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