无法按分类列过滤熊猫数据框

Question

pandas 0.16.1 I converted all columns in dataframe to categoricals so it takes MUCH less space when dumped to disk. Now i want to filter dataframe. It's ok with == and .isin but fails on <, <=, etc. operations with "Unordered Categoricals can only compare equality or not"

data[data["MONTH COLUMN"]<=3]

If i comment out the following lines in categorical.py everything works fine. Is it a bug in pandas?

if not self.ordered:
    if op in ['__lt__', '__gt__','__le__','__ge__']:
        raise TypeError("Unordered Categoricals can only compare equality or not")

I think it was a good idea to use Categorical datatype on column which has only 12 unique values in ~1'400'000 rows.)

Answer 1

The documentation states:

Note New categorical data are NOT automatically ordered. You must explicity pass ordered=True to indicate an ordered Categorical.

When you first create a category you want to be ordered, just specify this:

In [1]: import pandas as pd

In [3]: s = pd.Series(["a","b","c","a"]).astype('category', ordered=True)

In [5]: s
Out[5]: 
0    a
1    b
2    c
3    a
dtype: category
Categories (3, object): [a < b < c]

In [4]: s > 'a'
Out[4]: 
0    False
1     True
2     True
3    False
dtype: bool