import re
import argparse

def read_document(path):
    '''Reads a file when provided with its path, and returns a string 
    containing the lines of the file.'''
    file_given = open(path)
    f = ""
    for line in file_given:
        f += line + " "
    file_given.close()
    return f

def apply_preprocessing(f):
    '''Removes non-alphabetic characters, replaces all whitespace characters 
    with a single whitespace, and changes all uppercase characters to 
    lowercase'''
    f = re.sub(r'[^a-zA-Z\s]', r'', f)
    f = re.sub(r'\s+', r' ', f)
    f = f.lower()
    return f

def read_args():
    '''Creates an argparse ArgumentParser to read the command line 
    arguments.'''
    parser = argparse.ArgumentParser()
    parser.add_argument('path_to_file', help = 'path to document')
    args = parser.parse_args()
    return args

if __name__ == '__main__':
    args = read_args()
    f = read_document(args.path_to_file)
    f = apply_preprocessing(f)
    print(f)