read from source file one line at a time and write to target file one line at time.

def copy(source, target):
    source_f = open(source)
    target_f = open(target, 'w')
  
    while True:
        line = source_f.readline()
        if len(line) == 0:
            break
        target_f.write(line)
        
    source_f.close()
    target_f.close()

copy('source.txt', 'target.txt')