Lib/plat-irix5/panelparser.py

   1 # Module 'parser'
   2 #
   3 # Parse S-expressions output by the Panel Editor
   4 # (which is written in Scheme so it can't help writing S-expressions).
   5 #
   6 # See notes at end of file.
   7
   8
   9 whitespace = ' \t\n'
  10 operators = '()\''
  11 separators = operators + whitespace + ';' + '"'
  12
  13
  14 # Tokenize a string.
  15 # Return a list of tokens (strings).
  16 #
  17 def tokenize_string(s):
  18     tokens = []
  19     while s:
  20         c = s[:1]
  21         if c in whitespace:
  22             s = s[1:]
  23         elif c == ';':
  24             s = ''
  25         elif c == '"':
  26             n = len(s)
  27             i = 1
  28             while i < n:
  29                 c = s[i]
  30                 i = i+1
  31                 if c == '"': break
  32                 if c == '\\': i = i+1
  33             tokens.append(s[:i])
  34             s = s[i:]
  35         elif c in operators:
  36             tokens.append(c)
  37             s = s[1:]
  38         else:
  39             n = len(s)
  40             i = 1
  41             while i < n:
  42                 if s[i] in separators: break
  43                 i = i+1
  44             tokens.append(s[:i])
  45             s = s[i:]
  46     return tokens
  47
  48
  49 # Tokenize a whole file (given as file object, not as file name).
  50 # Return a list of tokens (strings).
  51 #
  52 def tokenize_file(fp):
  53     tokens = []
  54     while 1:
  55         line = fp.readline()
  56         if not line: break
  57         tokens = tokens + tokenize_string(line)
  58     return tokens
  59
  60
  61 # Exception raised by parse_exr.
  62 #
  63 syntax_error = 'syntax error'
  64
  65
  66 # Parse an S-expression.
  67 # Input is a list of tokens as returned by tokenize_*().
  68 # Return a pair (expr, tokens)
  69 # where expr is a list representing the s-expression,
  70 # and tokens contains the remaining tokens.
  71 # May raise syntax_error.
  72 #
  73 def parse_expr(tokens):
  74     if (not tokens) or tokens[0] != '(':
  75         raise syntax_error, 'expected "("'
  76     tokens = tokens[1:]
  77     expr = []
  78     while 1:
  79         if not tokens:
  80             raise syntax_error, 'missing ")"'
  81         if tokens[0] == ')':
  82             return expr, tokens[1:]
  83         elif tokens[0] == '(':
  84             subexpr, tokens = parse_expr(tokens)
  85             expr.append(subexpr)
  86         else:
  87             expr.append(tokens[0])
  88             tokens = tokens[1:]
  89
  90
  91 # Parse a file (given as file object, not as file name).
  92 # Return a list of parsed S-expressions found at the top level.
  93 #
  94 def parse_file(fp):
  95     tokens = tokenize_file(fp)
  96     exprlist = []
  97     while tokens:
  98         expr, tokens = parse_expr(tokens)
  99         exprlist.append(expr)
 100     return exprlist
 101
 102
 103 # EXAMPLE:
 104 #
 105 # The input
 106 #       '(hip (hop hur-ray))'
 107 #
 108 # passed to tokenize_string() returns the token list
 109 #       ['(', 'hip', '(', 'hop', 'hur-ray', ')', ')']
 110 #
 111 # When this is passed to parse_expr() it returns the expression
 112 #       ['hip', ['hop', 'hur-ray']]
 113 # plus an empty token list (because there are no tokens left.
 114 #
 115 # When a file containing the example is passed to parse_file() it returns
 116 # a list whose only element is the output of parse_expr() above:
 117 #       [['hip', ['hop', 'hur-ray']]]
 118
 119
 120 # TOKENIZING:
 121 #
 122 # Comments start with semicolon (;) and continue till the end of the line.
 123 #
 124 # Tokens are separated by whitespace, except the following characters
 125 # always form a separate token (outside strings):
 126 #       ( ) '
 127 # Strings are enclosed in double quotes (") and backslash (\) is used
 128 # as escape character in strings.