Python self-evaluating strings
Boris Borcic
bborcic at gmail.com
Sun Jan 27 14:00:18 EST 2008
Now there's always that style :
>>> print x
Traceback (most recent call last):
File "<pyshell#15>", line 1, in <module>
eval(x)
File "<string>", line 2
Traceback (most recent call last):
^
SyntaxError: invalid syntax
>>> eval(x)
Traceback (most recent call last):
File "<pyshell#15>", line 1, in <module>
eval(x)
File "<string>", line 2
Traceback (most recent call last):
^
SyntaxError: invalid syntax
>>>
Arnaud Delobelle wrote:
> Hi all,
>
> An earlier post today got me thinking about "quines" (programs that
> output themselves) in Python. I tried to find some on the web but
> didn't find many ([1]). In particular I didn't find any that
> corresponded to my instinctive (LISP-induced, probably) criterion:
>
> def self_evaluating(s):
> "Return True if string s evaluates to itself"
> return s == eval(s)
>
> Here is the result of my efforts so far:
>
> 1. Starting from the classic idea (lambda x:x%x)('lambda x:x%x') I got
> the following
> v=(lambda x:x%('"''""'+x+'"''""'))("""(lambda
> x:x%%('"''""'+x+'"''""'))(%s)""")
>
> 2. (Given that if s is a nonempty string, s*2 is a longer string).
> Starting from the idea "%s %s" % (("%s %s",)*2) I got the following
> u="\"%s\" %% ((r\"%s\",)*2)" % ((r"\"%s\" %% ((r\"%s\",)*2)",)*2)
>
> Most of my problems in creating these 2 was with finding a suitable way
> of quoting strings that propagates well. Both u and v are one-liners.
> I'm hoping for no funky line wrapping here.
>
> Note: I'm not quoting the string as it makes no difference since they
> evaluate to themselves:)
>
> I'd like to know if anyone on the list has indulged in this
> time-bending/mind-wasting activity before. If so, it would be nice to
> create a list of such expressions.
>
> Quining's-better-than-ironing'ly yours
>
> --Arnaud
>
> [1] http://www.nyx.net/~gthompso/self_pyth.txt
>
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