214 lines
6.3 KiB
Python
214 lines
6.3 KiB
Python
"""
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Parser and utilities for the smart 'if' tag
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"""
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# Using a simple top down parser, as described here:
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# http://effbot.org/zone/simple-top-down-parsing.htm.
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# 'led' = left denotation
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# 'nud' = null denotation
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# 'bp' = binding power (left = lbp, right = rbp)
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class TokenBase:
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"""
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Base class for operators and literals, mainly for debugging and for throwing
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syntax errors.
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"""
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id = None # node/token type name
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value = None # used by literals
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first = second = None # used by tree nodes
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def nud(self, parser):
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# Null denotation - called in prefix context
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raise parser.error_class(
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"Not expecting '%s' in this position in if tag." % self.id
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)
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def led(self, left, parser):
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# Left denotation - called in infix context
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raise parser.error_class(
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"Not expecting '%s' as infix operator in if tag." % self.id
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)
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def display(self):
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"""
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Return what to display in error messages for this node
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"""
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return self.id
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def __repr__(self):
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out = [str(x) for x in [self.id, self.first, self.second] if x is not None]
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return "(" + " ".join(out) + ")"
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def infix(bp, func):
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"""
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Create an infix operator, given a binding power and a function that
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evaluates the node.
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"""
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class Operator(TokenBase):
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lbp = bp
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def led(self, left, parser):
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self.first = left
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self.second = parser.expression(bp)
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return self
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def eval(self, context):
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try:
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return func(context, self.first, self.second)
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except Exception:
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# Templates shouldn't throw exceptions when rendering. We are
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# most likely to get exceptions for things like {% if foo in bar
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# %} where 'bar' does not support 'in', so default to False
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return False
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return Operator
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def prefix(bp, func):
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"""
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Create a prefix operator, given a binding power and a function that
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evaluates the node.
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"""
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class Operator(TokenBase):
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lbp = bp
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def nud(self, parser):
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self.first = parser.expression(bp)
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self.second = None
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return self
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def eval(self, context):
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try:
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return func(context, self.first)
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except Exception:
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return False
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return Operator
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# Operator precedence follows Python.
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# We defer variable evaluation to the lambda to ensure that terms are
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# lazily evaluated using Python's boolean parsing logic.
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OPERATORS = {
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"or": infix(6, lambda context, x, y: x.eval(context) or y.eval(context)),
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"and": infix(7, lambda context, x, y: x.eval(context) and y.eval(context)),
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"not": prefix(8, lambda context, x: not x.eval(context)),
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"in": infix(9, lambda context, x, y: x.eval(context) in y.eval(context)),
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"not in": infix(9, lambda context, x, y: x.eval(context) not in y.eval(context)),
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"is": infix(10, lambda context, x, y: x.eval(context) is y.eval(context)),
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"is not": infix(10, lambda context, x, y: x.eval(context) is not y.eval(context)),
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"==": infix(10, lambda context, x, y: x.eval(context) == y.eval(context)),
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"!=": infix(10, lambda context, x, y: x.eval(context) != y.eval(context)),
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">": infix(10, lambda context, x, y: x.eval(context) > y.eval(context)),
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">=": infix(10, lambda context, x, y: x.eval(context) >= y.eval(context)),
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"<": infix(10, lambda context, x, y: x.eval(context) < y.eval(context)),
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"<=": infix(10, lambda context, x, y: x.eval(context) <= y.eval(context)),
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}
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# Assign 'id' to each:
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for key, op in OPERATORS.items():
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op.id = key
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class Literal(TokenBase):
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"""
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A basic self-resolvable object similar to a Django template variable.
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"""
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# IfParser uses Literal in create_var, but TemplateIfParser overrides
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# create_var so that a proper implementation that actually resolves
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# variables, filters etc. is used.
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id = "literal"
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lbp = 0
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def __init__(self, value):
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self.value = value
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def display(self):
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return repr(self.value)
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def nud(self, parser):
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return self
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def eval(self, context):
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return self.value
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def __repr__(self):
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return "(%s %r)" % (self.id, self.value)
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class EndToken(TokenBase):
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lbp = 0
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def nud(self, parser):
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raise parser.error_class("Unexpected end of expression in if tag.")
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EndToken = EndToken()
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class IfParser:
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error_class = ValueError
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def __init__(self, tokens):
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# Turn 'is','not' and 'not','in' into single tokens.
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num_tokens = len(tokens)
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mapped_tokens = []
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i = 0
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while i < num_tokens:
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token = tokens[i]
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if token == "is" and i + 1 < num_tokens and tokens[i + 1] == "not":
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token = "is not"
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i += 1 # skip 'not'
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elif token == "not" and i + 1 < num_tokens and tokens[i + 1] == "in":
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token = "not in"
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i += 1 # skip 'in'
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mapped_tokens.append(self.translate_token(token))
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i += 1
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self.tokens = mapped_tokens
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self.pos = 0
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self.current_token = self.next_token()
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def translate_token(self, token):
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try:
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op = OPERATORS[token]
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except (KeyError, TypeError):
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return self.create_var(token)
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else:
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return op()
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def next_token(self):
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if self.pos >= len(self.tokens):
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return EndToken
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else:
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retval = self.tokens[self.pos]
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self.pos += 1
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return retval
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def parse(self):
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retval = self.expression()
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# Check that we have exhausted all the tokens
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if self.current_token is not EndToken:
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raise self.error_class(
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"Unused '%s' at end of if expression." % self.current_token.display()
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)
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return retval
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def expression(self, rbp=0):
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t = self.current_token
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self.current_token = self.next_token()
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left = t.nud(self)
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while rbp < self.current_token.lbp:
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t = self.current_token
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self.current_token = self.next_token()
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left = t.led(left, self)
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return left
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def create_var(self, value):
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return Literal(value)
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