Passing variables into ReportQueryBuilder - GoogleAdsAPI

I am using GoogleAdsApi - and I have the query below that works great. :)

report_query = (adwords.ReportQueryBuilder()                   .Select(                     'Date','AdGroupId','AdGroupName','CampaignId','CampaignName',                                      'Impressions','Clicks',                           'Cost'                           ,'Conversions','ConversionRate','Ctr','AverageCpc','CostPerConversion'                           )                    .From('CRITERIA_PERFORMANCE_REPORT')                   .Where('Status').In('ENABLED',                    'PAUSED')                   .During('20200101,20210331')                   .Build()) 

I would like to make this function more dynamic by creating a date variable (I have listed below) and possibly more variables let's say field as well. When I try to create & pass in variables ,it does not work

fields = "'Date','AdGroupId','AdGroupName','CampaignId','CampaignName','Impressions','Clicks','Cost','Conversions','ConversionRate','Ctr','AverageCpc','CostPerConversion'" today = date.today() yesterday = today - timedelta(days = 1) date_filter = f'\'{yesterday.strftime("%Y%m%d")},{today.strftime("%Y%m%d")}\''  report_query = (adwords.ReportQueryBuilder()                       .Select(                        fields                               )                        .From('CRITERIA_PERFORMANCE_REPORT')                       .Where('Status').In('ENABLED',                        'PAUSED')                       .During(date_filter)                       .Build()) 

I get the error below:

googleads.errors.AdWordsReportBadRequestError: Type: QueryError.INVALID_DURING_CLAUSE 

Even though when I pass in the printed value of my date variable, instead of the date variable into .During, it works. So a little confused.