Starting a Python program in the command line

On this platform your Python programs run in the browser: you press Run, and the output appears below. On your own computer it works differently. There, a program is a text file whose name ends in .py, and in the command line you hand it to Python, which reads it and carries it out line by line.

Is Python installed?

Ask Python for its version. On Windows the command is python, on the Mac python3:

python --version

python3 --version

If the shell answers with a line such as Python 3.13.1, everything is ready. If it does not know the command, install Python from python.org. On Windows the Microsoft Store sometimes opens instead of an error message; you can install Python there as well. Close the command line after the installation and open it again so that it knows the new command.

Saving the program as a file

For programs you need an editor that saves plain text. Good choices are Visual Studio Code or Positron, which also highlight Python code in colour. Word or Pages are no use, because they write formatting into the file as well.

Open the folder studium\programmieren\uebung-1 from the exercise in the editor, open the file hallo.py and write into it:

name = input("What is your name? ")
print(f"Hello, {name}!")

Save the file. Make sure that it really is called hallo.py and not hallo.py.txt; that is why you made the file extensions visible.

Starting the program

In the command line, change into the folder the file is in and give its name to Python as an argument:

cd ~\studium\programmieren\uebung-1
python hallo.py

cd ~/studium/programmieren/uebung-1
python3 hallo.py

Python reads the file and carries it out, and whatever the program prints with print appears in the command line. input waits until you type something and press Enter. After that the program has finished, and the prompt appears again:

PS C:\Users\anna\studium\programmieren\uebung-1> python hallo.py
What is your name? Anna
Hello, Anna!
PS C:\Users\anna\studium\programmieren\uebung-1>

If you change the program in the editor, you save it and start it again straight away with the up arrow key and Enter.

When Python cannot find the file

The most common error message at the beginning looks like this:

python: can't open file 'C:\\Users\\anna\\hallo.py': [Errno 2] No such file or directory

Python looks for hallo.py in the current directory, and it is not there. The path in the message tells you where Python looked. Change into the right folder with cd and check with ls that the file is there.

Stopping a program

Some programs run forever, for example when a loop never ends, or they wait for input you do not want to give. With Ctrl+C you stop a running program, on the Mac also with Control+C and not with Cmd. Python then reports KeyboardInterrupt, and the prompt comes back.