import urllib.request # import the code to access remote files. 

File_URL = 'http://www.uniprot.org/uniprot/P12931.fasta'
Sequence_file = urllib.request.urlopen(File_URL) # Open remote file for reading

while True:                             # Repeat ... 
    Line = Sequence_file.readline()     # Read the next line using readline()
    if not Line:                        # If at the end of the file,
        break                           # stop reading/printing,
    print(Line.decode("utf-8"), end='') # print Line, go round for more.

Sequence_file.close() # Close file tidily.