Вот шаги, чтобы увидеть, что происходит
>>> filename = 'Testfile_20190226114536.CSV.986466.1551204043175'
# split the string into a list at '.'
>>> l = filename.split('.')
>>> print(l)
['Testfile_20190226114536', 'CSV', '986466', '1551204043175']
# index the list to get all the elements before and including 'CSV'
>>> filtered_list = l[0:l.index('CSV')+1]
>>> print(filtered_list)
['Testfile_20190226114536', 'CSV']
# join together the elements of the list with '.'
>>> out_string = '.'.join(filtered_list)
>>> print(out_string)
Testfile_20190226114536.CSV
Вот полная функция:
def filter_filename(filename):
l = filename.split('.')
filtered_list = l[0:l.index('CSV')+1]
out_string = '.'.join(filtered_list)
return out_string
>>> filter_filename('Testfile_20190226114536.CSV.986466.1551204043175')
'Testfile_20190226114536.CSV'