What is the source code of the "this" module doing?

Question

If you open a Python interpreter, and type "import this", as you know, it prints:

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In the python source(Lib/this.py) this text is generated by a curious piece of code:

s = """Gur Mra bs Clguba, ol Gvz Crgref

Ornhgvshy vf orggre guna htyl.
Rkcyvpvg vf orggre guna vzcyvpvg.
Fvzcyr vf orggre guna pbzcyrk.
Pbzcyrk vf orggre guna pbzcyvpngrq.
Syng vf orggre guna arfgrq.
Fcnefr vf orggre guna qrafr.
Ernqnovyvgl pbhagf.
Fcrpvny pnfrf nera'g fcrpvny rabhtu gb oernx gur ehyrf.
Nygubhtu cenpgvpnyvgl orngf chevgl.
Reebef fubhyq arire cnff fvyragyl.
Hayrff rkcyvpvgyl fvyraprq.
Va gur snpr bs nzovthvgl, ershfr gur grzcgngvba gb thrff.
Gurer fubhyq or bar-- naq cersrenoyl bayl bar --boivbhf jnl gb qb vg.
Nygubhtu gung jnl znl abg or boivbhf ng svefg hayrff lbh'er Qhgpu.
Abj vf orggre guna arire.
Nygubhtu arire vf bsgra orggre guna *evtug* abj.
Vs gur vzcyrzragngvba vf uneq gb rkcynva, vg'f n onq vqrn.
Vs gur vzcyrzragngvba vf rnfl gb rkcynva, vg znl or n tbbq vqrn.
Anzrfcnprf ner bar ubaxvat terng vqrn -- yrg'f qb zber bs gubfr!"""

d = {}
for c in (65, 97):
    for i in range(26):
        d[chr(i+c)] = chr((i+13) % 26 + c)

print "".join([d.get(c, c) for c in s])

Answer 1

This is called rot13 encoding:

d = {}
for c in (65, 97):
    for i in range(26):
        d[chr(i+c)] = chr((i+13) % 26 + c)

Builds the translation table, for both uppercase (this is what 65 is for) and lowercase (this is what 97 is for) chars.

print "".join([d.get(c, c) for c in s])

Prints the translated string.

Answer 2

If you want to make the ROT13 substitution by hand - or in your head - you can check that because 13*2 = 26 (the number of the letters of the English alphabet), it's essentially an interchange:

a <-> n
b <-> o
c <-> p
...
m <-> z

A <-> N
B <-> O
C <-> P
...
M <-> Z 

Vs lbh cenpgvfr ybat rabhtu, lbh'yy riraghnyyl znfgre gur Mra bs EBG-13 nytbevguz naq ernq guvf Xyvatba ybbxvat grkgf jvgubhg pbzchgre uryc.

Answer 3

It uses ROT13 encoding. This is used because it's a joke.

You can also use Python functions to decode string.

Python 2 only:

import this
print(this.s.decode('rot13'))

Python 2 & 3:

import codecs
print(codecs.decode(this.s, 'rot-13'))

Answer 4

It's a substitution cipher, rot13.

Answer 5

It's a substitution cipher (as mentioned in previous answers). Historically speaking, it's the Caesar cipher.

https://www.google.de/search?q=caesar+cipher&cad=h