{"id":85,"date":"2021-05-06T19:56:53","date_gmt":"2021-05-06T19:56:53","guid":{"rendered":"http:\/\/juthon.com\/?p=85"},"modified":"2021-05-22T14:35:55","modified_gmt":"2021-05-22T14:35:55","slug":"passage-2-hello-world","status":"publish","type":"post","link":"http:\/\/juthon.com\/index.php\/2021\/05\/06\/passage-2-hello-world\/","title":{"rendered":"Passage 2: Hello World"},"content":{"rendered":"\n

Welcome to the second passage of our Python programming language series. We already have Python and IDLE installed on our computer. We can act!<\/p>\n\n\n\n

If you don't remember what IDLE is, take a look at the previous post.<\/p>\n\n\n\n

Jump to Polish version<\/a><\/p>\n\n\n\n

We will take our first steps in the IDLE main window called \"shell\". The shell is an interactive window that is great for writing very short programs (scripts) for purely training purposes. And that's where we start: writing mini-scripts in shell. In the future, when we want to write slightly more elaborate scripts, we will do so in Python files.<\/p>\n\n\n\n

There is such a superstition in computer science circles that you will not get lucky in programming if you don't write your first program and have the words Hello World! generated on the screen.<\/p>\n\n\n\n

If so, let's do it! The text that Python has to \"print\" (on the screen) for you is technically called a string. We must (obligatorily) mark the beginning and the end of the string with quotation marks (or if you prefer: with an apostrophe):<\/p>\n\n\n\n

\"Hello World!\"  or  'Hello World!'<\/p>\n\n\n\n

Now it's time to get to know the first Python \"syntax\" command. The first command we'll know is print(). Note the parentheses used after the word print. Within these parentheses you put the content you want to print.<\/p>\n\n\n\n

Therefore:<\/p>\n\n\n\n

open IDLE<\/p>\n\n\n\n

type: print('Hello World!')<\/p>\n\n\n\n

press ENTER<\/p>\n\n\n\n

The effect should be like this:<\/p>\n\n\n\n

<\/figure>\n\n\n\n

We told Python to type out (print) the phrase Hello World! \u2014 and Python did it.<\/p>\n\n\n\n

If you've achieved the above effect, congratulations! You just wrote your first Python program.
Let us note again how the shell works. The typed commands are executed immediately. That is why it is suitable for short, ad hoc exercises \u2014 and basically only for that.<\/p>\n\n\n

Writing and reading<\/h2>\n\n\n

Writing and reading are two basic skills in today's world, right? The third is definitely the ability to count. Likewise, with coding, \"writing\" and \"reading\" are the two most important ways you communicate with the computer. When you want the computer to \"expose information\", you will use the write<\/em> command. On the other hand, when the computer is supposed to \"receive information\", we will use the read<\/em> command. As it has been for many many years in the good old PASCAL\u2026 Unfortunately, in Python, there is no write<\/em> command and no read<\/em> command. Instead, we have print<\/em> and input<\/em>[1]<\/strong><\/em><\/a>. We have already learned how print works. So it's time for input.<\/p>\n\n\n\n

Remember about the parentheses for Python commands. The correct syntax looks like this:<\/p>\n\n\n

input()<\/p>\n\n\n

When you write the input () command, you assume that someone \u2014 some user \u2014 will open the script you wrote and that script will ask them for some information. Thanks to the input () command contained in the code, the information given by the user will be \"read\" by the program.<\/p>\n\n\n\n

For example, it might look like this:<\/p>\n\n\n

print('Enter your name:')<\/p>\n\n

input()<\/p>\n\n\n

As you can see, here we have a program consisting of two lines of code. The first line is the command to write a message to the user. The second line is the command to read the answer given by the user.<\/p>\n\n\n\n

Considering the user's involvement in the current task, it is difficult to imagine further work in the Python shell. Recall: the shell immediately executes the commands. Therefore, it is not suitable for activities in which you, as the author, create a program, and later someone else (the user) runs it on his computer. \"Later\" in this case means \"whenever he wants\". Whether it's in a minute, an hour or a year. Therefore, in order to make your script permanent, we will save it to a file. The user will need to run the file first to execute the commands.<\/p>\n\n\n\n

When you want to save the code as a file, in IDLE: select File menu, command New File. In the newly opened window, enter the two mentioned lines of code:<\/p>\n\n\n

print('Enter your name: ')
input()<\/p>\n\n\n

Now select \"Save as \u2026\" from the File menu, give the file a name such as Exercise01 and save it with the extension .py in the directory where you will store your training files.<\/p>\n\n\n\n

Great, you just created your first Python script which you saved as a file. We will check how our script works in a moment. A little more patience.<\/p>\n\n\n

Working with Python files<\/h2>\n\n\n

Working with Python files is very easy. We have just shown you how to save new content to a file.<\/p>\n\n\n

Create files<\/h3>\n\n\n

Developers work with files in two ways. When their creative inspiration comes and they want to write a piece of code, they use menu in IDLE. They open a new file (File \u2192 New File) and enter the code commands they invented, and then save it. Others, who do not like IDLE, use a simple text editor (for example Microsoft Notepad, which is a regular notebook probably known to every Windows user). We mentioned it in the first post and promised to get back to the topic.<\/p>\n\n\n\n

Yes it's true \u2014 you can save your code in a notepad file. When you write a few lines of code in your notebook and decide that you want to save the effect of your work, select the \"Save As \u2026\" command, give the file a name, but instead of the suggested .txt file extension, type py after the dot. This way you have a Python script ready to run at any time.<\/p>\n\n\n

Running existing files<\/h3>\n\n\n

How is it with running files? The easiest way to run a Python file is to find it in the correct directory (for example using the well-known File Explorer) and double-click the mouse. Remember that opening a file actually means running it: the Python interpreter will execute the commands you have written to the file.<\/p>\n\n\n\n

However, there may be times when you want to open a file in order to edit it further, without running it. Then, after selecting a file from the list, right-click it and select \"Open with\", and then select Notebook from the list of available programs.<\/p>\n\n\n\n

You can also run the file with IDLE. Select the file, right click it and select \"Edit with IDLE\". The file will be opened for editing. Now select the Run menu<\/em>, Run module<\/em> command. The file will run.<\/p>\n\n\n\n

In the early stages of learning Python, it's a good idea to run files this way. This has the advantage that the effect of the file (\"end screen\") remains visible in the shell until you close it yourself. On the other hand, when you run the file using the traditional method, the generated content appears and disappears immediately. For inexperienced programmers, this is extremely irritating; they describe the effect as follows: I found the Python file in the directory, chose it with the mouse, clicked twice. A black screen flashed before my eyes and disappeared.<\/p>\n\n\n\n

Once we get to know Python a little better, we promise to come back to the topic and present a simple trick to prevent the screen from disappearing. For today, we advise you to run files via IDLE. Considering that we will (for now) write rather short \"training\" scripts, this way of running them will be the most convenient.<\/p>\n\n\n

We check how our file works<\/h3>\n\n\n

It's time to run the script we wrote in Exercise01.py.<\/p>\n\n\n\n

Find this file on your computer, select it with a single mouse click, then right click and select \"Edit with IDLE\". Our file will open as for editing<\/p>\n\n\n\n

<\/figure>\n\n\n\n

Now select Run \u2192 Run Module<\/p>\n\n\n\n

The file is launched. The first command is carried out, which is the command to display the sentence \"Enter your name\" on the screen.<\/p>\n\n\n\n

<\/figure>\n\n\n\n

Type in any name and press Enter. Nothing special is going to happen because \u2014 so far \u2014 we haven't told Python how to react to a user entering a name. However, you can be sure that Python will remember the name you entered. It's always been a little success, isn't it?<\/p>\n\n\n\n

In the next post, we will expand our script and try to have a slightly longer conversation between our program and the user.<\/p>\n\n\n

Summary<\/h2>\n\n\n

In this passage, we wrote our first Python script saved as a file. The file can be run by the user at any time. The script contained in this file will exchange information with the user, writing messages to the user (print command) and reading the responses given by the user (input command).<\/p>\n\n\n\n

From now on, we will write and save our code in Python files, and the shell will only play an auxiliary role.<\/p>\n\n\n

Appendix<\/h2>\n\n\n

In this appendix, we'll try to take a deeper look at the input() command. So we will have to use a bit more technical (specialized) vocabulary.<\/p>\n\n\n\n

What we commonly call a command is in fact a Python function. There will be time to learn more about the functions later. For now, let's just remember that input() is one of Python's built-in (that is, ready-to-use) functions. When the input() function is called, the screen displays a cursor waiting for a string to be entered. In other words: when the input() function is executed, the program flow stops until the user enters the specified value.<\/p>\n\n\n\n

Another important observation: each programming language has strictly defined data types<\/em> that a program written in that language can accept. Python is no different. Examples of data types in Python are:<\/p>\n\n\n\n