Re: Using eval with substitutions
- From: Peter Otten <__peter__@xxxxxx>
- Date: Thu, 31 Aug 2006 14:54:50 +0200
abhishek@xxxxxxxxxxxxxxxx wrote:
Fredrik Lundh wrote:
(I'm afraid I don't really understand the point of your examples; what
is it you're really trying to do here ?)
A function is passed a bunch of string expressions like,
x = "a+b"
y= "x*a"
z= "x+y"
where a and b are assumed to have been assigned values in the local
namespace. Now I have to evaluate another string such as,
"z+y+x"
So as you say, I could do:
x=eval(x), y=eval(y), z=eval(z) and finally eval("z+y+x") but the
problem is that the initial strings are in no particular order, so I
don't know the sequence in which to perform the first 3 evaluations. I
was wondering if there was a simple way to 'pattern-match' so that the
required substitutions like z->x+y->x+x*a->(a+b)+(a+b)*a could be done
automatically.
Here is something to start with:
class EvalDict(object):
def __init__(self, namespace):
self.namespace = namespace
def __getitem__(self, key):
value = self.namespace[key]
if isinstance(value, str):
self.namespace[key] = value = eval(value, {}, self)
return value
def smart_eval(expr, namespace):
return eval(expr, {}, EvalDict(namespace))
def f():
a = 2
b = 3
x = "a+b"
y = "x*a"
z = "x+y"
return smart_eval("x + y + z", locals())
if __name__ == "__main__":
print f()
Beware of infinite recursion:
# RuntimeError: maximum recursion depth exceeded
smart_eval("a + b", dict(a="b", b="a"))
Peter
.
- References:
- Using eval with substitutions
- From: abhishek
- Re: Using eval with substitutions
- From: Fredrik Lundh
- Re: Using eval with substitutions
- From: abhishek
- Using eval with substitutions
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