Django: Create a variable in template

Question

I have a list of values I want to display in a template of django.

The list is more or less like this:

199801 string1
199802 string2
199904 string3
200003 string4
200011 string5

where the first column is a date in the form YYYYMM and the second is a generic string

the list is ordered by date desc

What I want to create is a list of stringx grouped by year something like this:

1998 string1, string2
1999 string3
2000 string4, string5

I gave a look to the documentation and I think the only thing I need is a way to create a variable where to store the "last year" I printed, so I can do something like:

if current_value.year != last_year
 #create a new row with the new year and the string
else
 #append the current string to the previous one

I think the only way I found is to write a custom templatetag and let it store the variable and the value... but before starting to write code I would like to know if there is a simpler way!

• mykhal: you should do it in view. from Django doc: the template system is meant to express presentation, not program logic.

Answer 1

Always do such thing in view, template system is not designed for such operations. Furthermore, it would be much harder to achieve this - the best idea that comes to my mind, is to create a filter. That, however, would be crazy - you would create a very specific filter just for one use. It's very easy to achieve in view:

last = None
result = []
for year, value in tuples:
 if year[0:4] == last:
 result[-1].append(value)
 else:
 result.append([value])
 last = year[0:4]

Answer 2

using itertools groupby

l = [
 (199801, 'string1'),
 (199802, 'string2'),
 (199904, 'string3'),
 (200003, 'string4'),
 (200011, 'string5'),
]
from itertools import groupby
iterator = groupby(l, key=lambda item: str(item[0])[:4])
for year, str_list in iterator:
 print year, list(str_list)

output

1998 [(199801, 'string1'), (199802, 'string2')]
1999 [(199904, 'string3')]
2000 [(200003, 'string4'), (200011, 'string5')]

• gruszczy: Wow, this groupby iterator is great :-)

Answer 3

You can't create a variable in a view, by design.

However the ifchanged tag probably does what you want:

{% for val in values %}
 {% ifchanged val.0 %} {{val.0}} {% endifchanged %}
 {{ val.1 }}
{% endfor %}