config.py

# I'm guessing this is the format of the spectrum config file in BNF
# <config_file> ::= <line>*
# <line> ::= <space>* <expr> <space>* <newline> | <space*>
# <expr> ::= <section> | <assignment>
# <section> ::= [<identifier>*]
# <assignment> ::= <identifier> <space>* = <space>* <value>


class SpectrumConfig:
	"""
	Represents spectrum2 configuration options.
	"""
	def __init__(self, path_to_config_file):
		"""
		Initialises configuration file.

		Args:
			path_to_config_file: The absolute path to the configuration file.
		"""
		self.config_path = path_to_config_file
		self.options = self.loadConfig(self.config_path)
		# Load backend_logging information
		self.options.update(self.loadConfig(self['logging.backend_config']))

	def loadConfig(self, file_name):
		section = {'a': ""} # Current section heading,
		# It's a dictionary because variables in python closures can't be
		# assigned to.
		options = dict()
		# Recursive descent parser
		def consume_spaces(line):
			i = 0
			for c in line:
				if c != ' ':
					break
				i += 1
			return line[i:]

		def read_identifier(line):
			i = 0
			for c in line:
				if c == ' ' or c==']' or c=='[' or c=='=':
					break
				i += 1
			# no identifier
			if i == 0:
				return (None, 'No identifier')
			return (line[:i], line[i:])

		def parse_section(line):
			if len(line) == 0 or line[0] != '[':
				return (None, 'expected [')
			line = line[1:]
			identifier, line = read_identifier(line)
			if len(line) == 0 or line[0] != ']' or identifier is None:
				return (None, line)
			return (identifier, line[1:])

		def parse_assignment(line):
			key, line = read_identifier(line)
			if key is None:
				return (None, None, line)
			line = consume_spaces(line)
			if len(line) == 0 or line[0] != '=':
				return (None, None, 'Expected =')
			line = consume_spaces(line[1:])
			value = line[:-1]
			return (key, value, '\n')

		def expr(line):
			sec, newline = parse_section(line)
			if sec is not None:
				section['a'] = sec
			else:
				key, value, newline = parse_assignment(line)
				if key is not None:
					if section['a'] != '':
						options[section['a'] + '.' + key] = value
					else:
						options[key] = value
				else:
					return (None, newline)
			return (newline, None)

		def parse_line(line, line_number):
			line = consume_spaces(line)
			if line == '\n':
				return
			newline, error = expr(line)
			if newline is None:
				raise ConfigParseError(str(line_number) + ': ' + error + ': ' + repr(line))
			newline = consume_spaces(newline)
			if newline != '\n':
				raise ConfigParseError(str(line_number) + ': Expected newline got ' + repr(newline))

		def strip_comments(line):
			i = 0
			for c in line:
				if c == '#' or c == '\n':
					break
				i += 1
			return line[:i] + '\n'

		with open(file_name, 'r') as f:
			i = 1
			while True:
				line = f.readline()
				if line == '':
					break
				parse_line(strip_comments(line), i)
				i += 1
		return options

	def __getitem__(self, key):
		return self.options[key]

class ConfigParseError(Exception):
	pass