Develop Reference

Python, django filter by kwargs or list, inclusive output

I want to get get account Ids that will be associated with determined list of ids, currently I filter by one exactly id and I would like to input various Ids so I can get a Wider result.
My code:
from typing import List
from project import models
def get_followers_ids(system_id) -> List[int]:
return list(models.Mapper.objects.filter(system_id__id=system_id
).values_list('account__id', flat=True))
If I run the code, I get the Ids associated with the main ID, the output will be a list of ids related to the main one (let's say, "with connection to"):
Example use:
system_id = 12350
utility_ids = get_followers_ids(system_id)
print(utility_ids)
output:
>>> [14338, 14339, 14341, 14343, 14344, 14346, 14347, 14348, 14349, 14350, 14351]
But I would like to input more variables avoiding to fell in a for loop, which will be slow because it will do many requests to the server.
The input I would like to use is a list or similar, it should be able to input various arguments at a time.
And the output should be a list of relations (doing the least number of requests to DB), example
if id=1 is related to [3,4,5,6]
and if id=2 is related to [5,6,7,8]
The output should be [3,4,5,6,7,8]

you can use the Field lookups, in your case use "In" lookup
so:
# system_ids is now a list
def get_followers_ids(system_ids) -> List[int]:
# here add in the end in filter field the "__in"
return list(models.Mapper.objects.filter(system_id__id__in=system_ids
).values_list('account__id', flat=True))
system_ids = [12350, 666]
utility_ids = get_followers_ids(system_ids)
print(utility_ids)

Conditionally change field value for lookup in Django ORM

I'm trying to conditionally change field value during lookup - I have some specific order in mind and I do not want to overwrite field value, just to sort it my way. Let's say, I have classProduct and every class object has product_code field. Now I want to get less than or equal, but it's not trivial - product_code is for most of the time like this A01, B02 and so on and Django lookup lte would work. But now I have fields 0001C01 which I would like to be the biggest value. So during lookup I would like to add 0000 at the begining of every string that does not have this prefix, so it would look like 0001C01, 0000B02, 0000A01.

You can conditionally annotate your queryset in order to get a new field that has de desired value and then use this field in your filter or order_by clause. For example you could do the following:
from django.db.models import CharField, Value as V, F, Q, Case, When
from django.db.models.functions import Concat
Product.objects.annotate(
new_product_code=Case(
When(product_code__iregex=r'^[A-Z]+.*', # If it starts with letters
then=Concat(V('0000'), 'product_code', output_field=CharField()) # Then prepend four 0's
),
default=F('product_code') # Else, the original value
)
).filter(new_product_code__lte='whatever you like') # Now filter by using your new value
Relevant parts of the documentation are conditional expressions, database functions and QuerySet API reference

This sounds fairly straightforward. Fetch the desired Product objects, and for each one, prepend 0000 to product_code if it doesn't start with that string.
products = Product.objects.filter(some_query_expression)
for product in products:
if not product.product_code.startswith('0000'):
product.product_code = '0000' + product.product_code
It's not clear if you want to save this value back to the database, or just use it for temporary comparisons. If you do want to save it, call product.save().

Sorting a list of dict from redis in python

in my current project i generate a list of data, each entry is from a key in redis in a special DB where only one type of key exist.
r = redis.StrictRedis(host=settings.REDIS_AD, port=settings.REDIS_PORT, db='14')
item_list = []
keys = r.keys('*')
for key in keys:
item = r.hgetall(key)
item_list.append(item)
newlist = sorted(item_list, key=operator.itemgetter('Id'))
The code above let me retrieve the data, create a list of dict each containing the information of an entry, problem is i would like to be able to sort them by ID, so they come out in order when displayed on my html tab in the template, but the sorted function doesn't seem to work since the table isn't sorted.
Any idea why the sorted line doesn't work ? i suppose i'm missing something to make it work but i can't find what.
EDIT :
Thanks to the answer in the comments,the problem was that my 'Id' come out of redis as a string and needed to be casted as int to be sorted
key=lambda d: int(d['Id'])

All values returned from redis are apparently strings and strings do not sort numerically ("10" < "2" == True).
Therefore you need to cast it to a numerical value, probably to int (since they seem to be IDs):
newlist = sorted(item_list, key=lambda d: int(d['Id']))

sqlalchemy print results instead of objects

I am trying to print the results of my query to the console, with the below code, but it keeps returning me the object location instead.
test = connection.execute('SELECT EXISTS(SELECT 1 FROM "my_table" WHERE Code = 08001)')
print(test)
Result that I get is - <sqlalchemy.engine.result.ResultProxy object at 0x000002108508B278>
How do I print out the value instead of the object?
Reason I am asking is because I want to use the value to do comparison tests.
EG:
if each_item not in test:
# do stuffs

Like this, test contains all the rows returned by your query.
If you want something you can iterate over, you can use fetchall for example. Like this:
test = connection.execute('SELECT EXISTS(SELECT 1 FROM "my_table" WHERE Code = 08001)').fetchall()
Then you can iterate over a list of rows. Each row will contain all the fields. In your example you can access fields by their position. You only have one at position one. So that's how you can access 1:
for row in test:
print(row[0])

test is an object containing the rows values. So if the column's name is value, you can call it using test.value.
If you're looking for more "convenient" way of doing so (like iterating through each column of test), you'd have to explicitly define these functions (either as methods of test or as other functions designed to iterate through those types of rows).

How to efficiently select entries by date in python?

I have emails and dates. I can use 2 nested for loops to choose emails sent on same date, but how can i do it 'smart way' - efficiently?
# list of tuples - (email,date)
for entry in list_emails_dates:
current_date = entry[1]
for next_entry in list_emails_dates:
if current_date = next_entry[1]
list_one_date_emails.append(next_entry)
I know it can be written in shorter code, but I don't know itertools, or maybe use map, xrange?

You can just convert this to a dictionary, by collecting all emails related to a date into the same key.
To do this, you need to use defaultdict from collections. It is an easy way to give a new key in a dictionary a default value.
Here we are passing in the function list, so that each new key in the dictionary will get a list as the default value.
emails = defaultdict(list)
for email,email_date in list_of_tuples:
emails[email].append(email_date)
Now, you have emails['2013-14-07'] which will be a list of emails for that date.
If we don't use a defaultdict, and do a dictionary comprehension like this:
emails = {x[1]:x[0] for x in list_of_tuples}
You'll have one entry for each date, which will be the last email for that that, since assigning to the same key will override its value. A dictionary is the most efficient way to lookup a value by a key. A list is good if you want to lookup a value by its position in a series of values (assuming you know its position).
If for some reason you are not able to refactor it, you can use this template method, which will create a generator:
def find_by_date(haystack, needle):
for email, email_date in haystack:
if email_date == needle:
yield email
Here is how you would use it:
>>> email_list = [('foo#bar.com','2014-07-01'), ('zoo#foo.com', '2014-07-01'), ('a#b.com', '2014-07-03')]
>>> all_emails = list(find_by_date(email_list, '2014-07-01'))
>>> all_emails
['foo#bar.com', 'zoo#foo.com']
Or, you can do this:
>>> july_first = find_by_date(email_list, '2014-07-01')
>>> next(july_first)
'foo#bar.com'
>>> next(july_first)
'zoo#foo.com'

I would do an (and it's good to try using itertools)
itertools.groupby(list_of_tuples, lambda x: x[1])
which gives you the list of emails grouped by the date (x[1]). Note that when you do it you have to sort it regarding the same component (sorted(list_of_tuples, lambda x: x[1])).
One nice thing (other than telling the reader that we do a group) is that it works lazily and, if the list is kind of long, its performance is dominated by n log n for the sorting instead of n^2 for the nested loop.