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Expression.py
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Expression.py
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# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this
# file, You can obtain one at http://mozilla.org/MPL/2.0/.
"""
Parses and evaluates simple statements for Preprocessor:
Expression currently supports the following grammar, whitespace is ignored:
expression :
and_cond ( '||' expression ) ? ;
and_cond:
test ( '&&' and_cond ) ? ;
test:
unary ( ( '==' | '!=' ) unary ) ? ;
unary :
'!'? value ;
value :
[0-9]+ # integer
| 'defined(' \w+ ')'
| \w+ # string identifier or value;
"""
import re
class Expression:
def __init__(self, expression_string):
"""
Create a new expression with this string.
The expression will already be parsed into an Abstract Syntax Tree.
"""
self.content = expression_string
self.offset = 0
self.__ignore_whitespace()
self.e = self.__get_logical_or()
if self.content:
raise Expression.ParseError, self
def __get_logical_or(self):
"""
Production: and_cond ( '||' expression ) ?
"""
if not len(self.content):
return None
rv = Expression.__AST("logical_op")
# test
rv.append(self.__get_logical_and())
self.__ignore_whitespace()
if self.content[:2] != '||':
# no logical op needed, short cut to our prime element
return rv[0]
# append operator
rv.append(Expression.__ASTLeaf('op', self.content[:2]))
self.__strip(2)
self.__ignore_whitespace()
rv.append(self.__get_logical_or())
self.__ignore_whitespace()
return rv
def __get_logical_and(self):
"""
Production: test ( '&&' and_cond ) ?
"""
if not len(self.content):
return None
rv = Expression.__AST("logical_op")
# test
rv.append(self.__get_equality())
self.__ignore_whitespace()
if self.content[:2] != '&&':
# no logical op needed, short cut to our prime element
return rv[0]
# append operator
rv.append(Expression.__ASTLeaf('op', self.content[:2]))
self.__strip(2)
self.__ignore_whitespace()
rv.append(self.__get_logical_and())
self.__ignore_whitespace()
return rv
def __get_equality(self):
"""
Production: unary ( ( '==' | '!=' ) unary ) ?
"""
if not len(self.content):
return None
rv = Expression.__AST("equality")
# unary
rv.append(self.__get_unary())
self.__ignore_whitespace()
if not re.match('[=!]=', self.content):
# no equality needed, short cut to our prime unary
return rv[0]
# append operator
rv.append(Expression.__ASTLeaf('op', self.content[:2]))
self.__strip(2)
self.__ignore_whitespace()
rv.append(self.__get_unary())
self.__ignore_whitespace()
return rv
def __get_unary(self):
"""
Production: '!'? value
"""
# eat whitespace right away, too
not_ws = re.match('!\s*', self.content)
if not not_ws:
return self.__get_value()
rv = Expression.__AST('not')
self.__strip(not_ws.end())
rv.append(self.__get_value())
self.__ignore_whitespace()
return rv
def __get_value(self):
"""
Production: ( [0-9]+ | 'defined(' \w+ ')' | \w+ )
Note that the order is important, and the expression is kind-of
ambiguous as \w includes 0-9. One could make it unambiguous by
removing 0-9 from the first char of a string literal.
"""
rv = None
m = re.match('defined\s*\(\s*(\w+)\s*\)', self.content)
if m:
word_len = m.end()
rv = Expression.__ASTLeaf('defined', m.group(1))
else:
word_len = re.match('[0-9]*', self.content).end()
if word_len:
value = int(self.content[:word_len])
rv = Expression.__ASTLeaf('int', value)
else:
word_len = re.match('\w*', self.content).end()
if word_len:
rv = Expression.__ASTLeaf('string', self.content[:word_len])
else:
raise Expression.ParseError, self
self.__strip(word_len)
self.__ignore_whitespace()
return rv
def __ignore_whitespace(self):
ws_len = re.match('\s*', self.content).end()
self.__strip(ws_len)
return
def __strip(self, length):
"""
Remove a given amount of chars from the input and update
the offset.
"""
self.content = self.content[length:]
self.offset += length
def evaluate(self, context):
"""
Evaluate the expression with the given context
"""
# Helper function to evaluate __get_equality results
def eval_equality(tok):
left = opmap[tok[0].type](tok[0])
right = opmap[tok[2].type](tok[2])
rv = left == right
if tok[1].value == '!=':
rv = not rv
return rv
# Helper function to evaluate __get_logical_and and __get_logical_or results
def eval_logical_op(tok):
left = opmap[tok[0].type](tok[0])
right = opmap[tok[2].type](tok[2])
if tok[1].value == '&&':
return left and right
elif tok[1].value == '||':
return left or right
raise Expression.ParseError, self
# Mapping from token types to evaluator functions
# Apart from (non-)equality, all these can be simple lambda forms.
opmap = {
'logical_op': eval_logical_op,
'equality': eval_equality,
'not': lambda tok: not opmap[tok[0].type](tok[0]),
'string': lambda tok: context[tok.value],
'defined': lambda tok: tok.value in context,
'int': lambda tok: tok.value}
return opmap[self.e.type](self.e);
class __AST(list):
"""
Internal class implementing Abstract Syntax Tree nodes
"""
def __init__(self, type):
self.type = type
super(self.__class__, self).__init__(self)
class __ASTLeaf:
"""
Internal class implementing Abstract Syntax Tree leafs
"""
def __init__(self, type, value):
self.value = value
self.type = type
def __str__(self):
return self.value.__str__()
def __repr__(self):
return self.value.__repr__()
class ParseError(StandardError):
"""
Error raised when parsing fails.
It has two members, offset and content, which give the offset of the
error and the offending content.
"""
def __init__(self, expression):
self.offset = expression.offset
self.content = expression.content[:3]
def __str__(self):
return 'Unexpected content at offset {0}, "{1}"'.format(self.offset,
self.content)
class Context(dict):
"""
This class holds variable values by subclassing dict, and while it
truthfully reports True and False on
name in context
it returns the variable name itself on
context["name"]
to reflect the ambiguity between string literals and preprocessor
variables.
"""
def __getitem__(self, key):
if key in self:
return super(self.__class__, self).__getitem__(key)
return key

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