[Tutor] question about listing variables defined since session started
steve at pearwood.info
Tue May 1 02:38:48 CEST 2012
Robert Sjoblom wrote:
> On 30 April 2012 23:25, Comer Duncan <comer.duncan at gmail.com> wrote:
>> I have a newbie type question. Say I have started a python (or
>> ipython) session and have done some imports and have also defined some
>> new variables since the session started. So, I have in my current
>> namespace a bunch of things. Suppose I want to list just those
>> variable names which have been defined since the session started but
>> not include the names of the objects that who and whos will return.
>> How to do that?
> Not entirely sure, but something like this might work (untested):
> for name in dir():
> myvalue = eval(name)
> print name, "is", type(name), "and is equal to ", myvalue
Please do not use eval unless you know what you are doing, and certainly don't
encourage newbies to use it without a word about the risks.
(I really wish eval and exec were hidden inside a module that you had to
import, to discourage people from using them unnecessarily.)
My advice is:
Never use eval.
For experts only: hardly ever use eval.
eval is slow. eval is tricky to use correctly for all but the simplest uses.
eval is dangerous.
In this *specific* case, using eval is probably safe. But as a matter of best
practice, you should not use eval when there is a simpler and safer alternative:
for name in dir():
print name, "is", vars()[name]
You can replace vars() with globals() if you prefer.
Possibly better still:
from pprint import pprint
Why is eval so dangerous?
Because it executes code.
The risk with eval is not using it at the interactive interpreter. If you want
to destroy your own data, there are easier ways than using eval. But the risk
is that you write a function that uses eval, and then some day that function
gets used in your web application, and you collect text from users on the
Internet who feed your application something that causes eval to execute code.
Suddenly, your web server is under their control and they can do *anything*.
Sound far-fetched? But it happens, and very frequently. Code injection attacks
are now the *most* common security vulnerability, more common than even buffer
overflows. Whenever you hear about some website being compromised, or a virus
or trojan horse taking over people's desktops, there is a high probability
that it is because some coder used the equivalent of "eval" incorrectly.
Here is a humorous look at the issue of code injection:
and a more serious discussion:
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