Application Startup

def run(main, argv=None, flags_parser=parse_flags_with_usage):
    """
    Run the main function with automatic flag parsing and exception handling.
    
    This function serves as the entry point for absl applications. It automatically
    parses command-line flags, calls initialization callbacks, handles exceptions,
    and ensures proper program termination.
    
    Args:
        main: Main function that takes argv as its single argument
        argv: Command line arguments (defaults to sys.argv if None)
        flags_parser: Function to parse flags (defaults to parse_flags_with_usage)
    
    The main function should have signature: main(argv: List[str]) -> None
    """

def usage(shorthelp=False, writeto_stdout=False, detailed_error=None, exitcode=None):
    """
    Write usage information to stderr and optionally exit.
    
    Args:
        shorthelp (bool): If True, print short help instead of full help
        writeto_stdout (bool): If True, write to stdout instead of stderr  
        detailed_error (str): Detailed error message to include in output
        exitcode (int): Exit code to use when exiting (if None, don't exit)
    """

def call_after_init(callback):
    """
    Register a callback to be called after flag parsing.
    
    Callbacks are executed in the order they were registered, after flags
    have been parsed but before the main function is called.
    
    Args:
        callback: Function to call after initialization, takes no arguments
    """

class Error(Exception):
    """Base exception class for the app module."""

class UsageError(Error):
    """
    Exception indicating a usage error (bad command-line arguments).
    
    When raised in the main function, automatically displays usage
    information and exits with code 1.
    """

def install_exception_handler(handler):
    """
    Install a custom exception handler.
    
    Args:
        handler: ExceptionHandler instance or callable that takes an exception
    """

class ExceptionHandler:
    """
    Base class for custom exception handlers.
    
    Subclass this to create custom exception handling behavior.
    """
    
    def wants(self, exc):
        """
        Determine if this handler wants to handle an exception.
        
        Args:
            exc: The exception to potentially handle
            
        Returns:
            bool: True if this handler wants to handle the exception
        """
    
    def handle(self, exc):
        """
        Handle an exception.
        
        Args:
            exc: The exception to handle
            
        Returns:
            bool: True if the exception was handled, False otherwise
        """

def parse_flags_with_usage(args):
    """
    Parse flags and display usage on error.
    
    Args:
        args: Command-line arguments to parse
        
    Returns:
        List of remaining non-flag arguments
        
    Raises:
        SystemExit: On flag parsing errors with usage information
    """

def define_help_flags():
    """Define standard help flags (--help, --helpshort, --helpfull, --helpxml)."""

# Help flag classes for advanced usage
class HelpFlag(flags.BooleanFlag):
    """Special boolean flag that displays usage and raises SystemExit."""
    NAME = 'help'
    SHORT_NAME = '?'

class HelpshortFlag(HelpFlag):
    """Flag for displaying short help information."""
    NAME = 'helpshort'

class HelpfullFlag(flags.BooleanFlag):
    """Flag for displaying full help information."""
    NAME = 'helpfull'

class HelpXMLFlag(flags.BooleanFlag):
    """Flag for displaying help in XML format."""
    NAME = 'helpxml'

from absl import app
from absl import flags
from absl import logging

FLAGS = flags.FLAGS
flags.DEFINE_string('input_file', None, 'Input file to process.')
flags.DEFINE_boolean('verbose', False, 'Enable verbose output.')

def main(argv):
    if len(argv) > 1:
        raise app.UsageError('Too many command-line arguments.')
    
    if FLAGS.verbose:
        logging.set_verbosity(logging.DEBUG)
    
    if FLAGS.input_file is None:
        raise app.UsageError('--input_file is required.')
    
    # Application logic here
    logging.info('Processing file: %s', FLAGS.input_file)

if __name__ == '__main__':
    app.run(main)

from absl import app
import logging

class CustomExceptionHandler(app.ExceptionHandler):
    def handle(self, exc):
        if isinstance(exc, FileNotFoundError):
            logging.error('File not found: %s', exc.filename)
            return True  # Exception handled
        return False  # Not handled

def main(argv):
    app.install_exception_handler(CustomExceptionHandler())
    # Application logic that might raise FileNotFoundError

if __name__ == '__main__':
    app.run(main)

from absl import app
from absl import flags
import logging

FLAGS = flags.FLAGS
flags.DEFINE_string('config_file', None, 'Configuration file.')

def initialize_config():
    """Initialize configuration after flags are parsed."""
    if FLAGS.config_file:
        logging.info('Loading config from: %s', FLAGS.config_file)
        # Load configuration logic here

def main(argv):
    # Configuration is already loaded via callback
    logging.info('Starting application with loaded config.')

if __name__ == '__main__':
    app.call_after_init(initialize_config)
    app.run(main)