Your First Program & How Python Remembers Things

A program is just a recipe. Python reads it one line at a time, from top to bottom, and does what each line says. The simplest instruction is print() — it shows something on the screen.

Press Play and watch the arrow move down the code while the output appears.

1print("Hello, world!")
2print("I am learning Python")
3print(2 + 3)

Many books say a variable is a "box" that holds a value. That picture breaks later. The real picture is simpler: Python makes an object in memory (the value), then sticks a name tag on it.

Step through this and watch each name point to its object. Every object gets an ID number — its address in memory.

1age = 21
2name = "Ada"
3print(name, "is", age)
4age = age + 1
5print(name, "is now", age)

x = 10
y = x
x = 20
print(y)

Numbers and text can never change once made (they're immutable). Lists can be changed in place (they're mutable). Combine that with name tags and you get Python's most famous surprise:

1a = [1, 2, 3]
2b = a
3b.append(4)
4print(a)
5print(a is b)

b = a doesn't copy anything — it puts a second tag on the same list. So adding to b also changes what a sees. is asks “are these the same object?”

Now compare with a real copy — watch a second list object appear:

1a = [1, 2, 3]
2b = a.copy()
3b.append(4)
4print(a)
5print(b)
6print(a is b)

nums = [5, 6]
more = nums
more.append(7)

Ali and Sam share a shopping list by accident. Edit the code so Sam gets "bread" but Ali's list stays ["milk", "eggs"]. Press Run & visualise — the diagram shows whether you really have two lists.

# Bug: changing the shopping list for Sam also changes it for Ali!
ali = ["milk", "eggs"]
sam = ali
sam.append("bread")
print("Ali:", ali)
print("Sam:", sam)

1# Bug: changing the shopping list for Sam also changes it for Ali!
2ali = ["milk", "eggs"]
3sam = ali
4sam.append("bread")
5print("Ali:", ali)
6print("Sam:", sam)

Want to keep your script? Paste it into Online Notepad++ and save it as shopping.py.

• Python runs your code top to bottom, one line at a time.
• A variable is a name tag pointing to an object in memory.
• x = y never copies — it adds another tag to the same object.
• Numbers and text are immutable; lists are mutable. Use .copy() when you need your own list.