Why contains can't select rows contains specified string?

Question

>>> y
1    2002-12-31
2    2003-12-31
3    2004-03-31
4    2004-06-30
Name: report_date, dtype: object

I want to extract rows which contain 12-31 .

>>> y.str.contains('12-31')
>>> y.str.contains('\.+12-31')
>>> y.str.contains('2002-12-31')

All the three expressions get same output:

1   NaN
2   NaN
3   NaN
4   NaN
Name: report_date, dtype: float64

How can i extract rows which contain string 12-31? My desired output:

1   True
2   True
3   NaN
4   NaN

Answer 1

Maybe in column are dates, so convert it to strings before:

m = y.astype(str).str.contains('12-31')
print (m)
0     True
1     True
2    False
3    False
Name: report_date, dtype: bool