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POPython: Using strings as an object argument?
 

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  1. POPython: Using strings as an object argument?
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    <p>Essentially my problem is as follows...</p> <p>In Python, I have a function that will return an output string in the following form:</p> <pre><code>'union(symbol(a), symbol(b))' </code></pre> <p>The function forms found within this string actually exist in an object class called RegExTree. Further this class contains a function to construct a tree data structure using the function "construct()" as shown below:</p> <pre><code>tree = RegExTree() tree.construct(union(symbol(a), symbol(b)) </code></pre> <p>The above two lines of code would work normally, constructing a tree based on parsing the arguments within the construct function. I want to pass in a string in a similar fashion, perhaps this line of code illustrates what I want:</p> <pre><code>tree = RegExTree() expression = 'union(' + 'symbol(' + 'a' + ')' + ', ' + 'symbol(' + 'b' + ')' + ')' tree.construct(expression) </code></pre> <p>Right now the way I have the code written as above it yields an error (in the Linux terminal) as follows: </p> <pre><code>$ Attribute Error: 'str' object has no attribute 'value' </code></pre> <p>Can you coerce Python to interpret the string as a valid argument/line of code. In essence, not as string, but as object constructors.</p> <p>Is there a way to get Python to interpret a string as rather something that would have been parsed/compiled into objects and have it construct the objects from the string as if it were a line of code meant to describe the same end goal? Is what I'm asking for some kind of back-door type conversion? Or is what I'm asking not possible in programming languages, specifically Python?</p> <p>EDIT: Using Michael's solution posited below that involves "eval()", there is one way to hack this into form:</p> <pre><code>tree = RegExTree() a = 'a' b = 'b' expression = 'union(' + 'symbol(' + a + ')' + ', ' + 'symbol(' + b + ')' + ')' tree.construct(eval(expression)) </code></pre> <p>Is there a better way of doing this? Or is it just that the nature of my output as string representing functions is just not a good idea? [Thanks martineau for the correction for my solution edit!]</p>
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    1. COYou can use the `eval` function to evaluate an expression in a string, but yes, it is a bit of a back door. There is probably a better way. What are you actually trying to accomplish?
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    2. COWhy is your function returning strings? You should be passing around functions instead. Evaluating strings is probably not the way to do it
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      1. USJoel Cornett
    3. COThe algorithm that created the string output is recursive in nature and it's purpose is to generate a completely randomized "regular expression" in the format I show above [i.e. union(x,y) represents x U y). Essentially, for every argument spot in a function, that could potentially be another function call. Take for example: union(x, y) could potentially be x = union(a,b) and y = star(c), yielding union(union(a,b), star(c))
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