"eval" function

Gerrit Holl gerrit at nl.linux.org
Sun Jun 22 22:20:43 CEST 2003


Hi Eirik,

Eirik wrote:
> How can I do what I would do like this:
> 
> set a 12
> set b 144
> eval a*a == b
> 
> in Tcl?

I'm not sure what this means in Tcl, but you can check for
equality using the '==' operator. You don't need to use
'eval' for that. You can simply do "a*a==b":

  3 >>> a=12
  4 >>> b=144
  5 >>> a*a==b
True

There is an 'eval' function in Python, but with a different
behaviour:

  6 >>> help(eval)
Help on built-in function eval:

eval(...)
    eval(source[, globals[, locals]]) -> value

    Evaluate the source in the context of globals and locals.
    The source may be a string representing a Python expression
    or a code object as returned by compile().
    The globals and locals are dictionaries, defaulting to the current
    globals and locals.  If only globals is given, locals defaults to it.

  8 >>> mystr = 'a*a==b'
  9 >>> eval(mystr)
True

Or:
 10 >>> mystr = 'open("/dev/zero", "r")'
 11 >>> eval(mystr).read(50)
'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

See also:

http://www.python.org/dev/doc/devel/lib/staticmethod.html#l2h-19

yours,
Gerrit.

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