Free Python course

What Python is

What it means. Python is a language for giving a computer instructions.

A computer does nothing on its own.

You tell it what to do, one line at a time.

Python is one of the clearest languages for doing that.

People use Python for

• counting and adding up figures
• sorting and tidying lists of data
• reading and writing files
• drawing charts
• building websites

The same four ideas cover all of those uses.

You will meet all four in this course.

print("Mill Lane Bakery")

Mill Lane Bakery

Showing something with print

What it means. print is an instruction that shows a value on the screen.

You write print, then round brackets.

Inside the brackets you put what you want shown.

Text goes inside quote marks.

A print line has three parts

• the word print
• a pair of round brackets
• the value you want to see

Miss a bracket and Python stops.

It tells you which line it could not read.

print("Open at 7am")
print("Four things on sale today")

Open at 7am
Four things on sale today

Code and output are different things

What it means. Code is what you write. Output is what the computer shows back.

The box on this page has two parts.

You type code at the top.

The output appears underneath after you press Run.

Watch for these

• code sits in the top box
• output sits in the bottom box
• quote marks are code, so they never appear in the output
• an error replaces the output

The quote marks tell Python where the text starts and ends.

They are not part of the text itself.

print("3 + 4")
print(3 + 4)

3 + 4
7

Putting a value in a variable

What it means. A variable is a name that holds a value you can use later.

You choose the name yourself.

One equals sign puts a value into it.

After that, the name stands for the value.

Rules for a variable name

• use letters, numbers and the underscore
• never start with a number
• no spaces in the name
• choose a name that says what it holds

A clear name saves you reading the whole program later.

Write loaves_sold rather than x.

loaves_sold = 12
print(loaves_sold)

12

The four basic types

What it means. A type is what kind of thing a value is.

Python treats text and numbers differently.

It needs to know which it has.

Most values you meet are one of four types.

The four types

• string, which is text in quote marks
• integer, which is a whole number
• float, which is a number with a decimal point
• boolean, which is True or False

A price needs a float, because it has pence.

A count of loaves needs an integer, because you cannot sell half a loaf.

A float is not always exact, so money sums sometimes need rounding.

item = "sourdough"
count = 12
price = 3.50
fresh = True
print(type(item))
print(type(count))
print(type(price))
print(type(fresh))

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

Changing what a variable holds

What it means. You can put a new value into a variable at any time.

Assigning again replaces the old value.

The old value is gone.

Python keeps only what you put in last.

Common ways to change a value

• assign a new value outright
• add to the value and assign the result
• use the shorter form with plus equals

The shorter form does the same job in fewer characters.

Both lines below leave the same number behind.

loaves_sold = 12
loaves_sold = loaves_sold + 5
print(loaves_sold)
loaves_sold += 4
print(loaves_sold)

17
21

Joining text

What it means. To concatenate is to join two strings end to end.

The plus sign joins two strings.

It does not add a space for you.

You put any space you want inside the quote marks.

What plus does

• between two strings, it joins them
• between two numbers, it adds them
• between a string and a number, it fails

That last case is one of the most common beginner errors.

You will see the message for it in topic 6.

first = "Mill Lane"
second = "Bakery"
print(first + " " + second)

Mill Lane Bakery

Putting a value inside a sentence

What it means. An f-string is a string that can hold values inside it.

Put the letter f before the opening quote.

Then put any value in curly brackets.

Python replaces the brackets with the value.

An f-string is useful because

• you do not have to join pieces with plus
• numbers go in without being turned into text first
• the line reads like the sentence it produces

This is the normal way to build a sentence in modern Python.

Use it instead of plus whenever a value is involved.

item = "sourdough"
count = 12
print(f"Ama sold {count} {item} loaves.")

Ama sold 12 sourdough loaves.

Methods that change text

What it means. A method is an instruction that belongs to a value.

You write the value, a full stop, then the method.

The method gives back a new string.

It never changes the original.

Three methods you will use often

• upper() gives the text in capital letters
• lower() gives the text in small letters
• strip() removes spaces from both ends

Text typed by people often has a stray space at the end.

strip() is how you tidy it before comparing it.

typed = "  Sourdough  "
print(f"[{typed.strip()}]")
print(typed.strip().lower())
print(typed.strip().upper())

[Sourdough]
sourdough
SOURDOUGH

Measuring text

What it means. len() gives the number of characters in a string.

Write len, then the value in round brackets.

Spaces count as characters.

The answer is always a whole number.

len() works on

• a string, giving the number of characters
• a list, giving the number of items
• anything else with a length

You will use len() on a list in the next topic.

It is the same instruction either way.

name = "Mill Lane Bakery"
print(len(name))
print(len("sourdough"))

16
9

Making a list

What it means. A list is a set of values kept in order under one name.

Square brackets hold the items.

Commas separate them.

The items stay in the order you wrote them.

A list can hold

• numbers
• strings
• True and False values
• a mixture of those

Keep one kind of thing in a list where you can.

A list of counts is easier to add up than a mixture.

sales = [12, 5, 9, 7]
print(sales)
print(len(sales))
print(sum(sales))

[12, 5, 9, 7]
4
33

Reading one item

What it means. An index is the position of an item, counted from zero.

The first item is at index 0.

The second is at index 1.

Square brackets after the name read one item.

Counting from zero

• index 0 is the first item
• index 1 is the second item
• index -1 is the last item
• an index past the end gives an error

Counting from zero catches everybody out at first.

A list of four items has indexes 0 to 3.

items = ["sourdough", "white tin", "seeded roll", "fruit scone"]
print(items[0])
print(items[2])
print(items[-1])

sourdough
seeded roll
fruit scone

Adding to a list

What it means. append() adds one item to the end of a list.

Write the list name, a full stop, then append.

Put the new item in the brackets.

The list grows by one.

append() in use

• it always adds to the end
• it adds one item at a time
• it changes the list itself
• it gives nothing back

This is different from the string methods in topic 3.

Those gave back a new value and left the original alone.

items = ["sourdough", "white tin"]
items.append("seeded roll")
print(items)
print(len(items))

['sourdough', 'white tin', 'seeded roll']
3

Doing something to every item

What it means. A loop repeats an instruction for each item in turn.

Write for, a name for one item, then in and the list.

End the line with a colon.

Indent the lines that belong to the loop.

A for loop has four parts

• the word for
• a name for the current item
• the word in and the list
• a colon, then indented lines

The indent is how Python knows what to repeat.

Four spaces is the normal amount.

items = ["sourdough", "white tin", "seeded roll"]
for item in items:
    print(f"Check stock of {item}")

Check stock of sourdough
Check stock of white tin
Check stock of seeded roll

Comparing two values

What it means. A condition is a question that is either True or False.

A comparison asks about two values.

The answer is always True or False.

Those are the boolean values from topic 2.

The comparisons you need

• == asks whether two values are the same
• != asks whether they are different
• > and < ask which is larger
• >= and <= allow the two to be equal

Two equals signs ask a question.

One equals sign stores a value instead.

sold = 12
print(sold > 10)
print(sold == 12)
print(sold != 12)

True
True
False

Taking one path or another

What it means. if runs a block of lines only when a condition is True.

Write if, then the condition, then a colon.

Indent the lines that should run.

Add else for what to do otherwise.

How Python reads it

• it works out the condition first
• if the answer is True it runs the indented lines
• if the answer is False it runs the else lines
• it never runs both

The shape is the same as the loop in topic 4.

A colon, then an indent, marks the lines that belong.

sold = 12
if sold > 10:
    print("Good morning for sourdough")
else:
    print("Quiet morning")

Good morning for sourdough

Asking a second question

What it means. elif asks another question when the one before it was False.

elif is short for else if.

You can use as many as you need.

Python stops at the first one that is True.

The order matters

• Python checks the if first
• then each elif in the order you wrote them
• it runs the first block that fits
• else catches everything left over

Put the narrowest test first.

A wide test at the top catches cases you meant for a later line.

sold = 7
if sold > 10:
    print("Busy")
elif sold > 5:
    print("Steady")
else:
    print("Quiet")

Steady

A decision inside a loop

What it means. You can put an if inside a for loop to test every item.

The loop takes each item in turn.

The if then asks about that item.

Lines inside both are indented twice.

Reading the indents

• four spaces means inside the loop
• eight spaces means inside the if as well
• no indent means outside both

This pattern finds the items that matter.

It is the most common shape in everyday Python.

sales = [12, 5, 9, 7]
items = ["sourdough", "white tin", "seeded roll", "fruit scone"]
for i in range(len(sales)):
    if sales[i] > 8:
        print(f"{items[i]} sold well at {sales[i]}")

sourdough sold well at 12
seeded roll sold well at 9

Writing a function

What it means. A function is a named piece of code you can run whenever you need it.

Write def, the name, then round brackets and a colon.

Indent the lines inside.

Nothing runs until you call it.

Why functions are worth it

• you write the steps once
• you give them a name that says what they do
• a fix in one place fixes every use
• the rest of your code gets shorter

Defining a function and calling it are two separate things.

A function you never call does nothing at all.

def greet():
    print("Welcome to Mill Lane Bakery")

greet()
greet()

Welcome to Mill Lane Bakery
Welcome to Mill Lane Bakery

Passing a value in

What it means. A parameter is a name in the brackets, standing for a value passed in.

Put a name inside the brackets when you define it.

Pass a value when you call it.

Inside the function, the name holds that value.

Two words that are easy to mix up

• a parameter is the name in the definition
• an argument is the value you pass in
• they meet when the function runs

One function can then serve many cases.

You change the value, not the function.

def greet(name):
    print(f"Good morning, {name}")

greet("Ama")
greet("Joe")

Good morning, Ama
Good morning, Joe

Sending a value back

What it means. return sends a value out of a function to whatever called it.

print shows a value on the screen.

return hands it back to your code.

You can then store it or use it in a sum.

print and return are not the same

• print puts text on the screen and gives nothing back
• return gives a value back and shows nothing
• a function stops the moment it returns

Use return when the answer is needed later.

Use print when a person needs to read it now.

def takings(count, price):
    return count * price

morning = takings(12, 3.50)
print(morning)
print(takings(5, 2.50) + takings(9, 0.75))

42.0
19.25

Reading an error

What it means. An error message names the problem and the line it happened on.

Python stops at the first line it cannot run.

It then tells you what went wrong.

Read the last line of the message first.

The errors you will meet most

• NameError, when Python does not know a name
• TypeError, when a value is the wrong type
• IndexError, when a position is past the end of a list
• SyntaxError, when a line cannot be read at all

Python often suggests the word you meant.

Fix the first error and run it again, because the rest may go with it.

price = 3.50
print("The price is " + price)

TypeError on line 2: can only concatenate str (not "float") to str