From the documentation of csv, the first argument to csv.reader or csv.DictReader is csvfile -

csvfile can be any object which supports the iterator protocol and returns a string each time its __next__() method is called — file objects and list objects are both suitable.

In your case when you give the string as the direct input for the csv.DictReader() , the __next__() call on that string only provides a single character, and hence that becomes the header, and then __next__() is continuously called to get each row.

Hence, you need to either provide an in-memory stream of strings using io.StringIO:

>>> import csv
>>> s = """a,b,c
... 1,2,3
... 4,5,6
... 7,8,9"""
>>> import io
>>> reader_list = csv.DictReader(io.StringIO(s))
>>> print(reader_list.fieldnames)
['a', 'b', 'c']
>>> for row in reader_list:
...     print(row)
... 
{'a': '1', 'b': '2', 'c': '3'}
{'a': '4', 'b': '5', 'c': '6'}
{'a': '7', 'b': '8', 'c': '9'}

or a list of lines using str.splitlines:

>>> reader_list = csv.DictReader(s.splitlines())
>>> print(reader_list.fieldnames)
['a', 'b', 'c']
>>> for row in reader_list:
...     print(row)
... 
{'a': '1', 'b': '2', 'c': '3'}
{'a': '4', 'b': '5', 'c': '6'}
{'a': '7', 'b': '8', 'c': '9'}

The csv.DictReader function expects a file object as its first argument, not a string. So, it always is used with normal file io.

for example,

with open('input.csv', 'r') as f:
    reader = csv.DictReader(f, delimiter=' ', fieldnames = ['Device', 'tps', 'kB_read/s' ,'kB_wrtn/s', 'kB_dscd/s', 'kB_read', 'kB_wrtn', 'kB_dscd'])

And, you can iterate the reader (i.e., iterable of dicts) row by row.

for row in reader:
    print(row)

The row variable contains dictionary-like object that you can easily access by key, for example, row['Device'].