Convert nominal (string, object) features in a pandas dataframe to integers

convert_nominal.py

import pandas as pd

def get_nominal_integer_dict(nominal_vals):
    d = {}
    for val in nominal_vals:
        if val not in d:
            current_max = max(d.values()) if len(d) > 0 else -1
            d[val] = current_max+1
    return d


def convert_to_integer(srs):
    d = get_nominal_integer_dict(srs)
    return srs.map(lambda x: d[x])


def convert_strings_to_integer(df):
    ret = pd.DataFrame()
    for column_name in df:
        column = df[column_name]
        if column.dtype=='string' or column.dtype=='object':
            ret[column_name] = convert_to_integer(column)
        else:
            ret[column_name] = column
    return ret