# Copyright (c) 2011, Thomas Paviot (tpaviot@gmail.com)

# All rights reserved.

# This file is part of the StepClassLibrary (SCL).

#

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# modification, are permitted provided that the following conditions are met:

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# this list of conditions and the following disclaimer.

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# this list of conditions and the following disclaimer in the documentation

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# be used to endorse or promote products derived from this software without

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# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

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from math import *

PI = pi #in EXPRESS, PI is uppercase

def EvalDerivedAttribute(class_instance, str_expr):

'''

This functions tries to evaluate EXPRESS math expr.

For instance in the following example:

==

ENTITY circle;

centre : point;

radius : REAL;

axis : vector;

DERIVE

area : REAL := PI*radius**2;

perimeter : REAL := 2.0*PI*radius;

END_ENTITY;

==

The following string should be passed to EvalExpr:

'PI*radius**2'

'''

# first, try to find class paramters

# in the previous example, we should replace radius with self.radius before eval'ing expr

props = []

for prop in dir(class_instance):

if not prop.startswith("_"): #means prop is an item that might be considered

props.append(prop)

for item in props:

str_expr = str_expr.replace(item,"class_instance.%s"%item)

# after that step, the expression should be:

# PI*class_instance.radius*class_instance.radius

# this can be evaluated with the eval function

# CAREFUL: eval is known to be unsafe. This should be changed in the future

# (using a parser, or simpy for instance)

try:

return eval(str_expr.lower())

except:

print 'Error evaluating expression'

return None