如何将数据框列名中的空格替换为"_"?
['join_date' 'fiscal_quarter' 'fiscal_year' 'primary_channel'
'secondary_channel' 'customer_count' 'new_members' 'revisit_next_day'
'revisit_14_day' 'demand_1yr' 'revisit_next_day_rate'
'revisit_14_day_rate' 'demand_1yr_per_new_member' u'ch_Ad Network'
u'ch_Affiliate' u'ch_Branded SEM' u'ch_DSP' u'ch_Daily Email'
u'ch_Daily Messaging' u'ch_Direct' u'ch_Direct Publisher' u'ch_Email'
u'ch_Feeds' u'ch_Native' u'ch_Non-Branded SEM' u'ch_Organic Search'
u'ch_Paid Social' u'ch_Site' u'ch_Special Email' u'ch_Television'
u'ch_Trigger Email' u'ch_UNMAPPED' u'ch_Unpaid Social' u'quarter_Q2'
u'quarter_Q3' u'quarter_Q4']