Count how many times a value appears per month in dataframe

I have the following time series with hourly data of several years:

    local time         ghi mean
0       2013-01-01 00:00:00 0.0
1       2013-01-01 01:00:00 0.0
2       2013-01-01 02:00:00 -9999
3       2013-01-01 03:00:00 0.0
4       2013-01-01 04:00:00 0.0
..         ...          ...
8754    2016-12-31 18:00:00 427.5
8755    2016-12-31 19:00:00 194.9
8756    2016-12-31 20:00:00 -9999
8757    2016-12-31 21:00:00 237.6
8758    2016-12-31 22:00:00 -9999
8759    2016-12-31 23:00:00 0.0

And I need to count how many times the value -9999 appears and group by year and month.The desired output would be something similar to:

    local time     ghi mean
0    2013-01   1
..         ...          ...
8    2016-12   2

I tried:

df.groupby(df["local time"].dt.strftime('%Y-%m')).df['ghi mean'].value_counts()[-9999]

But got:

AttributeError: 'Series' object has no attribute 'df'

>Solution :

Boolean select, groupby sum should give you what you need much easily

if df['ghi mean'] is a float or integer

  (df['ghi mean']==-9999).groupby(df['local time'].dt.strftime('%Y-%m')).sum()

if you made df['ghi mean']a string then

(df['ghi mean']=='-9999.0').groupby(df['local time'].dt.strftime('%Y-%m')).sum()