Python is a computer language that resembles Basic. It tries to be even easier to learn than Basic, though in some ways it’s harder.

Python is considered to be modern; Basic is considered to be old-fashioned. Many colleges teach students to program in Python instead of Basic.

Python was invented by a Dutchman, Guido van Rossum, in December 1989, as a hobby, to keep himself busy while his office was closed for Christmas vacation. He called it “Python” to honor the British comedy group Monty Python’s Flying Circus. In October 2000 he invented an improved version, Python 2. In December 2008, he invented a further improvement, Python 3.

This chapter explains the current version, Python 3.4.3, which is a slight improvement on Python 3.

In your Python program, you can put these commands —

and these functions (which have parentheses):

 

Let’s have fun programming in Python! If you have any difficulty, phone me at 603-666-6644 (day or night) for free help.

Get Python

Here’s how to copy Python (version 3.5.1) from the Internet to a Windows 10 computer, free (using Microsoft Edge).

Go to:

Tap the “Run” button then “Install Now” then “Yes”. The computer will say “Installing” then “Setup was successful.” Tap “Close”. Close the Internet Explorer window (by clicking its X button).

Start Python

To start Python (version 3.5.1), your first step is to tap “IDLE” by choosing one of these methods:

You see the Python Shell window, which is also called Python’s Integrated DeveLopment Environment (IDLE).

Math

In the Python Shell window, you see this Python prompt:

To the right of that prompt, you can type any Python command. For example, you can type 4+2, so the screen looks like this:

Try doing that: type 4+2.

After typing 4+2, press the Enter key. The Enter key makes the computer read what you typed and reply to it. The computer will reply by typing the answer, 6, like this:

So your screen looks like this:

Below that, the computer shows the Python prompt again, so your screen looks like this –

and you can give another command.

If you want to subtract 3 from 7, type 7-3, so your screen looks like this:

When you press the Enter key at the end of that line, the computer will reply:

You can use decimal points and negative numbers. For example, if you type -26.3+1, the computer will say:

Multiplication To multiply, use an asterisk. So to multiply 2 by 6, type this:

The computer will say:

Division To divide, use a slash. So to divide 8 by 4, type this:

Instead of saying the answer is 2, the computer will say —

because whenever the computer divides, its answer includes a decimal point.

If you try to divide by 0 (by giving a command such as 3/0 or 0/0), the computer will refuse: it will say “ZeroDivisionError”.

Last digit might be wrong To divide 5 by 3, type this:

The computer will say:

In that example, the computer gave the right answer, rounded to 17 digits. But in other calculations that have a decimal answer, the computer might accidentally say the last digit wrong. For example, suppose you say to divide 7 by 3, like this:

The computer will accidentally say:

In that answer, the 5 should be 3 instead. Moral: when Python makes the computer give a long decimal answer, don’t trust its last digit!
(I hope Python’s future versions hide that error, by showing just the 16 reliable digits and hiding the 17^th digit.)

To see an even scarier example, type this:

The answer should be simply .3, but Python makes the computer say this:

The last digit, the 17^th, should be 0, not 4.

Avoid commas Do not put commas in big numbers. To write four million, do not write 4,000,000; instead, write 4000000.

E notation If the computer’s answer is tiny (less than .0001) or “a huge number containing a decimal point” (at least 10000000000000000.0), the computer will put an e in the answer. The e means “move the decimal point”.

For example, suppose the computer says the answer to a problem is:

The e means “move the decimal point”. The plus sign means, “toward the right”. Altogether, the e+18 means “move the decimal point toward the right, 18 places.” So look at 1.3586281902638497 and move the decimal point toward the right, 18 places; you get —

which has the same meaning as:

So when the computer says the answer is 1.3586281902638497e+18, the computer really means the answer is 1358628190263849700.0, approximately. Since you can’t trust the computer’s last digit (the 7) and the zeros that belong after it, the exact answer might be 1358628190263849700.2 or 1358628190263849700.29 or 1358628190263849800.0 or some similar number, but the computer says just an approximation.

Suppose your computer says the answer to a problem is:

After the e, the minus sign means, “towards the left”. So look at 9.23 and move the decimal point towards the left, 6 places. You get:

.00000923

So when the computer says the answer is 9.23e-06, the computer really means the answer is:

.00000923

You’ll see e notation rarely: the computer uses it just if an answer involves decimals and tinier than .0001 or huge (at least 10 quadrillion). But when the computer does use e notation, remember to move the decimal point!

The highest number The highest number the computer can handle well is about 1e308, which is 1 followed by 308 zeros then a decimal point. If you try to go much higher, the computer will gripe by saying —

which means “infinity”.

The tiniest decimal The tiniest decimal the computer can handle well is about 1e-323, which is a decimal point followed by 323 digits (322 zeros then 1). If you try to go much tinier, the computer will give up and say just:

Order of operations What does “2 plus 3 times 4” mean? The answer depends on whom you ask.

To a clerk, it means “start with 2 plus 3, then multiply by 4”; that makes 5 times 4, which is 20. But to a scientist, “2 plus 3 times 4” means something different: it means “2 plus three fours”, which is 2+4+4+4, which is 14.

Since computers were invented by scientists, computers think like scientists. If you type —

the computer will think you mean “2 plus three fours”, so it will do 2+4+4+4 and say this answer:

The computer will not print the clerk’s answer, which is 20. So if you’re a clerk, tough luck!

Scientists and computers follow this rule: do multiplication and division before addition and subtraction. So if you type —

the computer begins by hunting for multiplication and division. When it finds the multiplication sign between the 3 and the 4, it multiplies 3 by 4 and gets 12, like this:

So the problem becomes 2+12, which is 14, which the computer prints.

For another example, suppose you type:

The computer begins by doing all the multiplications and divisions. So it does 2*3 (which is 6) and does 72/9*5 (which is 8.0*5, which is 40.0), like this:

So the problem becomes 10-6+40.0, which is 44.0, which is the answer the computer says:

You can use parentheses the same way as in algebra. For example, if you type —

the computer will compute 5-2 and say:

You can put parentheses inside parentheses. If you type —

the computer will compute 10-(5-2), which is 10-3, and will say:

Strings

Let’s make the computer fall in love. Let’s make it say, ‘I love you’.

To do that, type ‘I love you’, beginning and ending with a single-quote mark (which is the same mark as an apostrophe), so your screen looks like this:

At the end of that typing, when you press the Enter key, the computer will obey your command: it will say:

You can change the computer’s personality. For example, if you give this command —

the computer will reply:

Notice that to make the computer say a message, you must put the message between single-quote marks. The single-quote marks make the computer copy the message without worrying about what the message means. For example, if you misspell ‘I love you’ and type —

the computer will still copy the message (without worrying about what it means); the computer will say:

Jargon The word ‘joy’ consists of 3 characters: j and o and y. Programmers say that the word ‘joy’ is a string of 3 characters.

Strings versus numbers The computer can handle two types of expressions: strings and numbers. Put strings (such as ‘joy’ and ‘I love you’) in single-quote marks. Numbers (such as 4+2) do not go in single-quote marks.

Accidents Suppose you accidentally put the number 2+2 in single-quote marks, like this:

The single-quote marks make the computer think ‘2+2’ is a string instead of a number. Since the computer thinks ‘2+2’ is a string, it copies the string without analyzing what it means; the computer will say:

It will not say 4.

Suppose you want the computer to say the word ‘love’ but you accidentally forget to put the string ‘love’ in single-quote marks. You accidentally type this instead:

Since you forget to type the single-quote marks, the computer will try to figure out what you mean but will get confused, since it doesn’t know the meaning of love. Whenever the computer gets confused, it gripes by saying you have a “NameError” or “SyntaxError”.

String arithmetic You can add strings. For example, ‘fat’+‘her’ is ‘father’. So if you type —

the computer will say:

You can multiply a string by a number. For example, ‘fat’ multiplied by 3 is ‘fatfatfat’. So if you type —

the computer will say:

If you prefer, write the number before the string, like this:

The computer will still say:

Print

If you say print, the computer will print onto your screen more briefly.

For example, if you say —

The computer will print this onto your screen:

The computer will not print single-quote marks around that reply.

After the word print, you must type a parenthesis. If you forget to type the parenthesis and put a blank space instead, the computer will say “SyntaxError: Missing parentheses”.

If you say —

the computer prints the results of ‘love’ and 2+2 and ‘you’, all on the same line of your screen but separated by spaces, like this:

Yes, the computer produces love 4 you.

This command makes the computer do 6+2, 6-2, 6*2, and 6/2, all at once:

That makes the computer print the four answers, all on the same line:

The computer puts spaces between the answers.

Create a program

Here’s how to create a Python program.

At the top of the Python Shell window, you see this menu:

Click “File” then “New File”.

You see the program window, called “Untitled”. The program window is empty: it doesn’t contain any >>> prompt.

The program window partly covers up the Python Shell window. To make programming easier, drag the word “Untitled” toward the right; as you do so, the entire program window moves toward the right. Keep dragging toward the right until the program window no longer overlaps the Python Shell window. (If your screen isn’t wide enough to accomplish that, just drag as far as possible.)

In the program window, type your program. For example, let’s type a program that makes the computer say:

To do that, type this program:

At the end of each line, press the Enter key.

When you’ve done all that, click “File” then “Save”. Invent a name for your program, such as Joe; type the name then press the Enter key. That makes the computer copy the program to your hard disk. (If you’re using Python 3.5.1, the program will be in your hard disk’s Python35 folder. If you named the program Joe, the program’s name will actually be Joe.py, because the computer automatically puts “.py” at the end of the program’s name. The “.py” means “written in Python”.)

To run the program, tap the F5 key. (Exception: if the “F5” is blue or tiny or on a new computer by Microsoft, HP, Lenovo, or Toshiba, tap that key
while holding down the blue Fn key, which is left of the Space bar.) Then, in the Python Shell window, you see the result of the program running, so you see:

That writing is called the program’s output, since it’s what the program puts out.

Above the output, you see “RESTART”. Above and below the output, you see the >>> prompt, so you can give another Python command.

If you want to edit the program you wrote, click in the program window (which is to the right of the Python Shell window or at least peeks out behind the the Python Shell window) or do this in the Program Shell window:

You see your program again. Make whatever changes you wish, then save the program again (by clicking “File” then “Save”), then run the program (by tapping the F5 key).

To see an old program you created, go to the Python Shell window then click “File” then “Open” then double-click the program’s name. You see the program’s lines. To run the program, tap the F5 key.

Warning: in a normal program, you must say print. For example, to make a program say the answer to 2+2, you can’t type just 2+2; instead the program must say:

Saying just 2+2 is okay next to the >>> prompt, which means you’re in interactive mode, not in a program.

Polite versus fast To run a Python program, you must save it first. I showed you the polite way to do Python: save the program (by clicking “File” then “Save”) then run the program (by tapping the F5 key).

Here’s the faster way to run a Python program: tap the F5 key (which means you want to run the program), then watch the computer yell at you (for not saving the program first), then press the Enter key (which means you agree to save it). That’s impolite (so you get yelled at), but it’s faster than clicking “File” then “Save”.

Finish

When you finish using Python, close all windows (by clicking their X buttons).

Tricky printing

Printing can be tricky! Here are the tricks.

Indenting Suppose you want the computer to print this letter onto your screen:

This program prints it:

In the program, each line contains 2 single-quote marks.
To make the computer indent a line, put blank spaces AFTER the first single-quote mark.

Blank lines Life consists sometimes of joy, sometimes of sorrow, and sometimes of a numb emptiness. To express those feelings, run this program:

Altogether, the computer will print:

Apostrophe An apostrophe is this symbol:

Many words contain apostrophes:

To put an apostrophe in a string’s middle, use one of these tricks:

For example, suppose you want the computer to say:

This does not work:

Instead, you must use the backslash trick (putting a backslash before each apostrophe) —

or the double-quote trick (putting the string in double-quote marks instead of single-quote marks):

If you use the backslash trick, make sure you type a backslash (\), not a forward slash (/). The backslash key is above the Enter key.

New line In a string, \n means “create a new line”. For example, if you type —

the computer will print the word Love, then create a new line (by pressing the Enter key), then print the word Death, so you see this:

That’s how to make one print statement print 2 lines.

Separator If you say —

the computer will print the 4 words and put blank spaces between them, like this:

If instead you say —

the computer will print the 4 words and separate them with exclamation points instead of spaces, so you see this:

That’s because sep=‘!’ means “the separator is an exclamation point”.

If instead you say —

The computer will print those 4 words and separate them with nothing, so you see this:

If instead you say —

the computer will print:

End In your program, if you say —

the computer will print ‘fat’ and ‘her’ on separate lines, like this:

That’s because, at the end of printing ‘fat’, the computer presses the Enter key.

Suppose you say this instead:

The end=‘!’ means:

So after printing ‘fat’, the computer will print an exclamation point instead of pressing the Enter key. The computer will print:

Suppose you say this instead:

The end=‘ ’ means:

So after printing ‘fat’, the computer will press the space bar instead of the Enter key. The computer will print:

Suppose you say this instead:

The end=‘’ means:

So after printing ‘fat’, the computer won’t press the Enter key; instead, the computer will just obey the next command, which makes the computer print ‘her’, so ‘her’ appears next to ‘fat’, like this:

Lines that aren’t commands

Usually, each line you type is a command. Here’s how to change that.

Semicolon To type two commands on one line, put a semicolon between the commands:

The computer will say:

Backslash To type just part of a command on one line, put a backslash at the end of that part. Type the rest of the command on the line below.

For example, instead of typing —

you can type:

(The computer automatically indents the second line for you.) The computer will say the answer:

Instead of typing —

you can type:

(Since you put the backslash in the middle of a string, the computer does not indent the second line.) The computer will say:

If you want to type a command that’s too long to fit on your screen, put a backslash at the end of the command’s first part; underneath, type the rest of the command.

Don’t put a backslash in the middle of a computer word (such as “print”). Don’t put a backslash in the middle of a number (such as 57).

Comment Occasionally, jot a note to remind yourself what your program does. Slip the note into your program by putting a hashtag (the symbol “#”) before it:

When you run the program,
the computer ignores everything that’s to the right of a hashtag. So the computer ignores the top two lines; in the bottom line, the computer ignores the “because…”. The program makes the computer print just this:

Everything to the right of a hashtag is called a comment. While the computer runs the program, it ignores the comments. But the comments remain part of the program; they appear in the right-hand window with the rest of the program. Though the comments appear in the program, they don’t affect the run.

 

You can name a number. For example, you can make Joan be the name for the number 7, by typing this:

Then Joan+2 is 9, so if you type —

the computer will reply:

The name can be short (like Joan) or long (like PopeFrancisTheGreat) or
very short (like x) or technical (like TemperatureOfBasementFloor) or include digits (like LeaderOfThe3Musketeers)
or disgusting (like number_of_times_we_vomited).

When you invent a name, you face these restrictions:

Capitalization makes a difference.

Some companies (such as Google) prohibit employees from using capital letters in names, except in special circumstances. If you work at one of those companies, make a name be:

You can name any number. For example, you can say:

Then x is -34.1, so if you type —

the computer will multiply -34.1 by 2 and say:

You can name any string. For example, you can say:

Then the computer will multiply ‘go’ by 3 and say:

Beginners are usually too lazy to type long names, so beginners use names that are short (such as x). But when you become a pro and write a long, fancy program containing hundreds of lines and hundreds of names, you should use long names to help you remember each name’s purpose. A name can be as long as you wish. In this book, I’ll use short names in short programs (so you can type those programs quickly) but long names in long programs (so you can keep track of which name is which).

Jargon

A name (for a number or a string) is also called an identifier. It’s also called a variable.

For example, suppose you say:

That line makes Joan be a name, an identifier, a variable, whose value is 7. Since that line assigns 7 to Joan, that line is called an assignment statement. That line defines Joan to be 7.

If a variable (such as Joan) stands for a number, it’s called a numeric variable. If a variable stands for a string instead, it’s called a string variable.

Restart

When you run a program, the computer begins by doing a restart, which makes it forget any names you invented previously, so the program can start fresh.

After the program has run, the computer still remembers the names in the program. For example, if your program said Joan=7, the computer knows Joan is 7 even after the program has finished running, so if you say —

The computer will say:

A variable is a box

When you say Joan=7, here’s what happens inside the computer.

The computer’s random-access memory (RAM) consists of electronic boxes. The line Joan=7 makes the computer create a box named Joan and put 7 into it, like this:

box Joan                 7

Then when the computer encounters print(x+2), the computer prints what’s in box Joan, plus 2; so the computer prints 9.

Variable from variables

One variable can define another. For example, suppose you type:

The top line says n is 6. The next line says d is n+1, which is 6+1, which is 7; so d is 7. The bottom line says to print n*d, which is 6*7, which is 42; so the computer will print:

Change a value

A value can change:

The top line says k’s value is 4. The next line changes k’s value to 9, so the bottom line prints 18.

When you run that program, here’s what happens inside the computer’s RAM. The top line (k=4) makes the computer put 4 into box k:

box k                 4

The next line (k = 9) puts 9 into box k. The 9 replaces the 4:

box k                 9

That’s why k*2 prints 18.

Self-changing variable A variable can change itself. Look at this program:

The top line (x=7) says x starts by being 7. The next line (x=x+2) means: the new x is “what x was before, plus 2”. So the new x is 7+2, which is 9. The bottom line prints:

Let’s examine that program more closely. The top line (x=7) puts 7 into box x:

box x                 7

When the computer sees the next line (x=x+2), it examines the equation’s right side and sees the x+2. Since x is 7, the x+2 is 7+2, which is 9. So the line “x=x+2” means x=9. The computer puts 9 into box x:

box x                 9

The program’s bottom line prints 9.

Instead of typing —

you can type this shortcut:

You can pronounce “x+=2” this way:

Here’s another weirdo:

The second line (b+=1) says the new b gets added this amount: 1. So the new b is 6+1, which is 7. The bottom line prints:

In that program, the top line says b is 6; but the next line increases b, by adding 1; so b becomes 7. Programmers say that b has been increased or incremented. In the second line, the “1” is called the increase or the increment.

The opposite of “increment” is decrement:

The top line says j starts at 500; but the next line decreases j by subtracting 1, so the new j is 500-1, which is 499. The bottom line prints:

In that program, j was decreased (or decremented). In the second line, the “1” is called the decrease (or decrement).

Hassles

Variables can cause hassles.

Undefined variable If you type —

the computer tries to say Joan’s value (a number or string). If the computer fails (because you forgot to write a line such as Joan=7), the computer says “NameError”.

What’s before the equal sign? When writing an equation (such as x=47), put this before the equal sign: the name of just one box (such as x). So before the equal sign, put one variable:

The variable on the equation’s left side is the only one that changes. For example, the statement d=n+1 changes the value of d but not n. The statement b=c changes the value of b but not c:

The third line changes b, to make it equal c; so b becomes 7. Since both b and c are now 7, the bottom line prints 14.

“b=c” versus “c=b” Saying “b=c” has a different effect from “c=b”. That’s because “b=c” changes the value of b (but not c); saying “c=b” changes the value of c (but not b).

Compare these programs:

In the left program, the third line changes b to 7, so both b and c are 7. The bottom line prints 14.

In the right program, the third line changes c to 1, so both b and c are 1. The bottom line prints 2.

While you run those programs, here’s what happens inside the computer’s RAM. For both programs, the second and third lines do this:

box b                1

box c                7

In the left program, the third line makes the number in box b become 7 (so both boxes contain 7, and the bottom line prints 14). In the right program, the third line makes the number in box c become 1 (so both boxes contain 1, and the bottom line prints 2).

When to use variables

Here’s a practical example of when to use variables.

Suppose you’re selling something that costs $1297.43, and you want to do these calculations:

To do those four calculations, you could run this program:

But that program’s silly, since it contains the number 1297.43 four times. This program’s briefer, because it uses a variable:

So whenever you need to use a number several times, turn the number into a variable, which will make your program briefer.

Paranoid If you’re paranoid, you’ll love this program:

The top line says t stands for the string ‘They’re laughing at you!’ The later lines make the computer print:

Nursery rhymes The computer can recite nursery rhymes:

The top line says p stands for ‘Peas porridge’. The later lines make the computer print:

This program prints a fancier rhyme:

Lines 1-3 define h, m, and c. The later lines make the computer print:

 

Humans ask questions; so to turn the computer into a human, you must make it ask questions too. To make the computer ask a question, use the word “input”.

This program makes the computer ask for your name:

The top line says n is the answer to the question
‘What is your name?’ When you run the program and the computer sees that line, the computer asks ‘What is your name?’ then waits for you to answer the question; your answer will be called n. For example, if you answer Maria, then n is ‘Maria’. The bottom line makes the computer print:

When you run that program, here’s the whole conversation that occurs between the computer and you; I’ve underlined the part typed by you.…

Go ahead, type that program and run it, but be careful:
when you type the input line, leave a space after the question mark.

Just for fun, run that program again and pretend you’re somebody else.…

When the computer asks for your name, if you say something weird, the computer gives you a weird reply.…

That program begins by making the computer ask:

You can make the computer say this instead:

To do so, change the program’s top line to this:

The program’s bottom line makes the computer reply like this:

You can make the computer add an exclamation point, like this:

To do that, change the program’s bottom line to this:

The +‘!’ means:

College admissions

This program prints a letter, admitting you to the college of your choice:

When the computer sees the input line, the computer asks ‘What college would you like to enter?’ and waits for you to answer. Your answer will be called c. If you’d like to be admitted to Harvard, you’ll be pleased.…

The program’s 5^th line includes these symbols:

Those symbols mean:

You can choose any college you wish:

 

That program consists of three parts:

Input versus print

The word input is the opposite of the word print.

The word print makes the computer print information out. The word input makes the computer take information in.

What the computer prints out is called the output. What the computer takes in is called your input.

Input and Output are collectively called I/O, so the input and print statements are called I/O statements.

Once upon a time

Let’s make the computer write a story, by filling in the blanks:

To write the story, the computer must ask for your name, your friend’s name, and a verb. To make the computer ask, your program must say input:

Then make the computer print the story:

Here’s a sample run:

Here’s another run:

Try it: put in your own name, the name of your friend, and something you’d like to do to your friend.

Numeric input

To let you input a number (instead of a string), your program should say “eval(input” instead of just “input”.

For example, this program lets you input a number and makes the computer double it:

The top line says f is the answer, evaluated as a number, to the question ‘What number will you give me?’ When you run the program and the computer sees that line, the computer asks ‘What number will you give me?’ then waits for you to answer the question; your number will be called n. For example, if you say 3, then n is 3. The bottom line makes the computer print:

When you run that program, here’s the whole conversation that occurs between the computer and you; I’ve underlined the part typed by you.…

Go ahead, type that program and run it, but be careful:
at the end of the eval line, type TWO parentheses.

In that program, the eval tells the computer you’ll input a number.

 

This program makes the computer predict your future:

Here’s a sample run:

Suppose you’re selling tickets to a play. Each ticket costs $2.79. (You decided $2.79 would be a nifty price, because the cast has 279 people.) This program finds the price of multiple tickets:

This program tells you how much the “oil crisis” costs you, when you drive your car:

Here’s a sample run:

So the gas will cost a hair less than $33.28.

Conversion

This program converts feet to inches:

Here’s a sample run:

Trying to convert to the metric system? This program converts inches to centimeters:

Nice day today, isn’t it? This program converts the temperature from Celsius to Fahrenheit:

Here’s a sample run:

See, you can write the Guide yourself! Just hunt through any old math or science book, find any old formula (such as f=c*1.8+32), and turn it into a program.

 

Let’s write a program so if the human is less than 18 years old, the computer will say:

Here’s the program:

The top line makes the computer ask ‘How old are you?’ and wait for the human to type an age. Since the symbol for
“less than” is “<”, the bottom line says: if the age is less than 18, print ‘You are still a minor’.

Go ahead! Run that program! The computer begins the conversation by asking:

Try saying you’re 12 years old, by typing a 12, so the screen looks like this:

When you finish typing the 12 and press the Enter key at the end of it, the computer will reply:

Try running that program again, but this time try saying you’re 50 years old instead of 12, so the screen looks like this:

When you finish typing the 50 and press the Enter key at the end of it, the computer will not say “You are still a minor”. Instead, the computer will say nothing — since we didn’t teach the computer how to respond to adults yet!

In that program, the bottom line says:

That line begins with the word “if”. Whenever you say “if”, you must also write a colon (the symbol “:”).

What comes between “if” and the colon is called the condition. In that example, the condition is “age<18”. If the condition is true (if age is really less than 18), the computer does the action, which comes after the colon and is:

Else

Let’s teach the computer how to respond to adults.

Here’s how to program the computer so if the age is less than 18, the computer will say “You are still a minor”, but if the age is not less than 18 the computer will say “You are an adult” instead:

In programs, the word “else” means “otherwise”. That program’s 2^nd and 3^rd lines mean: if the age is less than 18, then print ‘You are still a minor’; otherwise (if the age is not less than 18), print ‘You are an adult’. So the computer will print ‘You are still a minor’ or else print ‘You are an adult’, depending on whether the age is less than 18.

Try running that program! If you say you’re 50 years old, so the screen looks like this —

the computer will reply by saying:

Multi-line

If the age is less than 18, here’s how to make the computer print “You are still a minor” and also print “Ah, the joys of youth”:

Type that correctly: put a colon after “if age<18” but a semicolon between the two print statements.

 

Here’s a more sophisticated way to say the same thing:

That sophisticated way (in which you type 3 short lines instead of a single long line) is called a multi-line “if” (or a block “if”).

Here’s how to type that multi-line “if”:

The indented lines are called the block. The line above them, which ends in a colon, is called the block’s header.

You can also create a multi-line “else”, so your program looks like this:

That means: if the age is less than 18, print ‘You are still a minor’ and ‘Ah, the joys of youth’; otherwise (if age not under 18) print ‘You are an adult’ and ‘We can have adult fun’.

Elif

Let’s say this:

Here’s another way to say the same thing, in English:

The Python word for “otherwise” is “else”, and the Python word for “otherwise if” is “elif” (which is short for “else if”), so the Python program is:

Double equal sign

Suppose you want to say:

Here’s how to say that:

Python doesn’t let the “if” condition have a simple equal sign (=); instead you must type a double equal sign (==). If you accidentally type a single equal sign there, the computer will say “SyntaxError”.

Therapist Let’s turn your computer into a therapist!

To make the computer ask the patient, “How are you?”, begin the program like this:

Make the computer continue the conversation by responding this way:

Here’s how:

Here’s a complete program:

The top line makes the computer ask the patient, “How are you?” The next several lines makes the computer analyze the patient’s answer and print ‘That’s good!’ or ‘Too bad!’ or else ‘I feel the same way!’ Regardless of what the patient and computer said, that program’s bottom line always makes the computer end the conversation by printing:

In that program, try changing the strings to make the computer print smarter remarks, become a better therapist, and charge even more money.

Fancy “if” conditions

Different relations You can make the “if” clause very fancy:

In the “if” clause, the symbols <, >, <=, >=, ==, and != are called relations.

Or The computer understands the word “or”. For example, here’s how to say, “If x is either 7 or 8, print the word wonderful”:

That example is composed of two conditions: the first condition is “x==7”; the second condition is “x==8”. Those two conditions combine, to form “x==7 or x==8”, which is called a compound condition.

If you use the word “or”, put it between two conditions.

And The computer understands the word “and”. Here’s how to say, “If p is more than 5 and less than 10, print tuna fish”:

Here’s how to say, “If s is at least 60 and less than 65, print you almost failed”:

Here’s how to say, “If n is a number from 1 to 10, print that’s good”:

You can make the computer repeat, again and again. Something repeated is called a loop. Here’s how to create a loop.

While True

This program makes the computer print the word '“love” once:

This fancier program makes the computer print the word “love” three times:

When you run that program, the computer will print:

Let’s make the computer print the word “love” many times. To do that, we must make the computer do this line many times:

To make the computer do the line many times, say “while True” above the line, so the program looks like this:

Here’s how to type that program:

When you run that program, the computer will print “love” many times, so it will print:

The computer will print “love” on every line of the Python Shell window.

But even when that window is full of “love”, the computer won’t stop: the computer will try to print even more loves onto your window! The computer will lose control of itself and try to devote its entire life to making love! The computer’s mind will spin round and round, always circling back to the thought of making love again!

Since the computer’s thinking keeps circling back to the same thought, the computer is said to be in a loop. In that program, the “while True” means “do repeatedly what’s indented”. The “while True” and the indented lines underneath form a loop, called a “while loop”.

To stop the computer’s lovemaking madness, you must give the computer a “jolt” that will put it out of its misery and get it out of the loop. To jolt the computer out of the program, abort the program. To abort the program, do this: while holding down the Control key (which says “Ctrl” on it), tap the C key. That makes the computer stop running your program; it will break out of your program; it will abort your program and say “KeyboardInterrupt”.

In that program, since the computer tries to go round and round the loop forever, the loop is called infinite. The only way to stop an infinite loop is to abort it.

Cats and dogs Run this program:

The computer will repeatedly print ‘cat’ and ‘dog’, so the screen will look like this:

Yes, on the screen it will be raining cats and dogs! The computer will keep printing “cat” and “dog” until you abort the program.

Interactive mode Instead of creating that program (which requires you to press the F5 key to run), you can create the cats and dogs by typing in interactive mode, at the >>> prompt, like this:

When you create the blank line under ‘dog’ (by pressing the Enter key again), the computer performs the while loop and prints lots of cats and dogs, until you abort. Here’s why:

Conversions This program, which you saw before, converts feet to inches:

Here’s a sample run:

Suppose you want to:

To do all that, you can run that conversion program many times: each time you want to run that conversion program, say “run” (by pressing the F5 key). But instead of pressing the F5 key so many times, you can make the computer rerun the program for you automatically! To do that, begin your program by by typing:

That means: automatically do, repeatedly, the indented lines underneath. Type those indented lines, so the program becomes like this:

When you run that program (by pressing the F5 key once), the computer will repeatedly convert feet to inches, each time asking you ‘How many feet?’ The computer will keep converting feet to inches until you abort the program.

Counting Suppose you want the computer to count, starting at 3, like this:

This program does it, by a special technique:

In that program, c is called the counter, because it helps the computer count.

The top line says c starts at 3. The “while” loop says to repeatedly do this:

So the computer prints c (which is 3), then increases c (so c becomes 4), then repeats the indented lines again, so the computer prints the new c (which is 4), then increases c again (so c becomes 5), then repeats the indented lines again, so the computer prints the new c (which is 5), then increases c again (so c becomes 6), etc. The computer repeatedly prints c and increases it. Altogether, the computer prints:

The program’s an infinite loop: the computer will print 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and so on, forever, unless you abort it.

Here’s the general procedure to make the computer count:

To read the printing more easily, say end=‘ ’:

That makes the computer print horizontally:

This program makes the computer count, starting at 1:

The computer will print 1, 2, 3, 4, etc.

This program makes the computer count, starting at 0:

The computer will print 0, 1, 2, 3, 4, etc.

For

Let’s make the computer print every number from 0 to 19, like this:

Here’s the program:

The top line says x will be every number under 20; so x will be 0, then 1, then 2, etc. The line underneath, which is indented, says what to do about each x; it says to print each x.

The computer will do the indented line repeatedly, so the computer will repeatedly print(x). To begin, x will be 0, so the computer will print:

The next time the computer prints x, the x will be 1, so the computer will print:

The computer will print every number from 0 up to 19. It will not print 20.

How to start at 1 If you want to print every number from 1 to 20, say this instead:

That makes the computer start at 1 (instead of 0) and print the numbers under 21, like this:

Interactive mode Instead of creating that program (which requires you to press the F5 key to run), you can type in interactive mode, at the >>> prompt, like this:

When you create the blank line under print(x) (by pressing the Enter key again), the computer performs the loop and prints the numbers under 21, starting at 1.

When men meet women Let’s make the computer print these lyrics:

To do that, type these lines —

after making x be every number from 2 up to 5 (so x starts at 2 but stays less than 6):

At the end of the song, print the closing couplet:

(The computer automatically indents every line under “for”, until you begin a line by pressing the Backspace key, which tells the computer to stop indenting.) That program makes the computer print the entire song.

Here’s an analysis:

Since the computer does the indented lines repeatedly, those lines form a loop. Here’s the general rule: the statements indented under “for” form a loop. The computer goes round and round the loop, for x=2, x=3, x=4, and x=5. Altogether, it goes around the loop 4 times, which is a finite number. Therefore, the loop is finite.

 

If you don’t like the letter x, choose a different letter. For example, you can choose the letter i:

When using the word “for”, most programmers prefer the letter i; most programmers say “for i” instead of “for x”. Saying “for i” is an “old tradition”. Following that tradition, the rest of this book says “for i” (instead of “for x”), except in situations where some other letter feels more natural.

Print the squares To find the square of a number, multiply the number by itself. The square of 3 is “3 times 3”, which is 9. The square of 4 is “4 times 4”, which is 16.

Let’s make the computer print the square of 3, 4, 5, etc., up to 20, like this:

To do that, type this line —

and make i be every number from 3 up to 20 (so below 21), like this:

Count how many copies This program, which you saw before, prints “love” on every line of your screen:

That program prints “love” again and again, until you abort the program by pressing Ctrl with C.

But what if you want to print “love” just 20 times? This program prints “love” 20 times —

because it makes i be 0 then 1 then 2 then 3, etc., up to 19.

As you can see, “for” resembles “while” but is more powerful: “for” makes the computer count!

Count to midnight This program makes the computer count to midnight:

The computer will print:

At the end of the indented line, let’s say end=‘ ’, like this:

That makes the computer print each item on the same line and separated by spaces, like this:

If you want the computer to press the Enter key before “midnight”, say \n:

That extra print line makes the computer press the Enter key just before “midnight”, so the computer will print “midnight” on a separate line, like this:

Let’s make the computer count to midnight 3 times, like this:

To do that, indent the entire program under the word “for”:

That version contains a loop inside a loop: the loop that says “for i” is inside the loop that says “for j”. The j loop is called the outer loop; the i loop is called the inner loop. The inner loop’s variable must differ from the outer loop’s. Since we called the inner loop’s variable “i”, the outer loop’s variable must not be called “i”; so I picked the letter j instead.

Programmers often think of the outer loop as a bird’s nest, and the inner loop as an egg inside the nest. So programmers say the inner loop is nested in the outer loop; the inner loop is a nested loop.

Step size The “for” statement can be varied.

If you say —

the i will start at 5, stay under 18, and keep increasing by 3.

In that example:

Programmers usually say “for i”, instead of “for x”, because the letter i reminds them of the word index.

If you say —

the i will start at 17, not get to 4, and keep decreasing by 3.

In that example, 3 is called the decrease (or decrement); -3 is the step size.

To count down, you must say a negative step size, such as -3 or -1. So to count every number from 17 down to 5, give this instruction:

This program prints a rocket countdown:

The computer will start at 10, count down (because the step size is -1), and stop the loop before saying 0; so the computer will print:

Suppose you want i to be 6.0, then 6.1, then 6.2, etc., up to 8.0. Python doesn’t let the range contain decimal points, so use this trick: make j be 60, then 61, then 62, etc., up to 80, then make i be a tenth of j, like this:

Break

If you’re stuck in jail, you hope to break out. Similarly, if a computer is stuck in a loop (doing the same thing again and again), the computer hopes to break out.

To let the computer break out of a loop, say “break”. Saying “break” lets the computer stop looping; it lets the computer break free from the loop and skip ahead to the rest of your program.

For example, suppose you say:

The “while” tells the computer to obey the indented lines repeatedly, to form a loop. The first indented line makes the computer print:

But the next line says “break”, which makes the computer break out of the loop, stop looping, do no more indented lines, and so not print ‘sandwiches under’; the computer will skip ahead to the next unindented line, which prints:

So the program makes the computer print just this:

Guessing game This program plays a guessing game, where the human tries to guess the computer’s favorite color, which is pink:

Here’s what the 2^nd line means. If ‘pink’ matches the human’s reply to the question ‘What is my favorite color?’, break out of the loop, so the computer skips ahead to unindented line, which prints:

But if ‘pink’ does not match the human’s reply, the computer will continue looping, by saying:

Here’s another way to program the guessing game:

That program’s loop makes the computer do this repeatedly: say ‘You have not guessed my favorite color yet!’ and then ask ‘What is my favorite color?’ The computer will repeat the indented lines again and again, until the guess is ‘pink’. When the guess is “pink”, the computer breaks out of the loop and proceeds to the bottom line, which prints ‘Congratulations!’.

Sex This program makes the computer discuss human sexuality:

The 2^nd line makes the computer ask, ‘Are you male or female?’ If the human claims to be “male”, the computer prints ‘So is Frankenstein!’ and stops looping. If the human says “female” instead, the computer prints ‘So is Mary Poppins!’ and stops looping. If the human says anything else (such as “not sure” or “super-male” or “macho” or “none of your business”), the computer prints ‘Please say male or female!’ and then does the loop again, so the computer asks again, ‘Are you male or female?’

That program’s bottom line is called an error handler (or error-handling routine or error trap), since its only purpose is to handle human error (a human who says neither “male” nor “female”). The error handler prints a gripe message (‘Please say male or female!’) and then lets the human try again (by having the human do the loop again).

Here’s how to accomplish the same goal without indenting so much:

That “while” loop says:

Let’s extend that program’s conversation. If the human says “female”, let’s make the computer say “So is Mary Poppins!”, then ask “Do you like her?”, then continue the conversation this way:

To accomplish all that, put the shaded lines into the program:

Rules Here are the rules about saying “break”:

You can combine numbers and strings, to build a data structure. Here’s how.

Lists

Here’s a list:

That list contains 5 items: ‘love’ and ‘death’ and 666 and ‘giggle’ and 3.14.

If you say —

or say —

the computer will say the list:

If you say —

The computer will do the math and say:

A list is also called an array.

Variable A variable can be a list. For example, you can say:

Adding You can add lists together. If you say —

The computer will add the list [‘dog’,‘cat’] to the list [‘mouse’,‘cheese’] and say:

For The “for” statement can use a list. If you say —

the i will be 18 then 21 then 100. If you say —

the i will be ‘Joe’ then ‘Fred’ then ‘Mary’.

Let’s make the computer print this message:

This program does that:

You can also write the program this way:

Subscripts Suppose you say:

That list contains 3 items: ‘Joe’, ‘Fred’, and ‘Mary’.

In x’s list, the starting item (which is ‘Joe’) is called x₀ (which is pronounced “x subscripted by zero” or “x sub 0” or just “x 0”). The next item (which is ‘Fred’) is called x₁ (which is pronounced “x subscripted by one” or “x sub 1” or just “x 1”). The next item is called x₂. So the 3 numbers in the list are called x₀, x₁, and x₂.

To make the computer say what x₂ is, type this:

The computer will say:

Notice this jargon:

You can change what’s in a list. For example, if you want to change x₂ to ‘Alice’, say:

Suppose you say:

The x starts as [‘Joe’,‘Fred’,‘Mary’], but ‘Mary’ changes to ‘Alice’; so if you say —

the computer will say:

Too long If you want to type a list that’s too long to fit on one line, do this:

List in a list A list can contain another list. For example, look at this list:

In that list, x[0] is ‘Joe’, x[1] is ‘Fred’, and x[2] is the list [‘dog’, ‘cat’]. Since ‘dog’ is the starting item of the list x[2], ‘dog’ is x[2][0]. Since ‘cat’ is the next item of the list x[2], ‘cat’ is x[2][1].

Blanks in a list You can create a list that’s full of blanks, then fill the blanks later.

To create a list that has 100 blank items, say:

The “None” means “blank”. Those 100 blank items are called x₀, x₁, x₂, etc., up through x₉₉.

After creating that x, you can give a command such as:

That command is legal just after you’ve created x.

Dictionaries

Suppose Jack is great, Jim is jolly, Sue is sweet, and Mary is smart.

To store that info, create a dictionary called d, like this:

When you type that, put the dictionary in braces, which are the symbols “{}” and require you to press the Shift key.

Then if you want to use that dictionary to look up Sue, type:

The computer will use the dictionary, look up Sue, discover Sue is sweet, and say:

Instead of the letter d, you can use any variable name you wish.

Here’s how to put a dictionary into a program:

The top line creates the dictionary. The next line makes the computer ask “What name should I look up?” and wait for the human to type a name. If the human typed a name (such as “Sue”) that’s in the dictionary, the program’s third line makes the computer print a message such as:

But if the human typed a name (such as “Alice”) that’s not in the dictionary, the program’s bottom line makes the computer print a message such as:

Besides storing comments such as “sweet”, you can make the dictionary store people’s addresses, phone numberd, social-security numbers, birthdays, debts, sexual orientiations, and methods by which they’d like to kill you when they discover you’ve stored that private data.

A dictionary is also called a lookup table.

Let’s make the computer translate English colors to French, by using this lookup table:

That lookup table becomes our multilingual dictionary. Here’s the program:

Too long If you want to type a dictionary (lookup table) that’s too long to fit on one line, do this: