raw facts as variables.

item = 'Coffee'
price = 3.5
qty = 4

Three variables, three types: a str, a float, and an int. Nothing prints yet. Next work out the total, which is a plain multiplication of the float by the int.

item = 'Coffee'
price = 3.5
qty = 4
total = price * qty
print(total)

14.0

The result is a float because one of the values was a float. Now try to build a readable line by joining the pieces with a plus sign, the way you joined strings earlier. This is the natural next step, and it fails on purpose.

print(item + ' costs ' + total)

Traceback (most recent call last):
  File "receipt.py", line 6, in <module>
    print(item + ' costs ' + total)
TypeError: can only concatenate str
(not "float") to str

Same lesson, new dress. The last line names the problem: you tried to join a str and a float. The value total is a number, and the plus sign will not glue a number onto text. Convert it with str() and the line comes together.

line = item + ' costs ' + str(total)
print(line)

Coffee costs 14.0

That works, and it is worth pausing on why. Every time text and a number meet, one of them has to change type. Here you pushed the number into text with str() so the whole line is one string. There is a cleaner way to stitch text and numbers together without all these conversions, and it arrives in Part 10. For now, the plus sign with str() shows you exactly what is happening under the hood, which is the better thing to understand first.

A few questions before Part 4

Do I have to say what type a variable is when I make it? No. Unlike some languages, Python reads the type from the value you assign. Write age = 25 and it is an int; write age = 25.0 and it is a float. You can even repoint the same name to a different type later, though that often signals a confusing design.

When do I use single quotes versus double quotes for text? They behave the same. Pick one and stay consistent. The one time it matters is when the text itself contains a quote: if your string has an apostrophe, wrap it in double quotes so the apostrophe does not end the string early.

Why did my division give 5.0 instead of 5? A single slash is always float division in Python 3, so it returns a float. Use two slashes, 10 // 2, for whole number division when you want a plain int back.

Is it bad that 0.1 + 0.2 is not exactly 0.3? It is not a Python flaw. Every language that uses standard floats behaves this way. For anything involving money or display, round the result to the number of decimal places you need.

What happens if I use a variable I never created? Python stops with a NameError telling you the name is not defined. Usually it is a typo in the name. Check the spelling against where you first assigned it.

You can now store values in named variables, tell Python's four everyday types apart, convert text to numbers and back, and read a TypeError instead of fearing it.

Part 4 opens the door to a real conversation with your program: you will ask the user a question, read what they type, and feed it into the tracker. Keep expenses.py exactly where it is. We build straight on top of it.

References

Python tutorial: an informal introduction, numbers and strings
Python docs: built-in types
Python docs: floating point arithmetic, issues and limitations