- Author:
- SmileyChris
- Posted:
- March 2, 2009
- Language:
- Python
- Version:
- 1.0
- Score:
- 65 (after 67 ratings)
Save this as smart_if.py
in the templatetags
folder of one of your apps. Then a simple {% load smart_if %}
replaces the boring built-in Django {% if %}
template with the new smart one.
7 May 2009: Was asked about whether it handles combination of and/or. It does, added a test to show it. I actually like how Django doesn't let you do this, but I'm not going to confuscate my code for a restriction like this.
15 June 2009: Fixed up a bug with boolean precedence (x or x == 0
was being parsed as (x or x) == 0
instead of x or (x == 0)
). Add some extra test cases, including some for invalid cases.
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A smarter {% if %} tag for django templates.
While retaining current Django functionality, it also handles equality,
greater than and less than operators. Some common case examples::
{% if articles|length >= 5 %}...{% endif %}
{% if "ifnotequal tag" != "beautiful" %}...{% endif %}
"""
import unittest
from django import template
register = template.Library()
#==============================================================================
# Calculation objects
#==============================================================================
class BaseCalc(object):
def __init__(self, var1, var2=None, negate=False):
self.var1 = var1
self.var2 = var2
self.negate = negate
def resolve(self, context):
try:
var1, var2 = self.resolve_vars(context)
outcome = self.calculate(var1, var2)
except:
outcome = False
if self.negate:
return not outcome
return outcome
def resolve_vars(self, context):
var2 = self.var2 and self.var2.resolve(context)
return self.var1.resolve(context), var2
def calculate(self, var1, var2):
raise NotImplementedError()
class Or(BaseCalc):
def calculate(self, var1, var2):
return var1 or var2
class And(BaseCalc):
def calculate(self, var1, var2):
return var1 and var2
class Equals(BaseCalc):
def calculate(self, var1, var2):
return var1 == var2
class Greater(BaseCalc):
def calculate(self, var1, var2):
return var1 > var2
class GreaterOrEqual(BaseCalc):
def calculate(self, var1, var2):
return var1 >= var2
class In(BaseCalc):
def calculate(self, var1, var2):
return var1 in var2
#==============================================================================
# Tests
#==============================================================================
class TestVar(object):
"""
A basic self-resolvable object similar to a Django template variable. Used
to assist with tests.
"""
def __init__(self, value):
self.value = value
def resolve(self, context):
return self.value
class SmartIfTests(unittest.TestCase):
def setUp(self):
self.true = TestVar(True)
self.false = TestVar(False)
self.high = TestVar(9000)
self.low = TestVar(1)
def assertCalc(self, calc, context=None):
"""
Test a calculation is True, also checking the inverse "negate" case.
"""
context = context or {}
self.assert_(calc.resolve(context))
calc.negate = not calc.negate
self.assertFalse(calc.resolve(context))
def assertCalcFalse(self, calc, context=None):
"""
Test a calculation is False, also checking the inverse "negate" case.
"""
context = context or {}
self.assertFalse(calc.resolve(context))
calc.negate = not calc.negate
self.assert_(calc.resolve(context))
def test_or(self):
self.assertCalc(Or(self.true))
self.assertCalcFalse(Or(self.false))
self.assertCalc(Or(self.true, self.true))
self.assertCalc(Or(self.true, self.false))
self.assertCalc(Or(self.false, self.true))
self.assertCalcFalse(Or(self.false, self.false))
def test_and(self):
self.assertCalc(And(self.true, self.true))
self.assertCalcFalse(And(self.true, self.false))
self.assertCalcFalse(And(self.false, self.true))
self.assertCalcFalse(And(self.false, self.false))
def test_equals(self):
self.assertCalc(Equals(self.low, self.low))
self.assertCalcFalse(Equals(self.low, self.high))
def test_greater(self):
self.assertCalc(Greater(self.high, self.low))
self.assertCalcFalse(Greater(self.low, self.low))
self.assertCalcFalse(Greater(self.low, self.high))
def test_greater_or_equal(self):
self.assertCalc(GreaterOrEqual(self.high, self.low))
self.assertCalc(GreaterOrEqual(self.low, self.low))
self.assertCalcFalse(GreaterOrEqual(self.low, self.high))
def test_in(self):
list_ = TestVar([1,2,3])
invalid_list = TestVar(None)
self.assertCalc(In(self.low, list_))
self.assertCalcFalse(In(self.low, invalid_list))
def test_parse_bits(self):
var = IfParser([True]).parse()
self.assert_(var.resolve({}))
var = IfParser([False]).parse()
self.assertFalse(var.resolve({}))
var = IfParser([False, 'or', True]).parse()
self.assert_(var.resolve({}))
var = IfParser([False, 'and', True]).parse()
self.assertFalse(var.resolve({}))
var = IfParser(['not', False, 'and', 'not', False]).parse()
self.assert_(var.resolve({}))
var = IfParser(['not', 'not', True]).parse()
self.assert_(var.resolve({}))
var = IfParser([1, '=', 1]).parse()
self.assert_(var.resolve({}))
var = IfParser([1, 'not', '=', 1]).parse()
self.assertFalse(var.resolve({}))
var = IfParser([1, 'not', 'not', '=', 1]).parse()
self.assert_(var.resolve({}))
var = IfParser([1, '!=', 1]).parse()
self.assertFalse(var.resolve({}))
var = IfParser([3, '>', 2]).parse()
self.assert_(var.resolve({}))
var = IfParser([1, '<', 2]).parse()
self.assert_(var.resolve({}))
var = IfParser([2, 'not', 'in', [2, 3]]).parse()
self.assertFalse(var.resolve({}))
var = IfParser([1, 'or', 1, '=', 2]).parse()
self.assert_(var.resolve({}))
def test_boolean(self):
var = IfParser([True, 'and', True, 'and', True]).parse()
self.assert_(var.resolve({}))
var = IfParser([False, 'or', False, 'or', True]).parse()
self.assert_(var.resolve({}))
var = IfParser([True, 'and', False, 'or', True]).parse()
self.assert_(var.resolve({}))
var = IfParser([False, 'or', True, 'and', True]).parse()
self.assert_(var.resolve({}))
var = IfParser([True, 'and', True, 'and', False]).parse()
self.assertFalse(var.resolve({}))
var = IfParser([False, 'or', False, 'or', False]).parse()
self.assertFalse(var.resolve({}))
var = IfParser([False, 'or', True, 'and', False]).parse()
self.assertFalse(var.resolve({}))
var = IfParser([False, 'and', True, 'or', False]).parse()
self.assertFalse(var.resolve({}))
def test_invalid(self):
self.assertRaises(ValueError, IfParser(['not']).parse)
self.assertRaises(ValueError, IfParser(['==']).parse)
self.assertRaises(ValueError, IfParser([1, 'in']).parse)
self.assertRaises(ValueError, IfParser([1, '>', 'in']).parse)
self.assertRaises(ValueError, IfParser([1, '==', 'not', 'not']).parse)
self.assertRaises(ValueError, IfParser([1, 2]).parse)
OPERATORS = {
'=': (Equals, True),
'==': (Equals, True),
'!=': (Equals, False),
'>': (Greater, True),
'>=': (GreaterOrEqual, True),
'<=': (Greater, False),
'<': (GreaterOrEqual, False),
'or': (Or, True),
'and': (And, True),
'in': (In, True),
}
BOOL_OPERATORS = ('or', 'and')
class IfParser(object):
error_class = ValueError
def __init__(self, tokens):
self.tokens = tokens
def _get_tokens(self):
return self._tokens
def _set_tokens(self, tokens):
self._tokens = tokens
self.len = len(tokens)
self.pos = 0
tokens = property(_get_tokens, _set_tokens)
def parse(self):
if self.at_end():
raise self.error_class('No variables provided.')
var1 = self.get_bool_var()
while not self.at_end():
op, negate = self.get_operator()
var2 = self.get_bool_var()
var1 = op(var1, var2, negate=negate)
return var1
def get_token(self, eof_message=None, lookahead=False):
negate = True
token = None
pos = self.pos
while token is None or token == 'not':
if pos >= self.len:
if eof_message is None:
raise self.error_class()
raise self.error_class(eof_message)
token = self.tokens[pos]
negate = not negate
pos += 1
if not lookahead:
self.pos = pos
return token, negate
def at_end(self):
return self.pos >= self.len
def create_var(self, value):
return TestVar(value)
def get_bool_var(self):
"""
Returns either a variable by itself or a non-boolean operation (such as
``x == 0`` or ``x < 0``).
This is needed to keep correct precedence for boolean operations (i.e.
``x or x == 0`` should be ``x or (x == 0)``, not ``(x or x) == 0``).
"""
var = self.get_var()
if not self.at_end():
op_token = self.get_token(lookahead=True)[0]
if isinstance(op_token, basestring) and (op_token not in
BOOL_OPERATORS):
op, negate = self.get_operator()
return op(var, self.get_var(), negate=negate)
return var
def get_var(self):
token, negate = self.get_token('Reached end of statement, still '
'expecting a variable.')
if isinstance(token, basestring) and token in OPERATORS:
raise self.error_class('Expected variable, got operator (%s).' %
token)
var = self.create_var(token)
if negate:
return Or(var, negate=True)
return var
def get_operator(self):
token, negate = self.get_token('Reached end of statement, still '
'expecting an operator.')
if not isinstance(token, basestring) or token not in OPERATORS:
raise self.error_class('%s is not a valid operator.' % token)
if self.at_end():
raise self.error_class('No variable provided after "%s".' % token)
op, true = OPERATORS[token]
if not true:
negate = not negate
return op, negate
#==============================================================================
# Actual templatetag code.
#==============================================================================
class TemplateIfParser(IfParser):
error_class = template.TemplateSyntaxError
def __init__(self, parser, *args, **kwargs):
self.template_parser = parser
return super(TemplateIfParser, self).__init__(*args, **kwargs)
def create_var(self, value):
return self.template_parser.compile_filter(value)
class SmartIfNode(template.Node):
def __init__(self, var, nodelist_true, nodelist_false=None):
self.nodelist_true, self.nodelist_false = nodelist_true, nodelist_false
self.var = var
def render(self, context):
if self.var.resolve(context):
return self.nodelist_true.render(context)
if self.nodelist_false:
return self.nodelist_false.render(context)
return ''
def __repr__(self):
return "<Smart If node>"
def __iter__(self):
for node in self.nodelist_true:
yield node
if self.nodelist_false:
for node in self.nodelist_false:
yield node
def get_nodes_by_type(self, nodetype):
nodes = []
if isinstance(self, nodetype):
nodes.append(self)
nodes.extend(self.nodelist_true.get_nodes_by_type(nodetype))
if self.nodelist_false:
nodes.extend(self.nodelist_false.get_nodes_by_type(nodetype))
return nodes
@register.tag('if')
def smart_if(parser, token):
"""
A smarter {% if %} tag for django templates.
While retaining current Django functionality, it also handles equality,
greater than and less than operators. Some common case examples::
{% if articles|length >= 5 %}...{% endif %}
{% if "ifnotequal tag" != "beautiful" %}...{% endif %}
Arguments and operators _must_ have a space between them, so
``{% if 1>2 %}`` is not a valid smart if tag.
All supported operators are: ``or``, ``and``, ``in``, ``=`` (or ``==``),
``!=``, ``>``, ``>=``, ``<`` and ``<=``.
"""
bits = token.split_contents()[1:]
var = TemplateIfParser(parser, bits).parse()
nodelist_true = parser.parse(('else', 'endif'))
token = parser.next_token()
if token.contents == 'else':
nodelist_false = parser.parse(('endif',))
parser.delete_first_token()
else:
nodelist_false = None
return SmartIfNode(var, nodelist_true, nodelist_false)
if __name__ == '__main__':
unittest.main()
|
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Comments
Hooray! I've been wanting one of these for literally years (even wrote part of it once before losing the code in a laptop theft). This design is exactly what I was after.
#
What about sneaking it into trunk? C'mon committers! ;-)
#
Please Please Please submit a ticket that kills the old if tag!! ;)
#
Seriously.
#
This snippet makes me happy
#
Very usefull snippets, you should really submit this for evalutation in the core.
Please also note that combinaison like {% if p > x and p < y %} or {% if x > i > y %} don't work.
#
{% if a > b and a < b %}
works now (I would have fixed sooner if I was notified of new comments on my snippets... I only saw this comment after I posted my fix).I realise that
{% if a > b > c %}
doesn't work - but I'm not really worried. That's getting pythonistic as opposed to basic boolean logic.#
Excellent! Although, would need EQ, NE, LT, GT, LE, GE, <> operators too.
#
i was looking for this, but i have error:
'smartif' is not a valid tag library: Could not load template library from django.templatetags.smartif, No module named smartif
thanks
#
ok i fixed
In you doc about use u have {% load smartif %} and need to fix to {% load smart_if %}
#
don't not support: {% if user.is_superuser != False %}
#
maplye: The better way to do the above is: {% if user.is_superuser %}...{% endif %}
#
This is now part of Django 1.2, so you don't need to use it there.
#
I am not even able to use it. Firebug throws the following error: "'smart_if' is not a valid tag library: Template library smart_if not found, tried django.templatetags.smart_if,django.contrib.admin.templatetags.smart_if"
I don't understand, I followed your instructions on top. What am I doing wrong???
I am not on Django 1.2 yet, since our ec2 instance only has 1.1, otherwise I wouldn't be using this plugin.
#
boralyl: while {% if x %} and {% if x != False %} are often the same, there are situations where you want to distinguish between False, None, [] and ''. I'm trying to figure out a workaround now.
#
It's nice that in Django 1.3 the builtin if tag can already do all this. :)
#
Just a test.
#
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