I know, I know, there are an infinite number of Python tutorials on the internet (and on everything else for that matter), but I haven't found a single one I think is really good, so I'm making this one. My goal is to make a tutorial as digestible, concise, and accurate as I can. (Most of the others I've seen I dislike because they talk about advanced stuff in the early chapters and put off certain essentials for too long.) The project we're aiming for in this first entry will be a text-based, single-operation calculator.

Prerequisite for this series: preferably a basic understanding of how programming works, but no actual ability.

Installing the Python interpreter

Regardless of your operating system, there's actually a good chance Python is preinstalled, but if not, it's easy to get:

If you're on Windows: you can download it from python.org (you probably want the "x86-64 executable installer"). The installer should include something called IDLE ("integrated development and learning environment"), which is a convenient (compared to Windows File Explorer) way of accessing the Python prompt and executing files. You can run IDLE from the Windows menu.

If you're on any Unix-like system: use your distribution's package manager to install it if needed. To use it, open a Terminal and run python <filename> to run a file named filename, or just python for interactive mode. (The command might not just be called python; it might be called python3, python3.8 or whatever version, so if python doesn't work, try that.)

What's with this "prompt"?

In most languages, to write a program, you have to save the source code to a file and then execute the file. But Python also features an interactive mode, where you can type instructions one by one and see them executed as you enter them. This is called the interactive prompt, the Python shell, or the REPL (read-evaluate-print loop). Please follow along with this for now by starting IDLE or the appropriate Python command.

When you start, you'll probably see a message about the Python version and stuff and then a line starting with >>>. The >>> means it's waiting for you to enter code. The result of each command you type is displayed on the next line without the >>>. The >>> is called a prompt (confusingly, that word refers to both this symbol and the interactive mode in general).

To write a saved program in IDLE, you can go to File -> New File in the top menu, and then when you run it with Run -> Run Module, the output will show up in the prompt window. But I highly recommend sticking with the interactive mode for now.

Most of my excerpts here are interactive sessions, and so lines that start with >>> are lines of code (but the >>> is not part of them), lines without it are output. If I show an excerpt with no >>> lines, then it's a saved program instead of an interactive session.

Math

First, let me demonstrate the calculator use:

>>> 5 + 2
7
>>> 7 / 5
1.4
>>> 11 * (3 - 1)
22

The interactive prompt interprets math expressions just like math.

Variables

A variable is like a name you can set to mean whatever you want. number = 5 assigns the value 5 to the name number. Hereafter, we could include number in our calculations and it would have this value:

>>> number = 5
>>> number + 2
7
>>> number
5

Note that Python's prompt doesn't print the result when you just assign to a variable. You can see the value of a variable by entering it on a line by itself, since Python reads it the same as if you just entered its value. Also note that number + 2 didn't change the value of number; it just printed the value plus 2. number = number + 2 would've changed the value of number.

A valid variable name can only use letters, numbers, and the underscore, and can't start with a number.

This lets us do slightly more interesting stuff:

>>> x = 12
>>> y = 2
>>> z = x + y
>>> z / y
7.0

Now let's take a break from the math. There are just a couple other prerequisites to our objective of making a calculator.

Print

First of all, if you save the above lines of code (minus the >>> prompt, obviously) to a file and run it, you'll notice the result is never displayed, even though it works in the prompt. In the shell, Python automatically displays the result of an expression, but not when it's in a saved program (there's a good reason for this). To make a program output text you need the print function:

x = 12
y = 2
z = x + y
print(z / y)

The parentheses after print are important - they tell Python what the print applies to, and it won't work without them.

You might've assumed this, but programming is case-sensitive: that function is called print, not Print or PRINT, and it matters. All functions and variable names are in lowercase by convention.

Strings

print can print anything, not just numbers, so now we're going to talk about strings. A "string" meaning a string of text. String literals (as opposed to a variable that holds a string) always have to be enclosed in quotes, so that Python knows to interpret them as literal text instead of as the name of a variable or something. The quotes are not themselves part of the string, though:

>>> print("hello")
hello

You can use either single or double quotes, as long as you use the same kind on both sides.

Before we get on with strings, now's a good time to talk a little about error messages, since you're very likely to run into some in the next section.

Reading error messages

Unless you're a way more accurate typist than me, there's a good chance you've already seen an error message, and it looked like a mess. If you haven't, try entering a line like blah and you should see something like this:

>>> blah
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'blah' is not defined

For now, don't worry about the line Traceback (most recent call last): - we won't get into what that means until much later in this series. The second line shows you the name of the file your program was saved to, or <stdin> if you were just in the prompt (it stands for 'standard input', and I explain the concept in my Unix tutorial track, but that's not important here); the line number that caused the error (or 1 if you're in the prompt); and the in <module> part is another thing you don't need to worry about until much later.

The most important line is the last one, which shows what the actual problem is. Here are some of the most common kinds of errors:

Escaping

The next logical question about strings should be (if you already started thinking this then you get a cookie): if enclosure in quotes is how we tell Python where the string starts and ends, how do we put literal quotes in a string? print("it's") works because the apostrophe is a single quote and that string is enclosed by double quotes, but if you need to print a string that has both types of quotes in them, it's not so simple. print("I said, "it's okay."") would look to Python like the string you want to print is "I said, " and for some reason you put it's okay."" after it. You would get a syntax error.

If you want to put literal quotation marks inside of a string that are the same kind as the ones enclosing the string, you need to "escape" them by putting a backslash before each one: print('I said, "it\'s okay."') or print("I said, \"it's okay.\"") will print I said, "it's okay.". A backlash inside of a string is a special signal to treat the next character literally, or vice versa if it's a character that's normally literal. To include a literal backlash, you can write \\ - the first one tells Python to treat the second one literally.

+ and * on strings

The + and * operators also work on strings. Check this out:

>>> 'hello' + 'friend'
'hellofriend'
>>> 'hello' * 3
'hellohellohello'

(Note that they weren't separated by spaces. You might've already noticed, but the spaces I've been typing around everything are unnecessary; spaces inside code are generally ignored. They're just there to make it easier to read. If you want to space-separate the output, put a space inside one of the string literals, like 'hello ' + 'friend'.)

You probably also noticed that this time the results of the operation were still printed with quotes around them. Entering a string by itself at the prompt just shows it in the form you'd have to type it in to get that value. print prints them without the quotes:

>>> 'hello'
'hello'
>>> print('hello')
hello

Jargon: concatenate: to add sequences together. Using + on strings is "concatenating" the stings. Using + on numbers is not concatenation, because it gives you the total of the numbers, not the numbers stuck together. (This distinction will start to make more sense when we get into sequences in part 3.)

As an aside, you can now figure out how to make backslash-escaping unnecessary. Try to find another way to print a line of text that has both single and double quotes.

Solution
print('I said, "' + "it's okay." + '"')

See how this works? I made it three separate strings: the first one is enclosed by single quotes, so the double quote inside it is literal; the second one is enclosed by double quotes, so the single-quote inside it is literal, and the last one just needs to contain the remaining double quote, so I put it in single quotes.

This isn't to say you shouldn't use backslash-escaping (of course it's better than that mess), I just wanted to point out that they're not strictly necessary.

Escape sequences

Okay, this one isn't essential to our immediate mission, but while we're talking about backlash escapes I better mention what they can do besides escape quotes. This won't take long, I promise.

There are several normal letters that, if escaped inside of a string, take on a special meaning instead of the other way around like quotes do. The most common example is n. \n inside of a string stands for 'newline':

>>> print('line 1\nline 2')
line 1
line 2

print is also necessary to interpret the escape sequences. If you just type 'line 1\nline 2', the string will, again, be shown back the way you would have to type it to get that value.

Another common escape sequence is \t for tab:

>>> print('one word\tanother')
one word	another

Here's a list of string escape sequences, but you don't need to memorize them or anything.

Input

input is the basic function for getting text input from the user. Just doing input() by itself will wait for the user to type something thing in (stopping when they press enter). After that, you can think of the input() as if it gets replaced by what the user typed in, so you can use it like a literal value:

print(input())

That one-line program will wait for the user to type something in, and then repeat it back to them. If the user types "hello", it's like that line becomes print('hello'). You could also store the input in a variable like a = input(), then do something else, and parrot it back later with print(a).

You can also give input a string value to use as a prompt:

text = input("give me some text: ")
print("here's the text you gave me: " + text)

(Note that input doesn't output a newline after its prompt, which print does. If you did print("give me some text:") and then input(), the user would be typing on the line below "give me some text".)

So with that done, can we use input to get numbers for our interactive calculator?

>>> num = input("give me a number: ")
give me a number: 5
>>> num
'5'
>>> num * 2
'55'

Hm... there's one problem with using input to make a calculator: it gives us a string, so when I tried to multiply it, it thought I was trying to multiply the text '5'.

5 and '5' are very different - one's a number and the other's a string. It's kinda like the difference between a number and a numeral. This is where the int function comes in: it converts a string to an int (integer).

>>> num = int('5')
>>> num
5
>>> num * 2
10

(The str function also exists to turn a number into a string. You don't need it for print, though, because print already prints a string representation of its parameter.)

Calculator time!

Now you've got all the knowledge you need to make a program that asks for two numbers and then prints their sum (or whichever operation you like - you can't ask the user which one they want, because that would require the if statement, which we'll get to in the next lesson). Combine what you've learned and you can do it in as little as three lines.

Solution
num1 = int(input('give me a number:'))
num2 = int(input('give me another number:'))
print(num1 + num2)

Other exercise: make it print the sum, difference, product, and quotient of the two numbers in sequence, as a substitute for asking the user which one they want.

More interesting exercise: How do you do the single-operation calculator in one line and without using any variables?

Solution
print(int(input('give me a number:')) + int(input('give me another number:')))

There's just one appendix worth tacking on to this first chapter:

Floats

You might've noticed that the calculator program doesn't accept decimal inputs, only integers. That's because we used the int function - int converts a string to an integer, and if it can't, it raises an error. (Interestingly, int can convert a decimal input: int(3.5) gives 3, but it can't convert the string '3.5' to an integer.)

A "floating-point number" or "float" is the programmer term for a decimal number, and the function to convert to them is float. Go ahead and try modifying the calculator program to accept float input.

(If you play with floats, you might also run into a strange inaccuracy like 0.1 + 0.2 adding up to 0.30000000000000004. This isn't Python's fault; it's due to an inherent consequence of the way your hardware stores decimal values in memory. There's a good explanation here, but it's not important unless you're curious.)

Next lesson: Flow control



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