Pandas - Filling missing dates within groups with different time ranges - python-3.x

Question

I'm working with a dataset which has monthly information about several users. And each user has a different time range. There is also missing "time" data for each user. What I would like to do is fill in the missing month data for each user based on the time range for each user(from min.time to max.time in months)
I've read approaches to similar situation using re-sample, re-index from here, but I'm not getting the desired output/there is row mismatch after filling the missing months.
Any help/pointers would be much appreciated.
-Luc
Tried using re-sample, re-index, but not getting desired output
x = pd.DataFrame({'user': ['a','a','b','b','c','a','a','b','a','c','c','b'], 'dt': ['2015-01-01','2015-02-01', '2016-01-01','2016-02-01','2017-01-01','2015-05-01','2015-07-01','2016-05-01','2015-08-01','2017-03-01','2017-08-01','2016-09-01'], 'val': [1,33,2,1,5,4,2,5,66,7,5,1]})
date id value
0 2015-01-01 a 1
1 2015-02-01 a 33
2 2016-01-01 b 2
3 2016-02-01 b 1
4 2017-01-01 c 5
5 2015-05-01 a 4
6 2015-07-01 a 2
7 2016-05-01 b 5
8 2015-08-01 a 66
9 2017-03-01 c 7
10 2017-08-01 c 5
11 2016-09-01 b 1
What I would like to see is - for each 'id' generate missing months based on min.date and max.date for that id and fill 'val' for those months with 0.

Answer 1

Create DatetimeIndex, so possible use groupby with custom lambda function and Series.asfreq:
x['dt'] = pd.to_datetime(x['dt'])
x = (x.set_index('dt')
.groupby('user')['val']
.apply(lambda x: x.asfreq('MS', fill_value=0))
.reset_index())
print (x)
user dt val
0 a 2015-01-01 1
1 a 2015-02-01 33
2 a 2015-03-01 0
3 a 2015-04-01 0
4 a 2015-05-01 4
5 a 2015-06-01 0
6 a 2015-07-01 2
7 a 2015-08-01 66
8 b 2016-01-01 2
9 b 2016-02-01 1
10 b 2016-03-01 0
11 b 2016-04-01 0
12 b 2016-05-01 5
13 b 2016-06-01 0
14 b 2016-07-01 0
15 b 2016-08-01 0
16 b 2016-09-01 1
17 c 2017-01-01 5
18 c 2017-02-01 0
19 c 2017-03-01 7
20 c 2017-04-01 0
21 c 2017-05-01 0
22 c 2017-06-01 0
23 c 2017-07-01 0
24 c 2017-08-01 5
Or use Series.reindex with min and max datetimes per groups:
x = (x.set_index('dt')
.groupby('user')['val']
.apply(lambda x: x.reindex(pd.date_range(x.index.min(),
x.index.max(), freq='MS'), fill_value=0))
.rename_axis(('user','dt'))
.reset_index())