[Chicago] dict to local vars?

[Toby, Brian H.]

Can any Python gurus help me find a cleaner way to attack a problem? I am of the opinion that anything can be done in Python (though not always cleanly or wisely), but so far how to define variables in the local namespace from a dict has so far eluded me.

Let me set the problem in context, in case there is a more pythonic way to approach this task: I am working on a set of routines that will evaluate an arbitrary expression. At least as I see myself framing the problem right now, I will end up with a str with the equation and a dict containing the values for each variable in the expression. I will not know what variable names to expect in the expression, so I must deal with them as strings at run-time.

I want to evaluate the expression reasonably quickly (as I will later need numerical derivatives). The code fragment below does almost exactly what I want, except the values end up in the module's global namespace, but I would prefer they were in the local namespace or that of an object --- so that the 3rd call to evaleq would produce an exception.

import numpy as np
def evaleq(eq,vals):
    for k,v in vals.iteritems(): globals()[k] = v
    return 'expression '+eq+' => '+str(eval(eq))
myeq = 'a*np.exp(b)'
myvals = {'a':1, 'b':0}
print evaleq(myeq,myvals)
print evaleq('2*Abc',{'Abc':21.})
print evaleq('2*Abc+a',{'Abc':21.})

Note that locals() is not supposed to be used in the way that I use globals() above, at least as far as I understand, so that is not an option.

Any suggestions?

Brian

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Brian H. Toby, Ph.D.                            office: 630-252-5488
Senior Physicist/Section Head for Scientific Software
Advanced Photon Source
9700 S. Cass Ave, Bldg. 401/B4192
Argonne National Laboratory
Argonne, IL 60439-4856         e-mail: brian dot toby at anl dot gov
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