Is it possible to read categorical columns with pandas' read_csv?

Question

I have tried passing the dtype parameter with read_csv as dtype={n: pandas.Categorical} but this does not work properly (the result is an Object). The manual is unclear.

Answer 1

In version 0.19.0 you can use parameter dtype='category' in read_csv:

data = 'col1,col2,col3\na,b,1\na,b,2\nc,d,3'
df = pd.read_csv(pd.compat.StringIO(data), dtype='category')
print (df)
  col1 col2 col3
0    a    b    1
1    a    b    2
2    c    d    3

print (df.dtypes)
col1    category
col2    category
col3    category
dtype: object

If want specify column for category use dtype with dictionary:

df = pd.read_csv(pd.compat.StringIO(data), dtype={'col1':'category'})
print (df)
  col1 col2  col3
0    a    b     1
1    a    b     2
2    c    d     3

print (df.dtypes)
col1    category
col2      object
col3       int64
dtype: object

Answer 2

Categorical is not a valid dtype.

This StackOverflow post contains details for how to store categorical data in a text file.