Unpacking list/tuple in for loop should raise exception on unpacking if length is different

[Peter Bengtsson]

In Python and Django you can do this:

points_3d = [(1,1,1), (2,2,0), ]

# python
for x, y, z in points_3d:
    pass

# django template
{% for x, y, z in points_3d %}
{% endfor %}

However, in Python you can't do this

for x, y in points_3d:
ValueError: too many values to unpack

# or

for x, y, z, w in points_3d:
ValueError: need more than 3 values to unpack

BUT Django allows it and doesn't raise a ValueError and I think this is bad. It allows you to do this:

{% for x, y, z, w in points_3d %}
  no probs :(
{% endfor %}

It lends itself to invalid and "incorrect" template code which can potentially hide programming mistakes that come back and bite you later.

[Chris Beaven]

I'm not sure if this falls into a backwards compatibility issue: The for tag was designed to not raise exceptions while rendering, and the current behaviour has specific tests.

[Peter Bengtsson]

I honestly don't know about the core tenants of the templating language. It might not be a bug then but my opinion is that it's a bad design decision and I'm questioning it.

[Aymeric Augustin]

This was initially marked as accepted by Russell, and I'm going to mark it as accepted again.

Idon't see much advantage in hiding this kind of errors.

We'll have to follow a deprecation path: start by raising warnings before we raise an actual exception.

[Ivan Kolodyazhny]

Here is proposed patch ​https://github.com/django/django/pull/1628. I'm not sure that warning message is good enough.

[Tim Graham <timograham@…>]

In 2aaa045c61a3d4b325f964aa01554107b47b9774:

Fixed #13408 -- Deprecated silent unpacking exception passing in for template tag.

Thanks peterbe for the suggestion.

[Tim Graham <timograham@…>]

In 3bbebd0:

Refs #13408 -- Made unpacking mismatch an exception in {% for %} tag per deprecation timeline.

[Tim Graham <timograham@…>]

In ba749f8:

Refs #13408 -- Removed obsolete code/comments from {% for %} unpacking deprecation.

[Tim Graham <timograham@…>]

In 54771f66:

[1.10.x] Refs #13408 -- Removed obsolete code/comments from {% for %} unpacking deprecation.

Backport of ba749f8f87dd60b20aeaefb84ee182f192746ebf from master