基于表单值对Django QuerySet进行排序的最有效方法

Question

Working on a project with another fella. This is some code I wrote in view.py to sort a QuerySet based on some form data:

# Get sort by value
sort_by = search_form.cleaned_data.get('sort_by', SORT_BY_LAST_VISIT)

# Gather stops
stops = Stops.approved_objects.filter(**query_attributes)

# Do neccessary sorting
if sort_by == SORT_BY_PRICE:
    stops = stops.order_by('price')
else: # By default, sort by last visted
    stops = stops.order_by('last_visited')

However, last night my colleague modified the code to this:

# Get sort by value
sort_by = search_form.cleaned_data.get('sort_by', SORT_BY_LAST_VISIT)

# Gather stops based on sort
if sort_by == SORT_BY_PRICE:
    stops = Stops.approved_objects.filter(**query_attributes).order_by('price')
else: # By default, sort by last visted
    stops = Stops.approved_objects.filter(**query_attributes).order_by('last_visited')

His SVN comment: More efficient .

According to Django's documentation , both will equate to one database query. It is possible that I'm missing something else. Perhaps the fact that I'm setting the stops variable ( stops = ... ) twice?

Because I cannot get a hold of him till Monday, thought I'll go to the SO community on this one.

Answer 1

Unnecessary optimization. Besides:

# settings.py
SORTS = {SORT_BY_PRICE: 'price'}
DEFAULT_SORT = 'last_visited'

# whatever.py
sort_field = settings.SORTS.get(sort_by, settings.DEFAULT_SORT)

stops = Stops.approved_objects.filter(**query_attributes).order_by(sort_field)

That's what you should be doing ;)

Answer 2

Your colleague's solution should only save one STORE_FAST instruction (assuming that this is in a function. If it's global than it's a STORE_GLOBAL ) and one LOAD_FAST (or LOAD_GLOBAL instruction).

I'm pretty militant about sweating the microseconds (when I know how to) but not at the cost of readability. Your version is much more readable.

Although, I would do

sort_field = 'price' if sort_by == SORT_BY_PRICE else 'last_visited'
stops = Stops.approved_objects.filter(**query_attributes).order_by(sort_field)`