Dummy variables when not all categories are present

Question

I have a set of dataframes where one of the columns contains a categorical variable. I'd like to convert it to several dummy variables, in which case I'd normally use get_dummies.

What happens is that get_dummies looks at the data available in each dataframe to find out how many categories there are, and thus create the appropriate number of dummy variables. However, in the problem I'm working right now, I actually know in advance what the possible categories are. But when looking at each dataframe individually, not all categories necessarily appear.

My question is: is there a way to pass to get_dummies (or an equivalent function) the names of the categories, so that, for the categories that don't appear in a given dataframe, it'd just create a column of 0s?

Something that would make this:

categories = ['a', 'b', 'c']

   cat
1   a
2   b
3   a

Become this:

  cat_a  cat_b  cat_c
1   1      0      0
2   0      1      0
3   1      0      0

Answer 1

TL;DR:

pd.get_dummies(cat.astype(pd.CategoricalDtype(categories=categories)))

• Older pandas: pd.get_dummies(cat.astype('category', categories=categories))

is there a way to pass to get_dummies (or an equivalent function) the names of the categories, so that, for the categories that don't appear in a given dataframe, it'd just create a column of 0s?

Yes, there is! Pandas has a special type of Series just for categorical data. One of the attributes of this series is the possible categories, which get_dummies takes into account. Here's an example:

In [1]: import pandas as pd

In [2]: possible_categories = list('abc')

In [3]: cat = pd.Series(list('aba'))

In [4]: cat = cat.astype(pd.CategoricalDtype(categories=possible_categories))

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

Then, get_dummies will do exactly what you want!

In [6]: pd.get_dummies(cat)
Out[6]: 
   a  b  c
0  1  0  0
1  0  1  0
2  1  0  0

There are a bunch of other ways to create a categorical Series or DataFrame, this is just the one I find most convenient. You can read about all of them in the pandas documentation.

EDIT:

I haven't followed the exact versioning, but there was a bug in how pandas treats sparse matrices, at least until version 0.17.0. It was corrected by version 0.18.1 (released May 2016).

For version 0.17.0, if you try to do this with the sparse=True option with a DataFrame, the column of zeros for the missing dummy variable will be a column of NaN, and it will be converted to dense.

It looks like pandas 0.21.0 added a CategoricalDType, and creating categoricals which explicitly include the categories as in the original answer was deprecated, I'm not quite sure when.