Python Pandas sum() the difference between two values - python-3.x

Question

In my python code, using pandas i have to resample a datetimedata series and calculate diffs between a column values (the sum of diffs between values), i write this piece of code:
import pandas as pd
import datetime
from .models import Results, VarsResults
start_date = datetime.date(2021, 6, 21)
end_date = datetime.date(2021, 6, 24)
def calc_q(start_d, end_d):
start_d = start_date
end_d = end_date
var_results = VarsResults.objects.filter(
id_res__read_date__range=(start_d, end_d)
).select_related(
"id_res"
).values(
"id_res__read_date",
"id_res__unit_id",
"id_res__device_id",
"id_res__proj_code",
"var_val",
)
df = pd.DataFrame(list(var_results))
df['id_res__read_date'] = pd.to_datetime(df['id_res__read_date'])
df = df.set_index('id_res__read_date')
df_15 = df.resample('15min').sum()
return df_15
but i get the sum of the values itself.
example
... | 5
... | 3
... | 1
i get 9
i would the sum of the difference between values not the sum of the values:
in this case 4 (5-3 = 2 + 3-1 = 2, 2+2)
Is there a method in pandas using resample for manage this kind of clcultion?
So many thanks in advance
Manuel

Answer 1

The sum of all the differences is equal to the difference between the first element and the last one: if you work it out, all the other elements cancel out. In your data for example the 3 cancels out:
(5-3) + (3-1)
= 5 - 3 + 3 - 1 # - 3 and + 3 cancel out
= 5 - 1
I don't know how Pandas works, but you can simply do the equivalent of first_value - last_value.