get default port optionally form environment var

If the default port 8080 is taken, you have to either change the app.run()
or  use --port <PORTNUM> on every invocation of the program. When trying
out examples I don't want to do the first and forget the second.

This patch allows you set the environment variables LONA_DEFAULT_HOST and
LONA_DEFAULT_PORT in your shell. These override the built-in defaults
('localhost' resp. 8080), and are in turn overridden by arguments
to ``app.run()`` and startup commandline parameters.

doc/content/tutorial/01-getting-started/index.rst

@@ -101,6 +101,9 @@ Run the script using:
 
 ``app.run()`` takes keywords like ``host`` or ``port`` for configuration, and
 also parses the command line. Run ``python example.py -h`` to print the help.
+If port 8080 is taken by another application, you can set environment
+variable ``LONA_DEFAULT_PORT`` to some other port, affecting all examples that 
+do not set the port explicitly.
 
 The script should print that it opened a webserver on
 ``http://localhost:8080``. If you navigate your browser there, you should see

lona/app.py

@@ -299,12 +299,14 @@ def parse_command_line(self) -> dict:
  from lona.command_line.handle_command_line import (
  parse_overrides,
  DESCRIPTION,
+ EPILOG,
  )
 
  parser = ArgumentParser(
  prog=str(self.script_path),
  formatter_class=RawTextHelpFormatter,
  description=DESCRIPTION,
+ epilog=EPILOG,
  )
 
  parser.add_argument(
@@ -428,8 +430,8 @@ def run(
 
  # setup arguments
  server_args = Namespace(
- host='localhost',
- port=8080,
+ host=os.environ.get('LONA_DEFAULT_HOST', 'localhost'),
+ port=int(os.environ.get('LONA_DEFAULT_PORT', 8080)),
  shell_server_url='',
  shutdown_timeout=0,
  log_level='info',

lona/command_line/handle_command_line.py

@@ -16,6 +16,10 @@
 Code: https://github.com/lona-web-org/lona
 """.strip()
 
+EPILOG = """\
+You can set environment variables LONA_DEFAULT_HOST and LONA_DEFAULT_PORT,
+to override built-in defaults ('localhost' resp. '8080').
+""".rstrip()
 
 def parse_overrides(raw_overrides):
  environment = Environment()