[Chicago] dict to local vars?

[Adam Forsyth]

You can't depend on updating locals inside a function or other inner scope.
The behavior is undefined. It works at module level because there is no
separate locals -- you're updating globals.


On Wed, Jan 22, 2014 at 3:35 PM, Michael Tobis <mtobis at gmail.com> wrote:

> oops I mean
>
> locals().update(mydict)
>
>
> On Wed, Jan 22, 2014 at 3:35 PM, Michael Tobis <mtobis at gmail.com> wrote:
>
>> locals.update(mydict)
>>
>> example:
>>
>> >>> mydict = {"foo" : "bazz"}
>> >>> mydict["foo"]
>> 'bazz'
>> >>> foo
>>
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> NameError: name 'foo' is not defined
>> >>> locals().update(mydict)
>> >>> foo
>> 'bazz'
>>
>>
>>
>> On Wed, Jan 22, 2014 at 2:22 PM, Adam Forsyth <adam at adamforsyth.net>wrote:
>>
>>> I'll assume you know this is bad and that there are existing libraries
>>> to solve this problem.
>>>
>>>
>>> There isn't a good way to modify locals. Instead, I think you want to
>>> use the optional arguments to eval that allow you to provide dictionaries
>>> to use in place of the real locals and globals.
>>>
>>> import math
>>> def evaleq(eq,vals):
>>>     return 'expression '+eq+' => '+str(eval(eq,globals(),vals))
>>> myeq = 'a*math.log(b)'
>>> myvals = {'a':3, 'b':2}
>>> print evaleq(myeq,myvals)
>>> print evaleq('2*Abc',{'Abc':21.})
>>> print evaleq('2*Abc+a',{'Abc':21.})
>>>
>>> expression a*math.log(b) => 2.07944154168
>>> expression 2*Abc => 42.0
>>>
>>> Traceback (most recent call last):
>>>   File "eval.py", line 8, in <module>
>>>
>>>     print evaleq('2*Abc+a',{'Abc':21.})
>>>   File "eval.py", line 3, in evaleq
>>>     return 'expression '+eq+' => '+str(eval(eq,globals(),vals))
>>>
>>>   File "<string>", line 1, in <module>
>>> NameError: name 'a' is not defined
>>>
>>>
>>>
>>>
>>> On Wed, Jan 22, 2014 at 2:13 PM, Toby, Brian H. <toby at anl.gov> wrote:
>>>
>>>>  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
>>>>
>>>>  ********************************************************************
>>>> 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
>>>>  ********************************************************************
>>>> "We will restore science to its rightful place, and wield technology's
>>>> wonders... We will harness the sun and the winds and the soil to fuel our
>>>> cars and run our factories...  All this we can do. All this we will do."
>>>>
>>>>
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>>
>
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