import os

train_files = os.listdir(train_data) # returns list of file in pathargument || should give you folder ['positive','negative']
train_file_positive = os.listdir(train_data + train_data_files[0]) # return just one text file path

Depending the the file format for your text you need to open it.
For normal .txt files it will be something like:
with open(train_file_positive, 'rb') as f:
    sentence = f.read()