Prohibit quotes and vertical bar in {% for %} unpacking variable names

[Stephen Kelly]

The 'for' tag does not validate names of unpacked variables, allowing things like

{% for k|upper, "v" in mapping.items %}

without throwing an error. Such 'variables' are not useful within the for block.

#!/usr/bin/env python

from django.template import Template, Context
from django.template.engine import Engine

e = Engine()

c = Context()
c["m"] = {"one": "1", "two": "2"}

t = e.from_string('{% for k|upper, v in m.items %}{{ k|upper }} : {{ v }}\n{% endfor %}')
print t.render(c) 
#  : 2
#  : 1

t = e.from_string('{% for "k", v in m.items %}{{ "k" }} : {{ v }}\n{% endfor %}')
print t.render(c)
# k : 2
# k : 1

The for tag should error on an attempt to unpack to variables which contain FILTER_SEPARATOR, double-quoted string or single-quoted string.

The underlying issue is that Context does not validate keys it is given, so the cycle tag also has this issue in the form of {% cycle 'a' 'b' 'c' as "letter" %}, as does widthratio and any other tag which has an 'as' form.

[Tim Graham]

We need to be careful as this sort of "helpful validation" may break working code, even if a bit odd. Accepting for further investigation.

[Tim Martin]

I've created a patch that fixes this by having the do_for function validate the variables against known failure cases. However, this isn't the most general solution, since there are lots of other cases of invalid syntax that won't be caught by this. Would it make sense instead to validate tokens against the requirements for Python identifiers as described ​here?

[Tim Graham]

I'm unsure about further changes, I think we can go with your patch for now.

[Tim Graham <timograham@…>]

In e3f095b0:

Fixed #26478 -- Made {% for %} reject invalid unpacking vars with quotes or vertical bars.