Python from absolute zero. Learning to code without boring books

If you have a modern Linux distribution on your computer, it already includes Python 3, and writing the first programs will be convenient in IDLE, a simple code editor that comes with Python. In Ubuntu, to install it, type in the console

On Windows, when installing Python, tick Add to Path on the first screen of the installer to allow running python3 from the command line in any convenient place.

After running IDLE, go to Options → Configure IDLE, go to the General tab, and tick Open Edit Window, click OK, and restart IDLE. You can now write programs, save them, and run them with the F5 key. Here we go!

Variables

Any programming language has a thing called variables. It’s like school algebra: here’s the variable a = 1, here’s the variable b = 2. I mean, these are such abstract things, inside of them lies a value that can change – for example, when you write after a variable, a sign equals and some new value.

Well, you have already realized that print(a) is a command that prints the current value of a variable on the screen. You wrote the variable + 2 again after the equals sign, so first, there was a value of 2 in the variable, then 2 more were added to this value. The screen proudly displays 4. Congratulations, you put two and two together!

And if it is not initially known, what numbers should be added? Then you would first have to ask the user to type them into the console and press Enter. Let’s do this:

Inside the parentheses at the input, write an explanation for the user, what exactly he is asked to enter. But the problem is, by default, everything entered via input is considered not a number, but a string, so before adding the number of liters of beer, you must first convert the entered strings into numbers using the int() function.

Okay, so I converted the strings into numbers, put them in the c variable, and then what’s the dash inside the parentheses at print? Here are strings (the strings are always written inside quotation marks) explaining what exactly is displayed, and the result of addition is passed to print() function.

For the strings to painlessly add to a c variable containing a number, you need to convert it into a string with the str() function, just as we converted strings into numbers, only the opposite.

In general, there are many types of variables, but you get the point — to do something with variables, you need to first lead them to one type — to a string, or to a numeric, or something else. Not to worry about it, Python doesn’t add up the numbers, but the strings, and the entered 2 and 3 liters of beer add up to not 5 but as many as 23. It would be nice to be real!

Here’s another example that calculates how long you can drink beer based on the average life expectancy in Russia:

Here, we call the function input() to get a value, subtract it from 73 (the average life expectancy of a Russian), remembering to turn a string into a number, and then print the result by turning the number back into a string and adding other strings.

So you learned what are integer and string variables, that you can convert these types into each other with int() and str(). In addition, now you can get variables from the user using the input('Enter something') function and print results using the print() function.

Conditions

Any program is based on conditions. Depending on whether they are implemented or not, the program may follow one or another path. Imagine driving and looking at the clock: if it’s already 10 p.m., you turn home, if not, you can stop by for a visit. The program also works the same way: checks a value and collapses there or here and executes the corresponding piece of code.

If means “if” in English, and else means “other” or “otherwise”. In the string after if, there is a condition that we check. If this is correct, the first block of code is executed (it is separated by four spaces at the beginning). If not, the one after else:.

Another important point here is the sign of equality in conditions. It is written as a double ‘equals’ (==) and thus differs from assignment, a single ‘equals’.

The lower() function makes all letters in a string small before comparing the condition, because a stupid user can enter the word YES with active Caps Lock, and this must be provided in advance.

Let’s try making a condition for checking the login and password using the AND statement, which is written as and. It is needed to verify that the first and second conditions are met simultaneously.

Let’s create an even more complicated condition using the or operator, which translates to OR.

Parentheses are used here — Python doesn’t require parentheses for simple conditions, but for complex ones they are used to explicitly define the order of operations. The program welcomes only two users, ivan or marina. So first, it checks if Ivan’s login and password didn’t match, and then after the or operator it checks for Marina.

Here’s another example, it uses elif, which means something like ELSE-IF. This is used to specify several blocks of commands: if one condition is not met, the following is checked using ELIF, and so on.

Different comparison operators can act as conditions:

• a == 9 (a is equal to 9)
• a != 7 (a is not equal to 7)
• a > 5 (a is more than 5)
• a < 5 (a is less than 5)
• a >= 3 (a is more than or equal to 3)
• a <= 8 (a is less than or equal to 8)

You can also invert the condition true to false and back with the word not.

For example, you need a person to enter a number at least (NOT) 5.

Lists

Regular variables are good for storing single values, whether a string or a number. But sometimes you need to store a group of variables. This is where lists come to help you.

For example, a list could be:

Each item in the list has its own index. To get one of the list values, you can refer to its serial number. Numbering in the lists goes not from one, but from zero, that is, 0, 1, 2, 3, 4…

print(a[2]) command will print the number 90, the third element (the number from scratch!) in the list above. There can be as many items as you want.

You can also make a list of strings:

Then print(b[1]) will print Vanya string.

You can add a new value to an existing list using the append method:

Now, the list looks like this:

If you need to refer to some list element, counting from the end of this list, you can write negative numbers. For example, the last item in the list is -1, and print(b[-1]) will print Dim.

Any list can be sorted ascending or alphabetically.

After performing a.sort() function, the list will take the form [5,20,30,67,90].

Now, a little bit about the slices. A slice is as if receiving some part of a list, which in turn is also a list. Slices are specified as follows:

Here, x is the number of the element from which the slice is taken, y is the last element of the slice, z is the interval at which we take the elements (an optional value).

We get a slice of list b elements from 1 to 3 (4 not included in the slice):

Get a slice of list b elements from 2 to the end:

Get every other item in the list:

We reverse the order of the list items:

By the way, ordinary strings also support slices, their result will also be a string. For example:

‘Hello’ will appear on the screen, because we printed the first five characters of the string.

Other lists can be elements of a list. To refer to the items in the list within the list, use another set of square brackets:

Such a list of lists is called two-dimensional and resembles a table. For example, to get the first number in the second string (indexes 0 and 1, because the number is zero), you write:

The result will be 44.

A list is a variable sequence. This means that if you create actions on the list, you do not have to override it and re-save it into a variable. But a string is an immutable sequence. If you do anything to it, you’ll have to put the new value somewhere.

Another invariable data type is the tuple. It’s the same list, but you can’t change it. So it takes less memory. You can declare a motorcade using parentheses:

A set is another sequence of elements, each of which does not have its own index. However, all elements of a set are unique and not repetitive. If you want a set of unique elements, you can put them in a set. Let’s convert an existing list into a set and see that there are no repeating elements left in it. l = [1, 2, 2, 3, 3, 4, 1]

Result: set([1, 2, 3, 4]), i. e. repeated elements disappeared. By the way, if you want to turn a set (or something else) into a list, use the list() function. # Loops

Loops

A loop is a block of commands that is repeated a certain number of times. Loops can be specified in different ways. For example, the loop for is often used to go through all elements of a sequence like a list.

Here you start with a list of numbers, and then you use the for x in lst construct to go through each element of this list one by one and do something with it. These actions, as with conditions, are indented.

The x variable , in this case, takes the value of each element of the lst list one by one, adds one to this value, prints the result, and then proceeds to the next round of the loop, i. e. takes the next value in the list and does the same with it, and so until the list ends.

If you just need to execute commands a specific, predetermined number of times, then use the for loop and range() function.

If you have any list, you can easily run through it in a loop:

Now it’s time to learn about the while list. The word while translates from English as “until” (not in the sense of “goodbye”, but in the sense of “until”). That is, the commands inside the loop will be executed as long as the condition specified below is met. For example, here is a loop that will print all even numbers from 1 to 100.

The while loop must explicitly specify the change in the variable that is responsible for the condition, otherwise the loop may become infinite and the program will hang.

Going forward, I’ll show you how to create a very small but malicious script called a fork bomb. It endlessly creates its copies in RAM, which can cause quite a few brakes:

Here, we do a new thing that is also worth remembering: we import a module, namely an os module that contains commands for accessing various functions of the operating system.

Then inside the while loop, we create endless copies. As a condition, we have written here simply True, that is, simply ‘true’. The loop body does not change this value, so the loop will execute over and over again, and os.fork() will produce new and new processes, clogging up RAM with them. So, mate, be careful with the while loop!

Practical task: monitor the clipboard

Now let’s move on from case studies to something interesting! After all, we’re not just studying programming out of curiosity. Let’s write a program that monitors what’s happening on the clipboard.

One of Python’s strengths is its huge database of off-the-shelf modules that you can pick up and plug into your programs. To install and use them within your programs, you can use a package manager called pip. Let’s assume you already have a pip installed.

First of all, we put a pip module that is responsible for working with the clipboard. In Windows, it will be a command like this:

In Linux — slightly different:

Proceed to coding. Our software will monitor the clipboard and print any text the user copies on the screen. I explained them in the form of comments, they are beaten in Python by a # sign at the beginning of the string.

Well, congratulations, you wrote a program that can catch anything a user copies to the clipboard. It can be further developed — for example, instead of displaying it, write the caught strings to a file with a log or send them over the network. But we just started, right?

Homework

Let me give you some homework you can do, so you can practice on your own.

1. Make a program that calculates your ideal weight based on height and age. Find the corresponding formula in search engines and fulfill the condition for calculation. Beer’s a beer, and you gotta watch your health!

2. Write a program that in a loop monitors the clipboard, periodically getting text from it using pyperclip.paste(), and if it sees that someone has copied email, replaces this email with another one pre-defined in the code, placing it on the clipboard with the command pyperclip.copy('coolhacker@xakep.ru').

If the topic turns out to be in demand, then in the next article, we will look in detail at working with strings, files, look at the Internet, and again dabble in writing the simplest possible hacker scripts.