Text19. Дан текстовый файл. Заменить в нем все прописные русские буквы на строчные, а все строчные — на прописные.

Решение на Python 3:

import codecs

file1 = "text19.txt"
print("Read from:",file1)
file2 = "text19_2.txt"
print("Write to:",file2)

ru_Lower = u"абвгдежзийклмнопрстуфхцчшщъыьэюя"
ru_Upper = u"АБВГДЕЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯ"

try:
    with codecs.open(file1, 'r', 'utf-8') as infile, \
         codecs.open(file2, 'w', 'utf-8') as outfile:
        for line in infile:
            s_new = ""
            for c in line:
                if c in ru_Lower:
                    c = c.upper()
                elif c in ru_Upper:
                    c = c.lower()
                s_new += c
            outfile.write(s_new)
            print(s_new)

except IOError:
    print('Open error: ',file1)