+= operation with non existing dataframes

Question

df_pairs:

city1   city2
0   sfo yyz
1   sfo yvr
2   sfo dfw
3   sfo ewr

output of df_pairs.to_dict(\'records\'):

Answer 1

Why don't you simply do this:

df_city1 = pd.merge(df_pairs['city1'], data_df, left_on='city1', right_on='city', how='left')
df_city2 = pd.merge(df_pairs['city2'], data_df, left_on='city2', right_on='city', how='left')
diff = df_city2.subtract(df_city1, fill_value=0)
pos_sum = diff[diff >= 0].sum(axis=1)
neg_sum = diff[diff <  0].sum(axis=1)

Instead of looping over all your columns, merging 2*(number of columns) times, not to mention indexing, then that complicated bit with np.sign and .clip... Your df_pairs and data_df have a one-to-one correspondence, right?