通过索引上的函数过滤熊猫数据框

Question

I have a dataframe with a Date used as index, I would like to extract all rows that are for a given month say January. Eg given

Date         Col1 
2000-01-01    1   
...
2000-12-31    12   
2001-01-01    1   
...
2001-12-31    12   
2002-01-01    1   
...
2002-12-31    12   

then this works

f = df.index.map(lambda x: x.month==1)
new_df = df[f]

Is there a simpler way of getting new_df?

I would have expected to be able to do something like df.loc[lambda x: x.month==1] but this does not work because x is actually df (which is counter-intuitive as .loc expect actual labels in all other forms of .loc calls so I would have though x would be each label of the index). Or df.filter(lambda x: x.month==1) but filter does not accept a callback.

Answer 1

You can use pandas.DatetimeIndex.month with loc to return index where condition matches

df.loc[df.index.month==1]
#df[df.index.month==1]

---

            Col1
Date            
2000-01-01   1.0
2001-01-01   1.0
2002-01-01   1.0