How to Extract Name of Return Variable from Python Function

python

I have a python function x that returns some module
x():
return y

I would like to parse the file x is located in and learn that "x returns this thing called 'y'". I have no idea how.

I'm not trying to get the type of the return type, just what the programmer calls exactly what is being returned.

Best Answer

Use the ast module

import ast                                                                                                  
                                                                                                            
def extract_return(file, fname):                                                                            
    for x in ast.walk(ast.parse(open(file).read())):                                                        
        if not(isinstance(x, ast.FunctionDef)):                                                             
            continue                                                                                        
        if not(x.name == fname):                                                                            
            continue                                                                                        
        for b in x.body:                                                                                    
            if isinstance(b, ast.Return):                                                                   
                if isinstance(b.value, ast.Name):                                                           
                    yield b.value.id

This defines a function which takes some python source's filename and a function name fname (your "x") and yields each identifier returned by function fname. Here, I assume that you are only interested in return statements that occur at the first level of function x and consequently, I don't visit subexpressions. Also, I'am using yield because in more general cases this could work like a filter which could be composed with other ones. In more complex cases, it can be preferable to subclass ast.NodeVisitor.

This is equivalent in size with the regex approach but far more reliable.

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