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# 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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