How can I insert data from a CSV file into a dataframe using pandas.read_csv?

Data Insert

You are putting the string of the filename into your DataFrame, i.e. RNA/4v6p.csv is your data in location row 0, col 0. You need to read in the file and store the data. This can be done by removing StringIO(str(...)) in your class

class StructureReader(object):
    def __init__(self, filename):
        self.filename = filename
    def read(self):
        self.data = pd.read_csv("RNA/"+self.filename), header=None, sep = ",")
        self.data

Code structure critique

I would also recommend removing the parent directory from being hardcoded by

1. Always passing in a full file path

   class StructureReader(object):
       def __init__(self, filepath):
           self.filepath = filepath
       def read(self):
           self.data = pd.read_csv(self.filepath), header=None, sep = ",")
           self.data
   

2. Making the directory an __init__() argument

   class StructureReader(object):
       def __init__(self, directory, filename):
           self.directory = directory
           self.filename = filename
       def read(self):
           self.data=pd.read_csv(self.directory+"/"+self.filename), header=None, sep = ",")
           # or import os and self.data=pd.read_csv(os.path.join(self.directory, self.filename)), header=None, sep = ",")
           self.data
   

3. Making the directory a constant attribute

   class StructureReader(object):
       def __init__(self, filename):
           self.directory = "RNA"
           self.filename = filename
       def read(self):
           self.data = pd.read_csv(self.directory+"/"+self.filename), header=None, sep = ",")
           # or import os and self.data=pd.read_csv(os.path.join(self.directory, self.filename)), header=None, sep = ",")
           self.data
   

This has nothing to do with reading your data, just a best practice commentary on structuring you code (Just my $0.02).

IIUC, you can just read it with:

df = pd.read_csv('yourfile.csv', header=None)

that for me returns:

         0        1       2   3   4
0  B/G/213  B/C/208  WW_cis NaN NaN
1  B/U/215  B/A/206  WW_cis NaN NaN
2  B/C/214  B/G/207  WW_cis NaN NaN
3  B/G/217  B/C/204  WW_cis NaN NaN
4  B/A/216  B/U/205  WW_cis NaN NaN
5  B/C/219  B/G/202  WW_cis NaN NaN
6  B/U/218  B/A/203  WW_cis NaN NaN
7  B/G/201  B/C/220  WW_cis NaN NaN
8  B/A/203  B/U/218  WW_cis NaN NaN

you can then select only the columns you want with:

df = df[[0,1,2]]

and operate as usual with dataframes.