The file editor
PythonAnywhere's built-in file editor allows you to edit your files with syntax highlighting and, for Python, simple pyflakes-based error-checking.
When you want to run your code, you can use the black "Save and run" button at the top right, with its three chevrons >>>.
The default Python version here is 3.4. If you want to change that, see the instructions for changing the default editor Python version.