Retaining categorical dtype upon dataframe concatenation

I don't think this is completely obvious from the documentation, but you could do something like the following. Here's some sample data:

df1=pd.DataFrame({'x':pd.Categorical(['dog','cat'])})
df2=pd.DataFrame({'x':pd.Categorical(['cat','rat'])})

Use union_categoricals1 to get consistent categories accros dataframes. Try df.x.cat.codes if you need to convince yourself that this works.

from pandas.api.types import union_categoricals

uc = union_categoricals([df1.x,df2.x])
df1.x = pd.Categorical( df1.x, categories=uc.categories )
df2.x = pd.Categorical( df2.x, categories=uc.categories )

Concatenate and verify the dtype is categorical.

df3 = pd.concat([df1,df2])

df3.x.dtypes
category

As @C8H10N4O2 suggests, you could also just coerce from objects back to categoricals after concatenating. Honestly, for smaller datasets I think that's the best way to do it just because it's simpler. But for larger dataframes, using union_categoricals should be much more memory efficient.

To complement JohnE's answer, here's a function that does the job by converting to union_categoricals all the category columns present on all input dataframes:

def concatenate(dfs):
    """Concatenate while preserving categorical columns.

    NB: We change the categories in-place for the input dataframes"""
    from pandas.api.types import union_categoricals
    import pandas as pd
    # Iterate on categorical columns common to all dfs
    for col in set.intersection(
        *[
            set(df.select_dtypes(include='category').columns)
            for df in dfs
        ]
    ):
        # Generate the union category across dfs for this column
        uc = union_categoricals([df[col] for df in dfs])
        # Change to union category for all dataframes
        for df in dfs:
            df[col] = pd.Categorical(df[col].values, categories=uc.categories)
    return pd.concat(dfs)

Note the categories are changed in place in the input list:

df1=pd.DataFrame({'a': [1, 2],
                  'x':pd.Categorical(['dog','cat']),
                  'y': pd.Categorical(['banana', 'bread'])})
df2=pd.DataFrame({'x':pd.Categorical(['rat']),
                  'y': pd.Categorical(['apple'])})

concatenate([df1, df2]).dtypes