Every Python Concept for Beginners in 10 Minutes

Your first Python program crashed, and the error makes no sense. Ana runs a small tea shop, and this is her first program: store two tins, then print them. Python stops with a wall of text, and a type error at the bottom.

cat order.pytins = 2
print("Tins: " + tins)

python3 order.pyTraceback (most recent call last):
  File "/home/ana/shop/order.py", line 2, in <module>
    print("Tins: " + tins)
          ~~~~~~~~~^~~~~~
TypeError: can only concatenate str (not "int") to str

Nothing here is hard. Almost every beginner program is built from the same eight ideas, and this error is two of them bumping into each other. By the end, you'll read that error at a glance, and every line of Ana's finished receipt program. One of the eight hides a trap that catches almost everyone: a function that prints the right answer, yet gives back nothing. We'll see why.

That trap is so common, one Stack Overflow question about it, what is the return statement for, and how is it different from printing, has been viewed over 710 thousand times since 2011.

How does a program remember anything? With variables: names that point at values. Every command and file here is linked in the description, to run yourself.

tins equals two doesn't say tins is two. The equals sign means: make the name tins point at the value two. And every value has a type. Two is an int, a whole number. Six point five is a float, a number with a decimal point. Text in quotes is a str, short for string.

Here it is in Python's own prompt, the three arrows: each line runs when you press Enter. tins plus one gives three. type of tins says int, the quoted two is a str, and six point five is a float.

python3Python 3.15.2 (main, Nov  5 2026, 08:02:30) [GCC 14.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> tins = 2

>>> tins + 13

>>> type(tins)<class 'int'>

>>> type("2")<class 'str'>

>>> type(6.5)<class 'float'>

Types decide what plus means. Two strings are glued together: two two, in quotes. Add two ints, and you get four. So Ana's error says: you can't glue text to a number. Turn the number into text with str first, and it works.

Types matter at any size. At Dropbox, Python is the most widely used language, and its engineers added type labels to almost four million lines of it, so a checker can catch mistakes like Ana's before the code even runs.

How do you build a sentence out of names and numbers? With strings, and one small letter: f.

A string is text between quotes. Put an f in front, and anything in curly braces is filled in with a variable's value. name becomes Ana, tins becomes two, and Python turns the number into text for you: the fix for Ana's error, with less typing.

For real: Ana ordered two tins. Strings also come with tools. Dot upper gives capitals, and len counts the characters: Sencha has six.

>>> name = "Ana"

>>> f"{name} ordered {tins} tins"'Ana ordered 2 tins'

>>> name.upper()'ANA'

>>> len("Sencha")6

What if the shop sells more than one tea? A list keeps many values, in order.

A list goes in square brackets, with commas between the items. Each item has a position number, its index, and Python counts from zero: Sencha is zero, Assam is one, Oolong is two. Minus one counts back from the end.

Index zero is Sencha, one is Assam, and minus one gives Oolong, the last. Append adds Earl Grey to the end, and len now says four. Four items. So what's at index four? Not Earl Grey. It's an index error. Four items means indexes zero to three.

>>> teas = ["Sencha", "Assam", "Oolong"]

>>> teas[0]'Sencha'

>>> teas[1]'Assam'

>>> teas[-1]'Oolong'

>>> teas.append("Earl Grey")

>>> len(teas)4

>>> teas[4]Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    teas[4]
    ~~~~^^^
IndexError: list index out of range

How does Ana find one tea's price without counting positions? With a dictionary: values you look up by a key.

A dictionary goes in curly braces: a key, a colon, then its value. Sencha costs six point five; Assam costs four. Ask for a key in square brackets, and you get its value. Assign to a new key, and it's added: Oolong, eight.

The price of Assam: four point oh. After adding Oolong, there are three teas. Now ask for Matcha, which Ana doesn't sell, and Python raises a key error. Dot get asks politely instead: when the key is missing, you get the default you chose, zero.

>>> prices = {"Sencha": 6.5, "Assam": 4.0}

>>> prices["Assam"]4.0

>>> prices["Oolong"] = 8.0

>>> prices{'Sencha': 6.5, 'Assam': 4.0, 'Oolong': 8.0}

>>> prices["Matcha"]Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    prices["Matcha"]
    ~~~~~~^^^^^^^^^^
KeyError: 'Matcha'

>>> prices.get("Matcha", 0)0

So far, every line runs. How does a program choose which lines run? With if, elif, and else.

Ana's shipping rule: twenty or more ships free, ten or more pays two, anything less pays three fifty. Python checks the conditions from the top, and runs only the first one that's true. For a fourteen order, the first check is false, and the second is true.

In code, each condition ends with a colon, and the lines that belong to it are indented, usually by four spaces: the indent is how Python knows what's inside. One equals sign stores a value; two equals signs compare.

Run it, and it prints shipping two. Now mix up the signs and write one equals in the if. Python stops before running anything, and even suggests the fix: maybe you meant two equals signs.

python3 shipping.pyShipping: 2.00

python3 typo.py  File "/home/ana/shop/typo.py", line 2
    if total = 20:
       ^^^^^^^^^^
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

How do you do the same thing for every tea, without copying the line? With a loop.

For tea in teas takes the list one item at a time. First, tea is Sencha, and the indented line runs. Then tea is Assam, and it runs again. Then Oolong. When the list runs out, the loop ends.

Here's that loop in a file, and under it a second one, over range three. Range makes numbers to loop over. What do you think range three prints? Zero, one, two.

Run it: pack the Sencha, the Assam, the Oolong, one line per tea. Then zero, one, two: three numbers, starting at zero like list indexes. The end number itself is never included.

python3 teas.pyPack the Sencha
Pack the Assam
Pack the Oolong
0
1
2

How do you reuse steps, without writing them twice? With a function. And this is where that trap is hiding.

def gives a name to some indented steps. Line total takes a price and a number of tins, and prints price times tins. Run it: thirteen point oh appears, so it looks right. But cost is None, Python's word for nothing.

cat line_total.pydef line_total(price, tins):
    print(price * tins)

cost = line_total(6.5, 2)
print("Cost:", cost)

python3 line_total.py13.0
Cost: None

Here's why. Print shows a value on the screen, for a person to read. The code that called the function gets nothing back. Return hands the value back to the caller. Python's own tutorial says it: a function without return still gives back a value, None.

Change print to return, and cost is thirteen point oh: a value the rest of the program can use. That's the answer to the Stack Overflow question: print is for people, return is for your code.

cat line_total_fixed.pydef line_total(price, tins):
    return price * tins

cost = line_total(6.5, 2)
print("Cost:", cost)

python3 line_total_fixed.pyCost: 13.0

And when the program crashes anyway? Read the error, then handle it.

Back to Ana's first error. A traceback reads from the bottom up. The last line says what went wrong: a type error, gluing a str to an int. Above it, the code that failed, and above that, the file and the line number: order dot py, line two.

Some errors aren't mistakes in the code: a person types the word two, where a number belongs. int can't turn that into a number, so it raises a value error, and the program stops.

python3 ask.pyHow many tins? two
Traceback (most recent call last):
  File "/home/ana/shop/ask.py", line 1, in <module>
    tins = int(input("How many tins? "))
ValueError: invalid literal for int() with base 10: 'two'

Try runs the risky lines. If a value error happens, Python jumps to except instead of crashing, and Ana's program asks politely for a number.

Same input, the word two. No traceback this time: please type a number.

python3 ask_safe.pyHow many tins? two
Please type a number, like 2

Here's Ana's receipt program, 23 lines, and you can now read every one. A dictionary of prices, a function that returns a line's cost, try and except around the input, a loop over a list of teas, an f-string for each line, and an if for shipping.

cat receipt.pyprices = {"Sencha": 6.5, "Assam": 4.0}

def line_total(tea, tins):
    return prices[tea] * tins

try:
    tins = int(input("Tins of each tea: "))
except ValueError:
    print("Not a number, so 1 tin each")
    tins = 1

total = 0
for tea in ["Sencha", "Assam"]:
    cost = line_total(tea, tins)
    print(f"{tea}: {cost:.2f}")
    total = total + cost

if total >= 20:
    print("Free shipping")
else:
    total = total + 3.5
    print("Shipping: 3.50")
print(f"Total: {total:.2f}")

Two tins of each: twenty-one, with free shipping. And type the word two: no crash, one tin each, and fourteen with shipping.

python3 receipt.pyTins of each tea: 2
Sencha: 13.00
Assam: 8.00
Free shipping
Total: 21.00

python3 receipt.pyTins of each tea: two
Not a number, so 1 tin each
Sencha: 6.50
Assam: 4.00
Shipping: 3.50
Total: 14.00

Eight ideas. Variables, strings, lists and dictionaries hold the data. If, loops and functions decide what runs. And errors tell you exactly where to look.

Every command and the lab files are in the description, to try yourself. If this made your first error less scary, a like helps YouTube show it to other new programmers, and subscribing helps a small channel reach more beginners.

Everything here ran for real on Python 3.15.2. Thanks for watching Cloud Explainers.

Sources

• Python 3.15 tutorial (indexes start at 0; range never includes its end point; a function without return returns None; a missing key raises KeyError)
• Dropbox engineering blog, Our journey to type checking 4 million lines of Python
• Stack Overflow: What is the purpose of the return statement? How is it different from printing? (asked 2011)