On Thu, 3 Nov 2005, Pujo Aji wrote:
> I have a dynamic functions which created by some algorithms during
> runtime. These functions are in string type. When I want to use it, I
> can use eval command. But can someone give me more suggestion about how
> to handle this problem, I want to avoid eval.
Hi Pujo,
To do this safely, many programmers will take a parsing + evaluation
approach.
A parser can take a string, such as:
"x + sin(x)"
and turn it into a data structure that can be evaluated safely.
As a concrete example of what it means to use data structures to represent
these dynamic functions:
##############################################
class Add:
"""Represents the addition operator."""
def __init__(self, lhs, rhs):
self.lhs, self.rhs = lhs, rhs
def evaluate(self, environment):
lval = self.lhs.evaluate(environment)
rval = self.rhs.evaluate(environment)
return lval + rval
class Variable:
"""Represents a variable name."""
def __init__(self, id):
self.id = id
def evaluate(self, environment):
return environment[self.id]
##############################################
Let's say that we have a string like:
"x + y"
We can capture the essence of the meaning there by building the data
structure:
Add(Variable('x'), Variable('y'))
The job of doing the string-to-data-structure thing is pretty automated if
we use parser tools like pyparsing. I won't go into parsing at the
moment; I'll treat that as a separate topic, but if you want to explore
parsers, see:
http://pyparsing.sourceforge.net/
Once we have a dynamic function in a data structure format like this, then
things aren't so bad. We can "evaluate" that data structure at multiple
points:
######
>>> equation = Add(Variable('x'), Variable('y'))
>>> equation
<__main__.Add instance at 0x403a7e2c>
>>> equation.evaluate(dict(x=3, y=4))
7
>>> equation.evaluate(dict(x=19, y=42))
61
>>>
>>> another = Add(Variable('x'), Add(Variable('y'), Variable('z')))
>>> another.evaluate(dict(x=3, y=4, z=5))
12
######
Does this make sense? We can talk about this in more detail if you'd
like.
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