1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
|
"""
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)
|
