def apply_cats(df, trn):
    """Changes any columns of strings in df into categorical variables using trn as
    a template for the category codes.
    Parameters:
    -----------
    df: A pandas dataframe. Any columns of strings will be changed to
        categorical values. The category codes are determined by trn.
    trn: A pandas dataframe. When creating a category for df, it looks up the
        what the category's code were in trn and makes those the category codes
        for df.
    Examples:
    ---------
    >>> df = pd.DataFrame({'col1' : [1, 2, 3], 'col2' : ['a', 'b', 'a']})
    >>> df
       col1 col2
    0     1    a
    1     2    b
    2     3    a
    note the type of col2 is string
    >>> train_cats(df)
    >>> df
       col1 col2
    0     1    a
    1     2    b
    2     3    a
    now the type of col2 is category {a : 1, b : 2}
    >>> df2 = pd.DataFrame({'col1' : [1, 2, 3], 'col2' : ['b', 'a', 'a']})
    >>> apply_cats(df2, df)
           col1 col2
        0     1    b
        1     2    a
        2     3    a
    now the type of col is category {a : 1, b : 2}
    """
    for n, c in df.items():
        if n in trn.columns and trn[n].dtype.name == 'category':
            df[n] = c.astype('category').cat.as_ordered()
            df[n].cat.set_categories(trn[n].cat.categories, ordered=True,
                inplace=True)
