Python First Steps

What a program is, and your first line

Goal run a Python program on this page and change what it says.

A program is a list of instructions for a computer. A computer cannot guess what you mean, so every instruction must be written in a language it understands. Python is one such language.

The computer follows the instructions one at a time, from the top line to the bottom line. Running a program means asking the computer to follow them. What the program shows you is its output.

Your first instruction is print. It makes Python show something on the screen. You put what to show inside round brackets. If it is text, you wrap the text in quote marks. The quote marks tell Python that these words are text and not an instruction.

Now run one. Press the Run button under the code below. The first time, the page downloads Python, which takes a few seconds. The output appears under the code. Then press Edit code, change the words between the quote marks, and press Run again. Nothing you type here can harm your computer, and Reset puts the original code back.

print("Hello, Leon!")

Hello, Leon!

Print("Hello")

print("Hello")

print("Hello)

print("Hello")

print(“Hello”)

print("Hello")

print "Hello"

print("Hello")

Values, names and print

Goal store numbers under names, do arithmetic with them and show the results.

A value is one piece of information, such as the number 12 or the word Mia. Python has different kinds of value. A whole number such as 12 is an integer. A number with a decimal point such as 2.5 is a float. Text is a string. You wrote one in lesson 1, between quote marks.

A variable is a name that you attach to a value, so that you can use the value later. You make one with the equals sign. The line price = 4 means: take the value 4 and give it the name price. In maths, = says that two sides are equal. In Python it is an instruction to store. Python works out the right side first and then stores the result under the name on the left. A name may contain letters, digits and underscores. It cannot start with a digit and it cannot contain spaces. Choose names that say what the value is.

After that, you can use the name wherever you could use the value. The line total = price * 3 works out 4 times 3 and stores 12 under the name total. The arithmetic symbols are + to add, - to subtract, * to multiply and / to divide. Round brackets control the order, as in school maths. A division with / always gives a float, so 10 / 4 is 2.5 and 10 / 2 is 5.0.

Be careful: a variable holds a value, not a rule. If total was worked out from count, and you change count later, total keeps its old value until you work it out again. The example shows this.

print can show several things at once. Put commas between them, and print writes a space between the pieces. A line that starts with # is a comment. Python ignores it. It is a note for the people who read the code.

# total cost of the notebooks
price = 4
count = 3
total = price * count
print("Total:", total)

count = 5
print("Still:", total)
total = price * count
print("Now:", total)

Total: 12
Still: 12
Now: 20

12 = price

price = 12

print(total)
total = 5

total = 5
print(total)

my score = 5

my_score = 5

price = 4
count = 3
total = price * count
count = 5
print(total)

price = 4
count = 3
total = price * count
count = 5
total = price * count
print(total)

Text: joining, measuring and changing

Goal join pieces of text, count its characters, change its case and put values inside text.

A string is text. You write it between quote marks, either "double" or 'single', as long as both ends match. A string can hold letters, digits, spaces and punctuation. The string "5" is text that looks like a number. It is not the number 5.

Join two strings with +. This is called concatenation. "Hello, " + "Mia" gives "Hello, Mia". The space is part of the first string, because Python adds no spaces for you. Strings and numbers do not mix with +. "Age: " + 5 is an error, because Python cannot tell whether you want to add or to join.

You have already used one function, print. A function is a named action that takes something in and, often, gives something back. Another function is len. It gives the number of characters in a string, spaces included. A method is an action that belongs to a value. You write the value, a dot, the method name and round brackets. "Mia".upper() gives "MIA". The method name must be spelled exactly, in small letters. A method never changes the original string. It gives back a new one.

A tidy way to put values inside text is an f-string. Write the letter f directly before the opening quote. Put names, or small calculations, inside curly brackets { }. Python replaces each pair of brackets with its value. f"{name} has {len(name)} letters" turns into a sentence with the real name and the real count.

name = "Mia"
greeting = "Hello, " + name + "!"
print(greeting)
print(len(name))
print(name.upper())
print(f"{name} has {len(name)} letters.")

Hello, Mia!
3
MIA
Mia has 3 letters.

age = 5
print("Age: " + age)

age = 5
print(f"Age: {age}")

print(Hello)

print("Hello")

print("5" + "5")

print(5 + 5)

name = "Mia"
print(name.Upper())

name = "Mia"
print(name.upper())

Decisions with if

Goal make a program choose between actions.

Programs often need to choose. A school app shows a pass or a fail message depending on a mark. A choice starts with a question that has a yes-or-no answer. Python writes the question as a comparison. score >= 80 asks: is score at least 80? The answer is a value called True or False. A value like that is a boolean.

The comparison symbols are: == equal to, != not equal to, < less than, > greater than, <= less than or equal to, and >= greater than or equal to. Take care with ==. One equals sign stores a value. Two equals signs ask a question.

The if statement uses a comparison. Write the word if, then the comparison, then a colon. Below it, write the lines to run, moved four spaces to the right. Moving lines to the right is called indentation. Python uses indentation to know which lines belong to the if. The indented lines run only when the comparison is True. A line that goes back to the left edge is no longer part of the if, and it always runs.

Add else: to say what to do when the answer is False. Add elif (short for else if) to ask a second question when the first answer was False. Python checks the questions from the top, runs the lines under the first True one, and skips the rest. So the order of the questions matters.

score = 72
if score >= 80:
    print("Grade: A")
elif score >= 60:
    print("Grade: B")
else:
    print("Grade: C")
print("Done")

Grade: B
Done

score = 80
if score = 80:
    print("Exactly 80")

score = 80
if score == 80:
    print("Exactly 80")

score = 80
if score >= 80
    print("A")

score = 80
if score >= 80:
    print("A")

score = 80
if score >= 80:
print("A")

score = 80
if score >= 80:
    print("A")

score = 90
if score >= 60:
    print("Grade: B")
elif score >= 80:
    print("Grade: A")

score = 90
if score >= 80:
    print("Grade: A")
elif score >= 60:
    print("Grade: B")

Repeating with for and while

Goal repeat steps with a for loop and a while loop, and know which to pick.

A loop repeats lines of code. Without loops, a program that adds up 100 numbers would need 100 lines. Python has two kinds of loop.

A for loop repeats once for each item in a group of values. The simplest group comes from range. range(1, 6) gives the whole numbers 1, 2, 3, 4 and 5. It starts at the first number and stops before the second one. range(3) is short for range(0, 3) and gives 0, 1 and 2. You write for number in range(1, 6): and then the lines to repeat, indented by four spaces. Each time round, number holds the next value. One pass through the indented lines is called an iteration.

A while loop repeats as long as a comparison stays True. Python checks the comparison before every pass. Something inside the loop must change what the comparison sees. Otherwise it stays True forever, which is called an infinite loop, and the program never stops. In a terminal, press Control and C to stop it. On this page, do not run a loop that never ends. Reload the page if that happens.

Use for when you know what to go through, or how many times. Use while when you repeat until something happens. A common pattern is a running total: start a variable at 0, and add to it inside the loop.

total = 0
for number in range(1, 6):
    total = total + number
    print("added", number, "total is", total)
print("Sum:", total)

count = 3
while count > 0:
    print("Countdown:", count)
    count = count - 1
print("Liftoff")

added 1 total is 1
added 2 total is 3
added 3 total is 6
added 4 total is 10
added 5 total is 15
Sum: 15
Countdown: 3
Countdown: 2
Countdown: 1
Liftoff

for n in range(5):
    print(n)

for n in range(1, 6):
    print(n)

count = 3
while count > 0:
    print(count)

count = 3
while count > 0:
    print(count)
    count = count - 1

for i in 5:
    print(i)

for i in range(5):
    print(i)

total = 0
for n in range(1, 4):
    total = total + n
print("running total", total)

total = 0
for n in range(1, 4):
    total = total + n
    print("running total", total)

Lists

Goal keep several values together, read one by its position and go through all of them.

A list keeps many values in order under one name. Write the values between square brackets, separated by commas: scores = [70, 85, 90]. A list can hold numbers, strings or a mix. The function len tells you how many items a list holds.

Each item has a position, called an index. Counting starts at 0, not 1. In scores, the item at index 0 is 70, at index 1 is 85 and at index 2 is 90. Write scores[0] to get the first item. A negative index counts from the end, so scores[-1] is the last item. If you ask for a position that does not exist, Python stops with an IndexError. Strings use the same idea: "Mia"[0] is "M".

A list can change. The method append adds one item at the end. scores.append(60) changes scores itself. It gives nothing back.

A for loop can go through a list item by item: for score in scores: is the same for loop as in lesson 5, with a list in place of range. With a list you can total and average a set of marks, which is what the example does.

scores = [70, 85, 90]
print(scores[0])
print(scores[-1])
scores.append(60)
print(len(scores))
total = 0
for score in scores:
    total = total + score
print("Average:", total / len(scores))

70
90
4
Average: 76.25

scores = [70, 85, 90]
print(scores[3])

scores = [70, 85, 90]
print(scores[2])

scores = [70, 85, 90]
print(scores[1])

scores = [70, 85, 90]
print(scores[0])

scores = [70, 85]
scores = scores.append(60)
print(len(scores))

scores = [70, 85]
scores.append(60)
print(len(scores))

scores = [70, 85, 90]
print(scores(0))

scores = [70, 85, 90]
print(scores[0])

Functions

Goal write a function that takes inputs and returns a result, and then call it.

You have used functions already: print, len and range. A function is a named set of steps that you can use again and again. You can write your own.

Write def, then the name, then round brackets holding the parameters, then a colon. A parameter is a name for an input. The function uses it like a variable. The steps of the function are indented below. The word return sends a result back to the code that used the function. That result is the return value.

Defining a function does not run it. The def line only teaches Python a new name. To run it, call the function: write its name and give it values in round brackets, such as average(class_a). The values you give are called arguments. Each argument is stored in the matching parameter, in order.

Names created inside a function exist only while it runs. They are local. That protects you: a function cannot accidentally change variables elsewhere in the program.

return and print are different. print shows something on the screen for a person. return hands a value back to the program, so that it can be stored, added or printed. If a function finishes without return, it gives back None, which means no value. A return also ends the function at once.

def average(numbers):
    total = 0
    for number in numbers:
        total = total + number
    return total / len(numbers)

def percent(part, whole):
    return part / whole * 100

class_a = [70, 85, 90, 60]
class_b = [50, 100]
print("Class A:", average(class_a))
print("Class B:", average(class_b))
print("Passed:", percent(3, 4), "percent")

Class A: 76.25
Class B: 75.0
Passed: 75.0 percent

def greet(name):
    print("Hello, " + name)

def greet(name):
    print("Hello, " + name)

greet("Mia")

def square(x):
    print(x * x)

y = square(4) + 1

def square(x):
    return x * x

y = square(4) + 1

def square(x):
    return x * x

print(square(2, 3))

def square(x):
    return x * x

print(square(2))

def total_of(numbers):
    total = 0
    for n in numbers:
        total = total + n
        return total

def total_of(numbers):
    total = 0
    for n in numbers:
        total = total + n
    return total

Reading an error message and fixing it

Goal find the kind, the place and the cause of an error, and fix a broken program one message at a time.

Every programmer sees error messages every day. They are not a sign that you failed. An error message is Python telling you where it stopped and why. The skill is to read it calmly, in the right order.

A mistake found while the program runs gives a message of this shape:
Traceback (most recent call last):
File "hello.py", line 4, in <module>
total = totl + price
NameError: name 'totl' is not defined. Did you mean: 'total'?

Read it from the bottom. The last line is the most useful. It has the kind of error (here NameError), a colon, and a sentence about what went wrong. Just above it is the line of your program where Python stopped. Above that are the file name and the line number. On this page the file is called main.py. Traceback means the trail of steps Python took to get there; in a short program the trail has one step. Newer versions of Python also print ^ marks under the exact part of the line, and the wording of messages differs a little between versions.

A mistake in the way a line is written is a syntax error. Python finds these before it runs anything. So the message has no Traceback, and the program prints nothing at all. Python shows the line and a ^ mark near the problem. The real mistake is at that spot or just before it.

Do the same four things every time. First, read the last line aloud. Second, go to the line number it names. Third, change one thing. Fourth, run again. Python stops at the first error, so fixing one often shows you the next. That is progress.

prices = [4, 6, 5]
total = 0
for price in prices:
    total = total + price
average = total / len(prices)
print("Total:", total)
print("Average:", average)

Total: 15
Average: 5.0

count = 0
print(10 / count)

count = 0
if count > 0:
    print(10 / count)
else:
    print("Nothing to divide")

print("Total:", (4 + 5)
print("Done")

print("Total:", (4 + 5))
print("Done")

print("a")
    print("b")

print("a")
print("b")