Variables and types — how a program remembers

The problem we are solving

So far our programs could remember nothing. Whatever was printed was gone.

Real work does not happen that way. In a billing program a price is calculated once and needed in three places. In a report a total has to be worked out first and used later. The program has to remember.

And there is a second problem alongside it. In the last mission "2" + "3" gave 23 while 2 + 3 gave 5 — the same symbol, two behaviours. That is because in Python's eyes those two things are of different kinds. The kind is called a type, and the largest share of a beginner's errors come from here.

By the end of this mission you can

• Store a value in a variable and change it later
• Choose names Python accepts, and that a human can read
• Recognise the four basic types: int, float, str, bool
• Inspect any value's type with type()
• Read a TypeError and tell exactly what it is complaining about

Prerequisites: Seeing with print, explaining with comments.

A variable is a name

city_count = 4

Read that line right to left: the value 4 came into existence, and the label city_count was attached to it. From now on, writing city_count hands you that value.

city_count = 4
print("Cities:", city_count)
print("Double:", city_count * 2)

Cities: 4
Double: 8

= does not mean equals. In mathematics x = 4 is a claim: x and 4 are the same. In Python = is an instruction: "store this value under that name." Which is why total = total + 5 — impossible as mathematics — is perfectly ordinary Python. It means "take what total holds now, add 5, and put the result back in total." Checking whether two things are equal has its own symbol, ==, which we will meet later.

Changing a value

Assigning a new value to a variable leaves nothing of the old one:

stock = 20
print("Before:", stock)

stock = stock - 3
print("After:", stock)

Before: 20
After: 17

One thing worth pinning down — the value is copied; the two labels do not stay wired together:

a = 10
b = a
a = 99
print(a, b)

99 10

On the line b = a, b received whatever a held at that moment, namely 10. Changing a afterwards does not change b — b was never watching a, it received a value.

Rules for names

What Python will not accept:

• Starting with a digit: 2nd_price is out, price_2 is fine
• A dash in the middle: total-price is out, Python reads it as subtraction
• A space in the middle: total price is out
• Python's reserved words: class, for, if, import, return, None cannot be used as names

What Python accepts but you should not write:

x1 = 4500           # clear today, meaningless in three weeks
Total_Price = 4500  # legal Python, but not the Python convention
l = 4500            # lowercase L and the digit one look almost identical

The professional Python convention is snake_case — lowercase, with underscores between words:

total_price = 4500
customer_name = "Rafi"
items_in_cart = 3

Fussing over names can feel like wasted time, but code is written once or twice and read twenty times. A name that explains itself does the work of a comment.

The four basic types

items = 7        # int   — a whole number
price = 249.50   # float — a number with a decimal point
name = "Rafi"    # str   — text, inside quotes
is_paid = True   # bool  — only True or False

print(type(items))
print(type(price))
print(type(name))
print(type(is_paid))

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

type() is your instrument for settling doubt. When a variable behaves strangely, the first move is to print its type. Most of the time the answer is right there.

Two small but important notes:

True and False start with a capital letter. Write true and Python treats it as a variable name and reports that it does not know it.

"7" and 7 are not the same thing. The first is text, the second is a number. They look alike and are completely distinct to Python:

a = 7
b = "7"
print(type(a), type(b))
print(a * 3)
print(b * 3)

<class 'int'> <class 'str'>
21
777

The error everyone gets

age = "22"
print(age + 1)

Traceback (most recent call last):
  File "D:\python-lab\lesson-04\demo.py", line 2, in <module>
    print(age + 1)
          ~~~~^~~
TypeError: can only concatenate str (not "int") to str

The message looks intimidating and is in fact precise: only a str can be joined to a str, not an int. Python cannot tell whether you meant to add or to join — so rather than guess, it stopped.

Read error messages from the bottom up. The last line says what is wrong; the line above it says where. Beginners often panic at the sight of a traceback and skip it entirely, but Python's error messages are the most helpful text on your screen.

The fix is to make the number a number:

age = "22"
print(int(age) + 1)

23

int() converts text to a whole number. When that works and when it collapses is covered properly in the mission on input(), because there this problem arrives every single time.

A complete example

invoice.py:

# Invoice for a single order, all figures in one place
customer_name = "Rafi"
unit_price = 249.50
quantity = 3
is_member = True

subtotal = unit_price * quantity
discount = 50.0
total = subtotal - discount

print("Customer  :", customer_name)
print("Quantity  :", quantity)
print("Subtotal  :", subtotal)
print("Discount  :", discount)
print("Total     :", total)
print("Member    :", is_member)

Customer  : Rafi
Quantity  : 3
Subtotal  : 748.5
Discount  : 50.0
Total     : 698.5
Member    : True

Notice where the real benefit is: to change the price you edit the unit_price line only, and every calculation below corrects itself. Had the numbers been typed straight into the print calls, you would have to hunt down every occurrence each time — and missing one makes the report wrong.

When it breaks

NameError: name 'total' is not defined The name you used has not been given a value yet, or it is misspelled. Python treats total and totl as two different names. Note that case matters too: Total is not total.

TypeError: can only concatenate str (not "int") to str You tried to join a number onto text. Either convert with int(), or use a comma — print("Age:", age) — because print handles mixed types itself.

SyntaxError: invalid syntax — a dash in a name On the line total-price = 5, Python sees an attempt to subtract price from total. Use an underscore: total_price.

SyntaxError: cannot assign to keyword You tried to use one of Python's reserved words as a name, such as class = 10. Pick another: class_name, grade.

NameError: name 'true' is not defined True and False start with a capital letter.

Your turn

Write a file called shipping.py containing:

1. Four variables — one int, one float, one str, one bool — all named meaningfully in snake_case
2. At least one calculation that uses two of them to produce a third variable
3. The type of all four variables, printed with type()
4. One variable that is given a value, then reassigned — printing the value at both steps

Then break it on purpose: take a str variable, write + 1 after it, and run the file. Read the whole error, especially the last line. You will meet that message many times; summoning it deliberately today makes it familiar later.