Django snippets: Smart {% if %} template tag

"""
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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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.

Author:: SmileyChris
Posted:: March 2, 2009
Language:: Python
Django Version:: 1.0
Tags:: if smart-if if-in greater-than less-than
Score:: 66 (after 66 ratings)

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Comments

simon (on March 3, 2009):

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.

lbolognini (on March 3, 2009):

What about sneaking it into trunk? C'mon committers! ;-)

telenieko (on March 3, 2009):

Please Please Please submit a ticket that kills the old if tag!! ;)

carljm (on March 3, 2009):

Seriously.

chrisrbennett (on March 3, 2009):

This snippet makes me happy

leehinde (on May 6, 2009):

Very nice. Thanks.

Krystal (on May 9, 2009):

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.

SmileyChris (on June 14, 2009):

{% 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.

showell (on June 17, 2009):

FYI I added an elif patch to snippets recently.

ges (on June 29, 2009):

Excellent! Although, would need EQ, NE, LT, GT, LE, GE, <> operators too.

asinox (on August 13, 2009):

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 %}

maplye (on August 15, 2009):

don't not support: {% if user.is_superuser != False %}

boralyl (on August 27, 2009):

maplye: The better way to do the above is: {% if user.is_superuser %}...{% endif %}

EmilStenstrom (on May 24, 2010):

This is now part of Django 1.2, so you don't need to use it there.

iganapolsky (on July 15, 2010):

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.

chadselph (on September 21, 2010):

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.

handaru (on January 24, 2011):

Are there smart_if template for django 0.96 on 'real' googleappengine [not on SDK]? Smart_if template from djangosnippets.org [http://djangosnippets.org/snippets/1350/] does not work properly with register = webapp.template.create_template_register().

ffsffd (on July 28, 2011):

It's nice that in Django 1.3 the builtin if tag can already do all this. :)

jezdez (on May 30, 2012):

Just a test.

RobertPattinson (on July 30, 2012):

The better way to do the above is: {% if user.is_superuser %}...{% endif %} Thanks! RP ---Samsung Galaxy S2 Cases--

KStevens (on December 19, 2012):

{% if p > x and p < y %} doesn't work. Is there a workaround? Kevin @ Injury Guide

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Smart {% if %} template tag

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