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POEval not only with dictionaries
 

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    <p>normaly I can use eval like that:</p> <pre><code>new_dict={'color':'green'} eval("color=='green'",new_dict) </code></pre> <p>in this case it would return true and therefore I would know that the color in new_dict is actually green. Now I found some code where somebody wanted to use eval but with a more general object instead of a dict. In the following code is basicly what this person did:</p> <pre><code>class myStuff(object): def __init__(self): self.color = "green" class Dummy(dict): def __init__(self, node): dict.__init__(self) self.node = node def __getitem__(self, key): return(getattr(self.node, key)) node=myStuff() new_dict = Dummy(node) print eval("color=='green'",new_dict) </code></pre> <p>I wonder now - how did the developer of the above code know that eval is using the method <code>__getitem__</code> on new_dict for color? I found the documentation and the python help function, but I couln't find a step-by-step documentation for a method (or the actual code), so that I would never ever came up on the idea to do just that what the above code does. Or is it bad to use the above method, because no one really knows how the eval method has been implemented and the code could therefore come up with some strange errors in future?</p> <p>EDIT: Here is why eval is used in the program: Imagine you got like 20 objects of myStuff in a list mylist and you want to filter them by color yellow, then one can simply call [ n for n in mylist if eval(query, Dummy(n) ] with `query="color=='yellow'". I am not an expert but I just want to know if this method can lead to problems.</p>
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    1. COWhy are you using `eval` this way? What's wrong with just `d['color'] == 'green'`?
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      1. USBlender
    2. COIt is a perfectly reasonable question to wonder how *eval* works (i.e. evaluations are performed in namespace that can be an arbitrary mapping using \_\_getitem\_\_). The technique is versatile and makes it possible to easily implement chained calculations such as those used in spreadsheets (something that can't be easily done with a regular dictionary).
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      1. USRaymond Hettinger
    3. CO@RaymondHettinger: Yes, it's reasonable to wonder how `eval` works, and it actually would be pretty cool if someone wrote the kind of tutorial the OP is searching for. But that doesn't change the fact that, like 99% of the people who have a problem with `eval`, the OP shouldn't have been using it here. That's why it's always worth asking "Why do you want to use `eval` here?" first. The worst-case scenario is that they have a good response (which can include "because I want to understand `eval`") which makes their question more useful for future readers, right?
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