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128 lines
2.8 KiB
Python
128 lines
2.8 KiB
Python
# Module 'parser'
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#
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# Parse S-expressions output by the Panel Editor
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# (which is written in Scheme so it can't help writing S-expressions).
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#
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# See notes at end of file.
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whitespace = ' \t\n'
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operators = '()\''
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separators = operators + whitespace + ';' + '"'
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# Tokenize a string.
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# Return a list of tokens (strings).
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#
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def tokenize_string(s):
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tokens = []
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while s:
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c = s[:1]
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if c in whitespace:
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s = s[1:]
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elif c == ';':
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s = ''
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elif c == '"':
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n = len(s)
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i = 1
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while i < n:
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c = s[i]
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i = i+1
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if c == '"': break
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if c == '\\': i = i+1
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tokens.append(s[:i])
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s = s[i:]
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elif c in operators:
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tokens.append(c)
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s = s[1:]
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else:
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n = len(s)
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i = 1
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while i < n:
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if s[i] in separators: break
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i = i+1
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tokens.append(s[:i])
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s = s[i:]
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return tokens
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# Tokenize a whole file (given as file object, not as file name).
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# Return a list of tokens (strings).
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#
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def tokenize_file(fp):
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tokens = []
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while 1:
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line = fp.readline()
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if not line: break
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tokens = tokens + tokenize_string(line)
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return tokens
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# Exception raised by parse_exr.
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#
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syntax_error = 'syntax error'
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# Parse an S-expression.
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# Input is a list of tokens as returned by tokenize_*().
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# Return a pair (expr, tokens)
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# where expr is a list representing the s-expression,
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# and tokens contains the remaining tokens.
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# May raise syntax_error.
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#
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def parse_expr(tokens):
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if (not tokens) or tokens[0] <> '(':
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raise syntax_error, 'expected "("'
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tokens = tokens[1:]
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expr = []
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while 1:
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if not tokens:
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raise syntax_error, 'missing ")"'
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if tokens[0] == ')':
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return expr, tokens[1:]
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elif tokens[0] == '(':
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subexpr, tokens = parse_expr(tokens)
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expr.append(subexpr)
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else:
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expr.append(tokens[0])
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tokens = tokens[1:]
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# Parse a file (given as file object, not as file name).
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# Return a list of parsed S-expressions found at the top level.
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#
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def parse_file(fp):
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tokens = tokenize_file(fp)
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exprlist = []
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while tokens:
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expr, tokens = parse_expr(tokens)
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exprlist.append(expr)
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return exprlist
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# EXAMPLE:
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#
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# The input
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# '(hip (hop hur-ray))'
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#
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# passed to tokenize_string() returns the token list
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# ['(', 'hip', '(', 'hop', 'hur-ray', ')', ')']
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#
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# When this is passed to parse_expr() it returns the expression
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# ['hip', ['hop', 'hur-ray']]
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# plus an empty token list (because there are no tokens left.
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#
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# When a file containing the example is passed to parse_file() it returns
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# a list whose only element is the output of parse_expr() above:
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# [['hip', ['hop', 'hur-ray']]]
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# TOKENIZING:
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#
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# Comments start with semicolon (;) and continue till the end of the line.
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#
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# Tokens are separated by whitespace, except the following characters
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# always form a separate token (outside strings):
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# ( ) '
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# Strings are enclosed in double quotes (") and backslash (\) is used
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# as escape character in strings.
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