FAQ: How do I calculate what quoted strings and numbers mean?

Fredrik Lundh fredrik at pythonware.com
Sun Nov 12 13:57:08 CET 2006

p.lavarre at ieee.org wrote:

> But those objections miss the point.  Having had those troubles
> explained to me now, I'm still leaving my code unchanged - it still
> does what I mean.  That is,
> eval(source, {'builtins': {}}) works enough like an evaluator of
> literals to let you duck the work of writing that evaluator until you
> need it.  Yagni.

until you forget about it, and someone uses the security hole to take 
down your company's site, or steal all the customer data from your 
database, or some such thing.

I think the PHP "I don't really get bound parameters; let's explain how 
to build SQL statements by hand first" shows that you should avoid doing 
things in stupid ways in documentation that's likely to be read by 
inexperienced programmers...

 > eval(source, {'builtins': {}}) works enough like an evaluator of
 > literals to l

eval(source, {'builtins': {}}) doesn't prevent you from using built-ins, 
though.  it's spelled __builtins__, not builtins:

 >>> eval("len('10')", {"builtins": {}})
 >>> eval("len('10')", {"__builtins__": {}}
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "<string>", line 1, in <module>
NameError: name 'len' is not defined

 > That's useful, and likely an FAQ.

A FAQ that discusses good ways to handle Python-like literals and 
expressions would definitely be a useful addition to the FAQ.  if nobody 
else does anything about it, I'll get there sooner or later.


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