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Is it possible to filter a Django queryset by model property?
i have a method in my model:
@property def myproperty(self): [..]
and now i want to filter by this property like:
is this somehow possible?
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Filter by property __del__: Questions
How can I make a time delay in Python?
I would like to know how to put a time delay in a Python script.
import time time.sleep(5) # Delays for 5 seconds. You can also use a float value.
Here is another example where something is run approximately once a minute:
import time while True: print("This prints once a minute.") time.sleep(60) # Delay for 1 minute (60 seconds).
You can use the
sleep() function in the
time module. It can take a float argument for sub-second resolution.
from time import sleep sleep(0.1) # Time in seconds
Filter by property __del__: Questions
How to delete a file or folder in Python?
How do I delete a file or folder in Python?
os.remove()removes a file.
os.rmdir()removes an empty directory.
shutil.rmtree()deletes a directory and all its contents.
List comprehension vs. lambda + filter
I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items.
My code looked like this:
my_list = [x for x in my_list if x.attribute == value]
But then I thought, wouldn"t it be better to write it like this?
my_list = filter(lambda x: x.attribute == value, my_list)
It"s more readable, and if needed for performance the lambda could be taken out to gain something.
Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way‚Ñ¢ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)?
It is strange how much beauty varies for different people. I find the list comprehension much clearer than
lambda, but use whichever you find easier.
There are two things that may slow down your use of
The first is the function call overhead: as soon as you use a Python function (whether created by
lambda) it is likely that filter will be slower than the list comprehension. It almost certainly is not enough to matter, and you shouldn"t think much about performance until you"ve timed your code and found it to be a bottleneck, but the difference will be there.
The other overhead that might apply is that the lambda is being forced to access a scoped variable (
value). That is slower than accessing a local variable and in Python 2.x the list comprehension only accesses local variables. If you are using Python 3.x the list comprehension runs in a separate function so it will also be accessing
value through a closure and this difference won"t apply.
The other option to consider is to use a generator instead of a list comprehension:
def filterbyvalue(seq, value): for el in seq: if el.attribute==value: yield el
Then in your main code (which is where readability really matters) you"ve replaced both list comprehension and filter with a hopefully meaningful function name.
This is a somewhat religious issue in Python. Even though Guido considered removing
reduce from Python 3, there was enough of a backlash that in the end only
reduce was moved from built-ins to functools.reduce.
Personally I find list comprehensions easier to read. It is more explicit what is happening from the expression
[i for i in list if i.attribute == value] as all the behaviour is on the surface not inside the filter function.
I would not worry too much about the performance difference between the two approaches as it is marginal. I would really only optimise this if it proved to be the bottleneck in your application which is unlikely.
Also since the BDFL wanted
filter gone from the language then surely that automatically makes list comprehensions more Pythonic ;-)
How do I do a not equal in Django queryset filtering?
In Django model QuerySets, I see that there is a
__lt for comparative values, but is there a
!= (not equals)? I want to filter out using a not equals. For example, for
Model: bool a; int x;
I want to do
results = Model.objects.exclude(a=True, x!=5)
!= is not correct syntax. I also tried
I ended up using:
results = Model.objects.exclude(a=True, x__lt=5).exclude(a=True, x__gt=5)
from myapp.models import Entry from django.db.models import Q Entry.objects.filter(~Q(id=3))
will return all entries except the one(s) with
3 as their ID:
[<Entry: Entry object>, <Entry: Entry object>, <Entry: Entry object>, ...]