Make new column in Panda dataframe by adding values from other columns
Building a little more on Anton's answer, you can add all the columns like this:
df['sum'] = df[list(df.columns)].sum(axis=1)
The simplest way would be to use DeepSpace answer. However, if you really want to use an anonymous function you can use apply:
df['C'] = df.apply(lambda row: row['A'] + row['B'], axis=1)