Given a wide dataframe, use melt to transform from wide to long. Declare the ID Columns and establish value columns as all remaining columns in dataframe.

quick_melt.py

import pandas as pd

def quick_melt(wide_df):
    '''
    Take wide dataframe and melt to long. Declare ID Columns (id_cols)
    and then establish all remaining columns as value columns
    '''

    id_cols = {'A', 'B', 'C'}
    value_cols = set(wide_df.columns) - id_cols
    long_df = pd.melt(wide_df, 
                      id_vars= list(id_cols), 
                      value_vars= list(value_cols),
                      var_name='Variable', 
                      value_name='Value')
       
    return long_df