panelparser.py :  » Mobile » Python-for-PalmOS » Python-1.5.2+reduced-1.0 » Lib » plat-irix6 » Python Open Source

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Python Open Source » Mobile » Python for PalmOS 
Python for PalmOS » Python 1.5.2 reduced 1.0 » Lib » plat irix6 » panelparser.py
# Module 'parser'
#
# Parse S-expressions output by the Panel Editor
# (which is written in Scheme so it can't help writing S-expressions).
#
# See notes at end of file.


whitespace = ' \t\n'
operators = '()\''
separators = operators + whitespace + ';' + '"'


# Tokenize a string.
# Return a list of tokens (strings).
#
def tokenize_string(s):
  tokens = []
  while s:
    c = s[:1]
    if c in whitespace:
      s = s[1:]
    elif c == ';':
      s = ''
    elif c == '"':
      n = len(s)
      i = 1
      while i < n:
        c = s[i]
        i = i+1
        if c == '"': break
        if c == '\\': i = i+1
      tokens.append(s[:i])
      s = s[i:]
    elif c in operators:
      tokens.append(c)
      s = s[1:]
    else:
      n = len(s)
      i = 1
      while i < n:
        if s[i] in separators: break
        i = i+1
      tokens.append(s[:i])
      s = s[i:]
  return tokens


# Tokenize a whole file (given as file object, not as file name).
# Return a list of tokens (strings).
#
def tokenize_file(fp):
  tokens = []
  while 1:
    line = fp.readline()
    if not line: break
    tokens = tokens + tokenize_string(line)
  return tokens


# Exception raised by parse_exr.
#
syntax_error = 'syntax error'


# Parse an S-expression.
# Input is a list of tokens as returned by tokenize_*().
# Return a pair (expr, tokens)
# where expr is a list representing the s-expression,
# and tokens contains the remaining tokens.
# May raise syntax_error.
#
def parse_expr(tokens):
  if (not tokens) or tokens[0] <> '(':
    raise syntax_error, 'expected "("'
  tokens = tokens[1:]
  expr = []
  while 1:
    if not tokens:
      raise syntax_error, 'missing ")"'
    if tokens[0] == ')':
      return expr, tokens[1:]
    elif tokens[0] == '(':
      subexpr, tokens = parse_expr(tokens)
      expr.append(subexpr)
    else:
      expr.append(tokens[0])
      tokens = tokens[1:]


# Parse a file (given as file object, not as file name).
# Return a list of parsed S-expressions found at the top level.
#
def parse_file(fp):
  tokens = tokenize_file(fp)
  exprlist = []
  while tokens:
    expr, tokens = parse_expr(tokens)
    exprlist.append(expr)
  return exprlist


# EXAMPLE:
#
# The input
#  '(hip (hop hur-ray))'
#
# passed to tokenize_string() returns the token list
#  ['(', 'hip', '(', 'hop', 'hur-ray', ')', ')']
#
# When this is passed to parse_expr() it returns the expression
#  ['hip', ['hop', 'hur-ray']]
# plus an empty token list (because there are no tokens left.
#
# When a file containing the example is passed to parse_file() it returns
# a list whose only element is the output of parse_expr() above:
#  [['hip', ['hop', 'hur-ray']]]


# TOKENIZING:
#
# Comments start with semicolon (;) and continue till the end of the line.
#
# Tokens are separated by whitespace, except the following characters
# always form a separate token (outside strings):
#  ( ) '
# Strings are enclosed in double quotes (") and backslash (\) is used
# as escape character in strings.
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