#26095: Same behaviour of dict.items and defaultdict.items in DTL
-------------------------------------+-------------------------------------
Reporter: kbr | Owner: kbr
Type: | Status: closed
Cleanup/optimization |
Component: Template system | Version: master
Severity: Normal | Resolution: wontfix
Keywords: templates dict | Triage Stage:
defaultdict | Unreviewed
Has patch: 1 | Needs documentation: 1
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by kbr):
Some final thoughts, but at the very beginning: I'm in no way annoyed that
the patch doesn't get merged!
As funkybob mentioned, isinstance is called for every step of every
lookup. So a template like this will cause 4 additional calls:
{{{
{% for k, v in d.items %}{{ k }}:{{ v }}{% endfor %}
}}}
I have set up a tiny script (with 1000 times the above snippet) for some
simple profiling to get a feeling for the impact:
{{{
from django.template.base import Context
from django.template.engine import Engine
s = '{% for k, v in d.items %}{{ k }}:{{ v }}{% endfor %}'
d = {'one': [1]}
n = 1000
s *= n
engine = Engine()
t = engine.from_string(s)
context = Context({'d': d,})
#res = t.render(context)
}}}
The last line is commented out to get the overhead for preparing the
template and the context. Calling 'python -m cProfile -s cumulativ
_profile.py' now gives a fairly large output:
{{{
322295 function calls (316792 primitive calls) in 0.420 seconds
...
20469 0.005 0.000 0.005 0.000 {built-in method isinstance}
}}}
isinstance() gets called 20469 times. Most of these calls are because of
's *= n'. Uncommenting the last line now renders with the unpatched
version:
{{{
464331 function calls (455828 primitive calls) in 0.543 seconds
...
50472 0.012 0.000 0.012 0.000 {built-in method isinstance}
}}}
shows that isinstance gets called excessively and takes about 7 ms
additional runtime.
Rendering with the patched version should produce even 4k more calls:
{{{
468330 function calls (459827 primitive calls) in 0.517 seconds
...
54471 0.013 0.000 0.013 0.000 {built-in method isinstance}
}}}
This is the case and runs 1 ms longer.
To prepare a context for 4k lookups it may take some additional time for
accessing a db and the template may not come from a prepared string but
from another io-channel. All in all I think that the impact of the patch
is insignificant.
@funkybob: you further mentioned a precedent. Well, that may be a point.
IMHO it's quiet ok to expect as least surprise using dicts and
defaultdicts as these datatypes are builtins resp. from the standard
library. One can not assume that individual modified classes derived from
these types will have no unexpected behaviour in the way the DTL-lookup
works. But that's just my opinion and there may be other ones.
--
Ticket URL: <https://code.djangoproject.com/ticket/26095#comment:9>
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