= comparison does work in templates although it shouldn't

[Philipp Metzler]

There's no = operator documented: ​https://docs.djangoproject.com/en/1.7/ref/templates/builtins/#operator

For comparisons we use the == operator as = does assign values in most languages. In JavaScript the = assignment for example always returns true here:

<script language="javascript">

a = 'foo';
b = 'bar';

if (a=b) {

alert('assignment always returns true here')

}

</script>

BUT in the Django templates = does work like a comparison operator:

{% if "a" = "a" %}TRUE but Django should throw an error because there's no = operator{% endif %}

{% if "a" = "b" %}Django doesn't return True here{% endif %}

I think this is highly misleading and confusing.

[Baptiste Mispelon]

Hi,

I was surprised to see that the = operator does appear to work so I did some digging:

• It only works when surrounded by spaces (eg {% if 1 = 1 %} works but not {% if 1=1 %}
• It's sortof documented in the {% if %} tag's docstring [1]
• it's not actually documented publicly (as you mentionned)
• It's not tested (removing the parsing code for it [2] doesn't break any test)

Given all this, I'm in favor of removing that "feature".
We still should go through the deprecation policy though, just in case anyone might be relying on this undocumented and untested feature.

Thanks!

[1] ​https://github.com/django/django/blob/c1552af1fe5832011e3b1c3e5b40c20ff3dbe6f9/django/template/defaulttags.py#L988
[2] ​https://github.com/django/django/blob/c1552af1fe5832011e3b1c3e5b40c20ff3dbe6f9/django/template/smartif.py#L102

[Aymeric Augustin]

I'm not convinced we should make this change. It looks like something that could break existing templates on stable websites.

I understand that the reporter was confused but I don't remember other similar reports. The advantages seem quite small to me in comparison with the drawbacks.

[Baptiste Mispelon]

For the record, this feature was introduced with the "smart if" (2c2f5aee4d44836779fcd74c7782368914f9cfd1) which landed in 1.2. Before that, you had to use {% ifequal %}.

I'll also note that this syntax is invalid in jinja2.

However, if we want to keep it around for stability's sake, we should at least have tests for it.
I remain doubtful about the need to document it though as I don't think we should encourage the practice.

[Philipp Metzler]

BTW - I used this grep now to find occurrences in our code:

grep -E "if[[:space:]]+(\w|\.)*[[:space:]]*={1}[^=]" . -H -r -i -n -s -C 1 --include=*.{htm*,} --exclude-dir="node_modules"

[Keryn Knight]

Replying to aaugustin:

I'm not convinced we should make this change. It looks like something that could break existing templates on stable websites.
I understand that the reporter was confused but I don't remember other similar reports.

I believe a change is required:

• Deprecate and eventually pull out the functionality (earliest opportunity being 1.9) or
• Stabilise the parsing ({% if 1 = 1 %} vs {% if 1=1 %}) test and document it, so that the principle of least surprise is minimised.

The fact that there may be code out there that might accidentally work because of the right combination of typos (one equals sign, enough spaces) seems a mis-step that could be rectified (or made authoritatively correct).

It's also a quirk that doesn't fit with equality vs assignment in Python (and thus Django).

[Ola Sitarska]

PR: ​https://github.com/django/django/pull/3660

[Collin Anderson]

At work we use " = " a bunch in our code for comparisons. I didn't realize it wasn't documented, but I always found it annoying that it worked both ways. Upgrading will be annoying, but deprecating it sounds like a good idea to me.

[Tim Graham]

​Updated PR

[Tim Graham <timograham@…>]

In d563e3be68369694a3bac1efd7779d8e03bb6a51:

Fixed #23913 -- Deprecated the = comparison in if template tag.

[Tim Graham <timograham@…>]

In 9f51d0c86d9348349d921c393d6ffcbbfd521192:

Fixed test from refs #23913 when running tests in reverse.

[Philipp Metzler]

​https://docs.djangoproject.com/en/1.8/releases/1.8/#as-comparison-operator-in-if-template-tag

[Tim Graham <timograham@…>]

In 2ccfac1a:

Refs #23913 -- Removed support for a single equals sign in {% if %} tag.

Per deprecation timeline.