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My First Program

>>> 3 + 4

You can issue commands in the Python interactive console. But it has a disadvantage: The program that you type is lost when the session ends. It is good for trying simple commands. But you need a way to save more complex programs somewhere.

Open your editor (You should have an editor installed. If not, follow the instructions in previous lesson.)

Create a new file in your editor and type the following code:

print("Hello world!")

Then save the file as

  • You have to create the subdirectory named 02 in the pyladies (or whatever folder name you've created last lecture) directory.
  • Then you can store the file as in it.

If you can choose the encoding in your editor, you should use UTF-8. If you can choose the file type, use .py or All Formats.

Some operating systems or editors hide extensions or add their own extensions. You can check the real name using the command line. Open your command line and change your current directory using the cd command to ~/pyladies/02. List what is in the directory using the command ls (Mac or Linux) or dir (Windows) and check that the filename is really and not, for example,

Executing Your Program

Open the command line and activate the virtual environment. Change to the ~/pyladies/02 directory and issue the following command:

$ python

You have learned about the command line in a previous lesson which shows how to change the current directory using the cd command. You know how to activate the virtual environment since the lesson about Python installation.

Do you see the greeting? If you do, you just wrote your first Python program!

If it does not work, make sure that:

  • You have activated the virtual environment. (You should see the (venv) in your command line prompt. If you do not see it, use the activate or source activate command that you have used in a previous lession.)
  • You are in the correct directory: ~/pyladies/02 (you need to replace ~/pyladies with the directory that you have created previously).
  • The file contains the correct command, including quotes and parentheses.
  • Do not type $ or > character in the command line – it is there to indicate that this is a command line. It is printed by the operating system after any program has finished. You type only: python

If it still does not work, ask your coach.

Note about code style

It does not usually matter where you use space in Python inside a command. The command print("Hello world!") has the same effect as:

print      (   "Hello world!"     )

It is however a good practice to follow some typical guidelines. In English, we do not write a space after an opening parenthesis. In Python, we do not write a space even between print and (. The recommended style is:

print("Hello world!")

The spaces between the quotes have a meaning: If you type " Hello world!", then the extra spaces are printed.

Toto je stránka lekce z kurzu, který probíhá nebo proběhl naživo s instruktorem.