Diff() function use with groupby for pandas - pandas

Question

I am encountering an errors each time i attempt to compute the difference in readings for a meter in my dataset. The dataset structure is this.
id paymenttermid houseid houseid-meterid quantity month year cleaned_quantity
Datetime
2019-02-01 255 water 215 215M201 23.0 2 2019 23.0
2019-02-01 286 water 193 193M181 24.0 2 2019 24.0
2019-02-01 322 water 172 172M162 22.0 2 2019 22.0
2019-02-01 323 water 176 176M166 61.0 2 2019 61.0
2019-02-01 332 water 158 158M148 15.0 2 2019 15.0
I am attempting to generate a new column called consumption that computes the difference in quantities consumed for each house(identified by houseid-meterid) after every month of the year.
The code i am using to implement this is:
water_df["consumption"] = water_df.groupby(["year", "month", "houseid-meterid"])["cleaned_quantity"].diff(-1)
After executing this code, the consumption column is filled with NaN values. How can I correctly implement this logic.
The end result looks like this:
id paymenttermid houseid houseid-meterid quantity month year cleaned_quantity consumption
Datetime
2019-02-01 255 water 215 215M201 23.0 2 2019 23.0 NaN
2019-02-01 286 water 193 193M181 24.0 2 2019 24.0 NaN
2019-02-01 322 water 172 172M162 22.0 2 2019 22.0 NaN
2019-02-01 323 water 176 176M166 61.0 2 2019 61.0 NaN
2019-02-01 332 water 158 158M148 15.0 2 2019 15.0 NaN
Many thank in advance.
I have attempted to use
water_df["consumption"] = water_df.groupby(["year", "month", "houseid-meterid"])["cleaned_quantity"].diff(-1)
and
water_df["consumption"] = water_df.groupby(["year", "month", "houseid-meterid"])["cleaned_quantity"].diff(0)
and
water_df["consumption"] = water_df.groupby(["year", "month", "houseid-meterid"])["cleaned_quantity"].diff()
all this commands result in the same behaviour as stated above.
Expected output should be:
Datetime houseid-meterid cleaned_quantity consumption
2019-02-01 215M201 23.0 20
2019-03-02 215M201 43.0 9
2019-04-01 215M201 52.0 12
2019-05-01 215M201 64.0 36
2019-06-01 215M201 100.0 20
what steps should i take?

Answer 1

Sort values by Datetime (if needed) then group by houseid-meterid before compute the diff for cleaned_quantity values then shift row to align with the right data:
df['consumption'] = (df.sort_values('Datetime')
.groupby('houseid-meterid')['cleaned_quantity']
.transform(lambda x: x.diff().shift(-1)))
print(df)
# Output
Datetime houseid-meterid cleaned_quantity consumption
0 2019-02-01 215M201 23.0 20.0
1 2019-03-02 215M201 43.0 9.0
2 2019-04-01 215M201 52.0 12.0
3 2019-05-01 215M201 64.0 36.0
4 2019-06-01 215M201 100.0 NaN