# -*- coding: utf-8 -*-
#
# Copyright (C) 2006-2008 Edgewall Software
# All rights reserved.
#
# This software is licensed as described in the file COPYING, which
# you should have received as part of this distribution. The terms
# are also available at http://genshi.edgewall.org/wiki/License.
#
# This software consists of voluntary contributions made by many
# individuals. For the exact contribution history, see the revision
# history and logs, available at http://genshi.edgewall.org/log/.

"""Support for "safe" evaluation of Python expressions."""

import __builtin__

from textwrap import dedent
from types import CodeType

from genshi.core import Markup
from genshi.template.astutil import ASTTransformer, ASTCodeGenerator, \
                                    _ast, parse
from genshi.template.base import TemplateRuntimeError
from genshi.util import flatten

__all__ = ['Code', 'Expression', 'Suite', 'LenientLookup', 'StrictLookup',
           'Undefined', 'UndefinedError']
__docformat__ = 'restructuredtext en'


# Check for a Python 2.4 bug in the eval loop
has_star_import_bug = False
try:
    class _FakeMapping(object):
        __getitem__ = __setitem__ = lambda *a: None
    exec 'from sys import *' in {}, _FakeMapping()
except SystemError:
    has_star_import_bug = True
del _FakeMapping


def _star_import_patch(mapping, modname):
    """This function is used as helper if a Python version with a broken
    star-import opcode is in use.
    """
    module = __import__(modname, None, None, ['__all__'])
    if hasattr(module, '__all__'):
        members = module.__all__
    else:
        members = [x for x in module.__dict__ if not x.startswith('_')]
    mapping.update([(name, getattr(module, name)) for name in members])


class Code(object):
    """Abstract base class for the `Expression` and `Suite` classes."""
    __slots__ = ['source', 'code', 'ast', '_globals']

    def __init__(self, source, filename=None, lineno=-1, lookup='strict',
                 xform=None):
        """Create the code object, either from a string, or from an AST node.
        
        :param source: either a string containing the source code, or an AST
                       node
        :param filename: the (preferably absolute) name of the file containing
                         the code
        :param lineno: the number of the line on which the code was found
        :param lookup: the lookup class that defines how variables are looked
                       up in the context; can be either "strict" (the default),
                       "lenient", or a custom lookup class
        :param xform: the AST transformer that should be applied to the code;
                      if `None`, the appropriate transformation is chosen
                      depending on the mode
        """
        if isinstance(source, basestring):
            self.source = source
            node = _parse(source, mode=self.mode)
        else:
            assert isinstance(source, _ast.AST), \
                'Expected string or AST node, but got %r' % source
            self.source = '?'
            if self.mode == 'eval':
                node = _ast.Expression()
                node.body = source
            else:
                node = _ast.Module()
                node.body = [source]

        self.ast = node
        self.code = _compile(node, self.source, mode=self.mode,
                             filename=filename, lineno=lineno, xform=xform)
        if lookup is None:
            lookup = LenientLookup
        elif isinstance(lookup, basestring):
            lookup = {'lenient': LenientLookup, 'strict': StrictLookup}[lookup]
        self._globals = lookup.globals

    def __getstate__(self):
        state = {'source': self.source, 'ast': self.ast,
                 'lookup': self._globals.im_self}
        c = self.code
        state['code'] = (c.co_nlocals, c.co_stacksize, c.co_flags, c.co_code,
                         c.co_consts, c.co_names, c.co_varnames, c.co_filename,
                         c.co_name, c.co_firstlineno, c.co_lnotab, (), ())
        return state

    def __setstate__(self, state):
        self.source = state['source']
        self.ast = state['ast']
        self.code = CodeType(0, *state['code'])
        self._globals = state['lookup'].globals

    def __eq__(self, other):
        return (type(other) == type(self)) and (self.code == other.code)

    def __hash__(self):
        return hash(self.code)

    def __ne__(self, other):
        return not self == other

    def __repr__(self):
        return '%s(%r)' % (self.__class__.__name__, self.source)


class Expression(Code):
    """Evaluates Python expressions used in templates.

    >>> data = dict(test='Foo', items=[1, 2, 3], dict={'some': 'thing'})
    >>> Expression('test').evaluate(data)
    'Foo'

    >>> Expression('items[0]').evaluate(data)
    1
    >>> Expression('items[-1]').evaluate(data)
    3
    >>> Expression('dict["some"]').evaluate(data)
    'thing'
    
    Similar to e.g. Javascript, expressions in templates can use the dot
    notation for attribute access to access items in mappings:
    
    >>> Expression('dict.some').evaluate(data)
    'thing'
    
    This also works the other way around: item access can be used to access
    any object attribute:
    
    >>> class MyClass(object):
    ...     myattr = 'Bar'
    >>> data = dict(mine=MyClass(), key='myattr')
    >>> Expression('mine.myattr').evaluate(data)
    'Bar'
    >>> Expression('mine["myattr"]').evaluate(data)
    'Bar'
    >>> Expression('mine[key]').evaluate(data)
    'Bar'
    
    All of the standard Python operators are available to template expressions.
    Built-in functions such as ``len()`` are also available in template
    expressions:
    
    >>> data = dict(items=[1, 2, 3])
    >>> Expression('len(items)').evaluate(data)
    3
    """
    __slots__ = []
    mode = 'eval'

    def evaluate(self, data):
        """Evaluate the expression against the given data dictionary.
        
        :param data: a mapping containing the data to evaluate against
        :return: the result of the evaluation
        """
        __traceback_hide__ = 'before_and_this'
        _globals = self._globals(data)
        return eval(self.code, _globals, {'__data__': data})


class Suite(Code):
    """Executes Python statements used in templates.

    >>> data = dict(test='Foo', items=[1, 2, 3], dict={'some': 'thing'})
    >>> Suite("foo = dict['some']").execute(data)
    >>> data['foo']
    'thing'
    """
    __slots__ = []
    mode = 'exec'

    def execute(self, data):
        """Execute the suite in the given data dictionary.
        
        :param data: a mapping containing the data to execute in
        """
        __traceback_hide__ = 'before_and_this'
        _globals = self._globals(data)
        exec self.code in _globals, data


UNDEFINED = object()


class UndefinedError(TemplateRuntimeError):
    """Exception thrown when a template expression attempts to access a variable
    not defined in the context.
    
    :see: `LenientLookup`, `StrictLookup`
    """
    def __init__(self, name, owner=UNDEFINED):
        if owner is not UNDEFINED:
            message = '%s has no member named "%s"' % (repr(owner), name)
        else:
            message = '"%s" not defined' % name
        TemplateRuntimeError.__init__(self, message)


class Undefined(object):
    """Represents a reference to an undefined variable.
    
    Unlike the Python runtime, template expressions can refer to an undefined
    variable without causing a `NameError` to be raised. The result will be an
    instance of the `Undefined` class, which is treated the same as ``False`` in
    conditions, but raise an exception on any other operation:
    
    >>> foo = Undefined('foo')
    >>> bool(foo)
    False
    >>> list(foo)
    []
    >>> print foo
    undefined
    
    However, calling an undefined variable, or trying to access an attribute
    of that variable, will raise an exception that includes the name used to
    reference that undefined variable.
    
    >>> foo('bar')
    Traceback (most recent call last):
        ...
    UndefinedError: "foo" not defined

    >>> foo.bar
    Traceback (most recent call last):
        ...
    UndefinedError: "foo" not defined
    
    :see: `LenientLookup`
    """
    __slots__ = ['_name', '_owner']

    def __init__(self, name, owner=UNDEFINED):
        """Initialize the object.
        
        :param name: the name of the reference
        :param owner: the owning object, if the variable is accessed as a member
        """
        self._name = name
        self._owner = owner

    def __iter__(self):
        return iter([])

    def __nonzero__(self):
        return False

    def __repr__(self):
        return '<%s %r>' % (self.__class__.__name__, self._name)

    def __str__(self):
        return 'undefined'

    def _die(self, *args, **kwargs):
        """Raise an `UndefinedError`."""
        __traceback_hide__ = True
        raise UndefinedError(self._name, self._owner)
    __call__ = __getattr__ = __getitem__ = _die


class LookupBase(object):
    """Abstract base class for variable lookup implementations."""

    def globals(cls, data):
        """Construct the globals dictionary to use as the execution context for
        the expression or suite.
        """
        return {
            '__data__': data,
            '_lookup_name': cls.lookup_name,
            '_lookup_attr': cls.lookup_attr,
            '_lookup_item': cls.lookup_item,
            '_star_import_patch': _star_import_patch,
            'UndefinedError': UndefinedError,
        }
    globals = classmethod(globals)

    def lookup_name(cls, data, name):
        __traceback_hide__ = True
        val = data.get(name, UNDEFINED)
        if val is UNDEFINED:
            val = BUILTINS.get(name, val)
            if val is UNDEFINED:
                val = cls.undefined(name)
        return val
    lookup_name = classmethod(lookup_name)

    def lookup_attr(cls, obj, key):
        __traceback_hide__ = True
        try:
            val = getattr(obj, key)
        except AttributeError:
            if hasattr(obj.__class__, key):
                raise
            else:
                try:
                    val = obj[key]
                except (KeyError, TypeError):
                    val = cls.undefined(key, owner=obj)
        return val
    lookup_attr = classmethod(lookup_attr)

    def lookup_item(cls, obj, key):
        __traceback_hide__ = True
        if len(key) == 1:
            key = key[0]
        try:
            return obj[key]
        except (AttributeError, KeyError, IndexError, TypeError), e:
            if isinstance(key, basestring):
                val = getattr(obj, key, UNDEFINED)
                if val is UNDEFINED:
                    val = cls.undefined(key, owner=obj)
                return val
            raise
    lookup_item = classmethod(lookup_item)

    def undefined(cls, key, owner=UNDEFINED):
        """Can be overridden by subclasses to specify behavior when undefined
        variables are accessed.
        
        :param key: the name of the variable
        :param owner: the owning object, if the variable is accessed as a member
        """
        raise NotImplementedError
    undefined = classmethod(undefined)


class LenientLookup(LookupBase):
    """Default variable lookup mechanism for expressions.
    
    When an undefined variable is referenced using this lookup style, the
    reference evaluates to an instance of the `Undefined` class:
    
    >>> expr = Expression('nothing', lookup='lenient')
    >>> undef = expr.evaluate({})
    >>> undef
    <Undefined 'nothing'>
    
    The same will happen when a non-existing attribute or item is accessed on
    an existing object:
    
    >>> expr = Expression('something.nil', lookup='lenient')
    >>> expr.evaluate({'something': dict()})
    <Undefined 'nil'>
    
    See the documentation of the `Undefined` class for details on the behavior
    of such objects.
    
    :see: `StrictLookup`
    """
    def undefined(cls, key, owner=UNDEFINED):
        """Return an ``Undefined`` object."""
        __traceback_hide__ = True
        return Undefined(key, owner=owner)
    undefined = classmethod(undefined)


class StrictLookup(LookupBase):
    """Strict variable lookup mechanism for expressions.
    
    Referencing an undefined variable using this lookup style will immediately
    raise an ``UndefinedError``:
    
    >>> expr = Expression('nothing', lookup='strict')
    >>> expr.evaluate({})
    Traceback (most recent call last):
        ...
    UndefinedError: "nothing" not defined
    
    The same happens when a non-existing attribute or item is accessed on an
    existing object:
    
    >>> expr = Expression('something.nil', lookup='strict')
    >>> expr.evaluate({'something': dict()})
    Traceback (most recent call last):
        ...
    UndefinedError: {} has no member named "nil"
    """
    def undefined(cls, key, owner=UNDEFINED):
        """Raise an ``UndefinedError`` immediately."""
        __traceback_hide__ = True
        raise UndefinedError(key, owner=owner)
    undefined = classmethod(undefined)


def _parse(source, mode='eval'):
    source = source.strip()
    if mode == 'exec':
        lines = [line.expandtabs() for line in source.splitlines()]
        if lines:
            first = lines[0]
            rest = dedent('\n'.join(lines[1:])).rstrip()
            if first.rstrip().endswith(':') and not rest[0].isspace():
                rest = '\n'.join(['    %s' % line for line in rest.splitlines()])
            source = '\n'.join([first, rest])
    if isinstance(source, unicode):
        source = '\xef\xbb\xbf' + source.encode('utf-8')
    return parse(source, mode)


def _compile(node, source=None, mode='eval', filename=None, lineno=-1,
             xform=None):
    if isinstance(filename, unicode):
        # unicode file names not allowed for code objects
        filename = filename.encode('utf-8', 'replace')
    elif not filename:
        filename = '<string>'
    if lineno <= 0:
        lineno = 1

    if xform is None:
        xform = {
            'eval': ExpressionASTTransformer
        }.get(mode, TemplateASTTransformer)
    tree = xform().visit(node)

    if mode == 'eval':
        name = '<Expression %r>' % (source or '?')
    else:
        lines = source.splitlines()
        if not lines:
            extract = ''
        else:
            extract = lines[0]
        if len(lines) > 1:
            extract += ' ...'
        name = '<Suite %r>' % (extract)
    new_source = ASTCodeGenerator(tree).code
    code = compile(new_source, filename, mode)

    try:
        # We'd like to just set co_firstlineno, but it's readonly. So we need
        # to clone the code object while adjusting the line number
        return CodeType(0, code.co_nlocals, code.co_stacksize,
                        code.co_flags | 0x0040, code.co_code, code.co_consts,
                        code.co_names, code.co_varnames, filename, name,
                        lineno, code.co_lnotab, (), ())
    except RuntimeError:
        return code


def _new(class_, *args, **kwargs):
    ret = class_()
    for attr, value in zip(ret._fields, args):
        if attr in kwargs:
            raise ValueError('Field set both in args and kwargs')
        setattr(ret, attr, value)
    for attr, value in kwargs:
        setattr(ret, attr, value)
    return ret


BUILTINS = __builtin__.__dict__.copy()
BUILTINS.update({'Markup': Markup, 'Undefined': Undefined})
CONSTANTS = frozenset(['False', 'True', 'None', 'NotImplemented', 'Ellipsis'])


class TemplateASTTransformer(ASTTransformer):
    """Concrete AST transformer that implements the AST transformations needed
    for code embedded in templates.
    """

    def __init__(self):
        self.locals = [CONSTANTS]

    def _extract_names(self, node):
        names = set()
        def _process(node):
            if isinstance(node, _ast.Name):
                names.add(node.id)
            elif isinstance(node, _ast.alias):
                names.add(node.asname or node.name)
            elif isinstance(node, _ast.Tuple):
                for elt in node.elts:
                    _process(node)
        if hasattr(node, 'args'):
            for arg in node.args:
                _process(arg)
            if hasattr(node, 'vararg'):
                names.add(node.vararg)
            if hasattr(node, 'kwarg'):
                names.add(node.kwarg)
        elif hasattr(node, 'names'):
            for elt in node.names:
                _process(elt)
        return names

    def visit_Str(self, node):
        if isinstance(node.s, str):
            try: # If the string is ASCII, return a `str` object
                node.s.decode('ascii')
            except ValueError: # Otherwise return a `unicode` object
                return _new(_ast.Str, node.s.decode('utf-8'))
        return node

    def visit_ClassDef(self, node):
        if len(self.locals) > 1:
            self.locals[-1].add(node.name)
        self.locals.append(set())
        try:
            return ASTTransformer.visit_ClassDef(self, node)
        finally:
            self.locals.pop()

    def visit_Import(self, node):
        if len(self.locals) > 1:
            self.locals[-1].update(self._extract_names(node))
        return ASTTransformer.visit_Import(self, node)

    def visit_ImportFrom(self, node):
        if [a.name for a in node.names] == ['*']:
            if has_star_import_bug:
                # This is a Python 2.4 bug. Only if we have a broken Python
                # version do we need to apply this hack
                node = _new(_ast.Expr, _new(_ast.Call,
                    _new(_ast.Name, '_star_import_patch'), [
                        _new(_ast.Name, '__data__'),
                        _new(_ast.Str, node.module)
                    ], (), ()))
            return node
        if len(self.locals) > 1:
            self.locals[-1].update(self._extract_names(node))
        return ASTTransformer.visit_ImportFrom(self, node)

    def visit_FunctionDef(self, node):
        if len(self.locals) > 1:
            self.locals[-1].add(node.name)

        self.locals.append(self._extract_names(node.args))
        try:
            return ASTTransformer.visit_FunctionDef(self, node)
        finally:
            self.locals.pop()

    # GeneratorExp(expr elt, comprehension* generators)
    def visit_GeneratorExp(self, node):
        gens = []
        # need to visit them in inverse order
        for generator in node.generators[::-1]:
            # comprehension = (expr target, expr iter, expr* ifs)
            self.locals.append(set())
            gen = _new(_ast.comprehension, self.visit(generator.target),
                            self.visit(generator.iter),
                            [self.visit(if_) for if_ in generator.ifs])
            gens.append(gen)
        gens.reverse()

        # use node.__class__ to make it reusable as ListComp
        ret = _new(node.__class__, self.visit(node.elt), gens)
        #delete inserted locals
        del self.locals[-len(node.generators):]
        return ret

    # ListComp(expr elt, comprehension* generators)
    visit_ListComp = visit_GeneratorExp

    def visit_Lambda(self, node):
        self.locals.append(self._extract_names(node.args))
        try:
            return ASTTransformer.visit_Lambda(self, node)
        finally:
            self.locals.pop()

    def visit_Name(self, node):
        # If the name refers to a local inside a lambda, list comprehension, or
        # generator expression, leave it alone
        if isinstance(node.ctx, _ast.Load) and \
                node.id not in flatten(self.locals):
            # Otherwise, translate the name ref into a context lookup
            name = _new(_ast.Name, '_lookup_name', _ast.Load())
            namearg = _new(_ast.Name, '__data__', _ast.Load())
            strarg = _new(_ast.Str, node.id)
            node = _new(_ast.Call, name, [namearg, strarg], [])
        elif isinstance(node.ctx, _ast.Store):
            if len(self.locals) > 1:
                self.locals[-1].add(node.id)

        return node


class ExpressionASTTransformer(TemplateASTTransformer):
    """Concrete AST transformer that implements the AST transformations needed
    for code embedded in templates.
    """

    def visit_Attribute(self, node):
        if not isinstance(node.ctx, _ast.Load):
            return ASTTransformer.visit_Attribute(self, node)

        func = _new(_ast.Name, '_lookup_attr', _ast.Load())
        args = [self.visit(node.value), _new(_ast.Str, node.attr)]
        return _new(_ast.Call, func, args, [])

    def visit_Subscript(self, node):
        if not isinstance(node.ctx, _ast.Load) or \
                not isinstance(node.slice, _ast.Index):
            return ASTTransformer.visit_Subscript(self, node)

        func = _new(_ast.Name, '_lookup_item', _ast.Load())
        args = [
            self.visit(node.value),
            _new(_ast.Tuple, (self.visit(node.slice.value),), _ast.Load())
        ]
        return _new(_ast.Call, func, args, [])