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<div>Can any Python gurus help me find a cleaner way to attack a problem? I am of the opinion that <i>anything</i> 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. </div>
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<div>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. </div>
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<div>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. </div>
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<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; ">import numpy as np</span></font></div>
<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; ">def evaleq(eq,vals):</span></font></div>
<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; "> for k,v in vals.iteritems(): globals()[k] = v</span></font></div>
<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; "> return 'expression '+eq+' => '+str(eval(eq))</span></font></div>
<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; ">myeq = 'a*np.exp(b)'</span></font></div>
<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; ">myvals = {'a':1, 'b':0}</span></font></div>
<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; ">print evaleq(myeq,myvals)</span></font></div>
<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; ">print evaleq('2*Abc',{'Abc':21.})</span></font></div>
<div><font class="Apple-style-span" face="Courier"><span class="Apple-style-span" style="font-size: 11px; ">print evaleq('2*Abc+a',{'Abc':21.})</span></font></div>
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<div>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. </div>
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<div>Any suggestions? </div>
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<div>Brian</div>
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