From Blocks to Python

Blocks were a good way to start. You couldn’t misspell a block, and blocks that didn’t fit wouldn’t snap. But blocks get slow when a program grows to hundreds or thousands of instructions: there is a lot of dragging, and a big program no longer fits on the screen. So programmers mostly use a text-based language: the program is typed, like a message.

Python is a text-based programming language. A program written in Python is plain text: you can type it on a phone, send it in a message, or print it on paper. To run it, the computer needs a Python interpreter, a program that reads your Python one line at a time, from the top, and carries out each line. People all over the world use Python, from students writing their first program to scientists studying the weather and engineers building websites.

Here is the tea-shop bill from Grade 7, once as blocks and once as Python. You don’t need to understand every part of the Python yet; by the end of the unit you will.

Look at what stayed the same: the same two questions, the same variables (name, cups, bill), the same calculation, in the same order. A block program and a Python program can say exactly the same thing. Only the way of writing it changed.

Running Python on this page

Every Python program in these pages has a Run button under it. Press it, and the program runs here on the page, on a phone or a computer, with nothing to install. What it prints appears under it, in a box like a computer’s screen. When a program asks a question, type your answer in that box and press Enter.

One thing works differently here. Python on a computer waits while you type an answer. This page can’t make Python wait, so it stops the program at the question, and when you press Enter it runs the program again from the first line, with all your answers so far typed in for you. For the programs in this book you won’t see the difference, except perhaps a short flicker after each answer.

Press Edit to change the program: tap where you want to type, as in a message. Press Run again to run your version, and Reset to get the book’s program back. Change things, break things and see what happens. You can’t harm anything: the program runs only inside this page.

The first time you press Run, the page downloads Python itself, about 6 MB, a little less than a short video. After that it comes from your browser’s memory, so later runs start quickly and use almost no data. Python needs the internet that first time, so press Run once while you have Wi-Fi.

In blocks, say showed something in the output box. In Python, print does it. Press Run:

print("Namaste!")print("Welcome to Python.")

Namaste!Welcome to Python.

Each print shows one line, in order from the top. Every part of a line matters:

• print is written in small letters.
• The round brackets ( ) hold what is to be printed.
• The text inside the double quotes " " is printed exactly as it is, spaces and all. The quotes themselves are not printed: they only mark where the text starts and stops. Single quotes, 'Namaste!', work too, as long as both ends match.

Numbers are printed without quotes, and Python works out a sum before printing it:

print(5 + 3)print("5 + 3")

85 + 3

Without quotes, 5 + 3 is a sum, and Python prints its answer. With quotes, it is just text, three characters with spaces between them, and Python prints it as it is.

To print several things on one line, separate them with commas. Python puts a space between them:

print("Cups of tea:", 4)print("Price per cup: Rs", 30)print()print("Thank you!")

Cups of tea: 4Price per cup: Rs 30 Thank you!

print() with nothing inside prints an empty line.

Comments

A line, or the end of a line, that starts with # is a comment: a note for people reading the program. Python skips it.

# My first program, by Ashaprint("Namaste!")   # greet the user

Namaste!

Comments say what a program is for, or why a line is there. They are useful in your own programs, and even more in someone else’s.

In Grade 7, set score to 0 put 0 into the variable score. In Python you write:

score = 0

Read = as “is set to”, or “gets”: score gets 0. It is not the equals of a math lesson. In math, x = 5 says that two things are equal; in Python, score = 0 is an instruction: put the value on the right into the variable on the left. Python makes the variable the first time you set it, so there is no Make a variable button.

To use a variable’s value, write its name, without quotes:

runs = 4print(runs)print("runs")

4runs

print(runs) looks in the variable and prints what it holds. print("runs") prints the word runs.

Change by

There is no change block in Python. To add 6 to runs, you write:

runs = runs + 6

That would be nonsense in math, but read it as Python does, right side first: take the value in runs, add 6, and put the answer back into runs. If runs held 4, it now holds 10. Here is Grade 7’s cricket scorer:

runs = 0runs = runs + 4runs = runs + 6runs = runs + 1print("Runs:", runs)

Runs: 11

Python also has a short way to write the same thing: runs += 6 means runs = runs + 6, and lives -= 1 means lives = lives - 1. You will see both in programs.

Naming variables

The rules for names are strict:

• A name is made of letters, digits and the underscore _, with no spaces. Write bus_fare, not bus fare.
• It can’t start with a digit: cups2 is fine, 2cups is not.
• Capital letters matter: score, Score and SCORE are three different variables.
• A few words are Python’s own, called keywords, and can’t be names: if, else, and, or, not, True, False, class, for, while and some others.

Choose names that say what is inside, as in Grade 7. Python programmers write names in small letters, with _ between the words: price_per_cup, total_marks.

In Grade 7 values came in three types: numbers, text and true/false. Python has the same three, but it splits numbers in two, so there are four:

A few things decide the type, and they are worth remembering:

• Quotes make a string. 12 is an int, but "12" is a string: two characters, the digit 1 and the digit 2. "True" with quotes is a string too.
• A decimal point makes a float. 12.0 is a float, even though it is a whole number.
• True and False start with a capital letter, and have no quotes. true is not a value in Python: Python looks for a variable called true and doesn’t find one.
• "" is a string with nothing in it, the empty string. It is still a string.

To ask Python a value’s type, use type():

print(type(77))print(type(7.5))print(type("77"))print(type(True))

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

Comparisons make True or False

A comparison works out to True or False, a bool, just as conditions did in Grade 7. Python has six comparison operators. A keyboard has no ≤, ≥ or ≠ keys, so three of them are written with two characters:

With >= the rule “40 or more” is simply marks >= 40; you don’t need Grade 7’s trick of writing > 39.

Grade 7’s and, or and not are Python’s and, or and not, written small, and they work just the same:

age = 13grade = 8print(age >= 13)print(grade > 5 and grade < 9)print(not grade == 8)paid = Falseprint(paid)

TrueTrueFalseFalse

Python is a very good calculator. A keyboard has no × or ÷ keys, so multiplying is * (a star) and dividing is /:

Two of these need a closer look.

/ always gives a float, even when the answer is whole: 6 / 2 is 3.0, not 3.

// and % are the two halves of the division you learned in primary school, where 17 ÷ 5 = 3 remainder 2. // (integer division) gives the 3, and % (the remainder, also called modulo) gives the 2. (% has nothing to do with percent in Python.) They are more useful than they look: anything that is shared out, packed, or counted in groups needs them.

Which comes first?

Python works out a sum in the same order as in your math lessons: powers first, then multiplying and dividing (*, /, //, %), then adding and taking away, from left to right. Brackets come before everything, so use them when you want a different order, or just to make the order clear.

print(7 + 3 * 2)print((7 + 3) * 2)

1320

Grade 7’s ask … and keep the answer in … is Python’s input(). It shows the question, waits for the user to type an answer and press Enter, and gives back what they typed, which = keeps in a variable:

name = input("What is your name? ")print("Namaste,", name)

What is your name? PembaNamaste, Pemba

In this book an output with an input shows the question and what the user typed on the same line, as the screen shows it. The space at the end of the question keeps the answer from sticking to the question mark.

input() always gives a string

Here is a careful-looking tea-shop bill. Run it and buy 2 cups:

cups = input("How many cups of tea? ")bill = cups * 30print("Please pay Rs", bill)

How many cups of tea? 2Please pay Rs 222222222222222222222222222222

Python didn’t multiply 2 by 30. input() always gives a string, whatever is typed, because what you type are characters. So cups holds "2", the text, and "2" * 30 means “the text "2", thirty times”. There is no error message; the program just gives the wrong answer, which is the worst kind of bug.

Grade 7’s blocks quietly turned an answer that looked like a number into a number. Python doesn’t guess: you tell it. These functions convert a value from one type to another:

Put int() round input(), and the answer is turned into a whole number as soon as it is typed:

cups = int(input("How many cups of tea? "))bill = cups * 30print("Please pay Rs", bill)

How many cups of tea? 2Please pay Rs 60

Read int(input("How many cups of tea? ")) from the inside out: input() asks and gives back the string "2", then int() turns "2" into the number 2. Use float() instead of int() when the answer can have a decimal point, such as a weight in kilograms or a price in rupees and paisa.

Putting values into text

The bill should say Please pay Rs 60., with a period right after the number. With a comma, print adds a space we don’t want. There are two other ways.

Join strings with +, as Grade 7’s join did. Both sides must be strings, so a number is turned into one with str() first:

bill = 60print("Please pay Rs " + str(bill) + ".")

Please pay Rs 60.

Or, more neatly, an f-string: put f just before the opening quote, and write each value’s name in curly brackets { } where it should go. Python fills them in:

name = "Pemba"bill = 60print(f"{name}, please pay Rs {bill}.")

Pemba, please pay Rs 60.

An f-string turns the values into text by itself, so there is no str() to forget. Most Python programmers use f-strings for messages like this.

Sooner or later, your programs will stop with an error message. That is normal: professional programmers see them every day. The difference between a beginner and a programmer is that the programmer reads the message. Python is telling you where it stopped and why, and usually that is enough to fix it in a minute.

Here is the tea-shop bill with a mistake, and the message Python gives:

cups = 2bill = cups * 30print("Please pay Rs " + bill)

Read an error message in this order:

1. The last line: the kind of error, then a colon, then what went wrong in Python’s words. Here, a TypeError: text can only be joined to text, and bill holds an int.
2. The line number above it: where Python was when it stopped, line 3 here, and that line printed under it. The marks under the line (~~~^~~~ or ^^^) point at the part that went wrong.
3. Fix one thing and run again. Here, print("Please pay Rs " + str(bill)), or an f-string.

Five errors you will meet

Python finds a SyntaxError or an IndentationError before it runs anything, so nothing at all is printed. The other three happen while the program runs, when Python reaches the line, so the lines before it have already done their work.

Each program below has one mistake. Press Run, read the message, then press Edit, fix it and run it again until it works. Try each one before you open its answer.

Now you can read and write every block from Grade 7’s first programs in Python. Here they are, side by side:

The mouth of an if is an indent

An if block had a mouth, and the blocks inside it ran only when the condition was true. Python has no mouths; it uses indentation: the lines inside an if start four spaces further right. The if line ends with a colon :, which says “the indented lines under me belong to me”, and so does else:. When the lines go back to the left edge, the if is over. Here is Grade 7’s bus fare:

Because the spaces mean something, they must be exact. A missing colon is a SyntaxError (expected ':'), and a line that should be inside but isn’t indented is an IndentationError (expected an indented block after 'if' statement). In this book’s editor, press Enter after a line ending in a colon, and the next line starts four spaces in by itself.

You will learn much more about if and else, and about loops, in Unit 2. For now it is enough to read them, and to turn a block program into Python.

The quiz

Here is the first question of Grade 7’s quiz. Notice the ==, and what it means for the answer kathmandu with a small k.

A typed program goes wrong in more ways than a block program, so plan it before you type, as you did in Grade 7:

1. Say what it does, in a sentence: what it asks, what it works out, what it prints.
2. List the variables, each with its type: which are ints, floats, strings, bools? Every number that comes from input() needs int() or float().
3. Write the steps in plain words, then turn each into a line of Python.
4. Type a little, run it, then type more. Run the program after every two or three lines. A mistake found in a three-line program takes seconds to fix; in a thirty-line one, much longer.
5. Test it with answers you can check in your head, and with an awkward one: 0, a large number, a decimal.