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Per the exec docs:

Be aware that the return and yield statements may not be used outside of function definitions even within the context of code passed to the exec statement.

So exec is explicitly off-limits. And that wording is global, not specific to exec; on checking, while eval using code compile-ed in 'single' mode has the same error; you can't dynamically insert return statements like this.

If you absolutely must allow executing arbitrary code, I strongly recommend limiting it to expressions, not statements, and implicitly returning the result of said expressions. So instead of storing u"return response.xpath('/text()')", you'd store u"response.xpath('/text()')", and your code that performs dynamic invocation would change to: