Note: This page is about running the Python lab code, using the command line. For general information on how to do the labs, in particular where to find them on GitLab Chalmers, see the page Doing the labs.

To run the labs, at the very least you need to have Python version 3.6 or later installed on your system. The computers at Chalmers should have everything you need installed. Make sure that the command python3 works. Note that just running python might give you Python 2 instead!

Setting up

Make sure you have set up an SSH key with Chalmers GitLab (see page Doing the labs). Clone the lab project using

git clone <address>

where <address> is the repository SSH URL from the project page on Chalmers GitLab. That’s it if you followed our recommendation not to add a passphrase to your SSH key. Otherwise, you will be asked for this passphrase here.

Note: if you see the prompt git@git.chalmers.se's password:, your SSH key is configured incorrectly.

Navigate the command line to the project directory lab-N.

Git workflow

Follow the standard git workflow explained on the page Doing the labs. Briefly:

git add -u to add all of your changes to git’s index.
git commit to form a commit of all added changes.
git push to send your commit(s) to the shared repository on Chalmers GitLab.
git pull to receive updates from the shared repository on Chalmers GitLab.

Running

Suppose there is a Python file Program.py that has the following clause at the end:

if __name__ == '__main__':
    ...do something nice...

Run the program as follows:

python3 Program.py

This executes the main method. The program will print its output in the command-line interface. If it expects input, you may type in the command-line interface (following each line by Enter to flush).

Quitting

To abort the execution of a program, you press Ctrl-C from the command line. You might get an error message saying that the program was aborted.

When a program expects input, you can also send it the “end-of-input” signal (closing the input stream of the program). It will typically interpret this as a request to exit. This is done by pressing Ctrl-D (on Mac/Linux), or by pressing Ctrl-Z followed by Enter (on Windows).

Arguments

A program may take arguments, which can be given on the command line, separated by spaces. Running

python3 Program.py arg0 arg1 arg2

results in the equivalent of this execution:

sys.argv = ["Program.py", "arg0", "arg1", "arg2"]

Command-line arguments are typically used to pass metadata to the program, e.g. options or paths to files to process. If an argument contains a special character, e.g. a whitespace character in a file path, it must be escaped, e.g. by enclosing the argument in quotation marks. The details depend on the specific command-line interface.