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POPython Introspection: Defining dynamic class methods during runtime
 

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    <p>I'm trying to create a unit test, that checks that every function in <code>mymodule</code> has its own <code>TestCase</code> instance.<br> To reduce boiler-plate code and manual effort I wanted to use introspection/reflection to dynamically add <code>lambda</code> functions as class methods to the initially empty class <code>Test_TestCases</code>.<br> The following code kind of works - it does indeed add lambdas as class methods and <code>unittest.main()</code> finds and correctly calls them.</p> <pre><code>import unittest from unittest import TestCase import mymodule class Test_TestCases(TestCase): """Class whose test_* methods will be created during runtime.""" pass ################################################################################ if __name__ == "__main__": for item in dir(mymodule): attr = getattr(pws, item) if callable(attr): testname = "Test_%s" % item setattr(Test_TestCases, "test_%s_is_tested" % item, lambda self: self.assertTrue(testname in globals()) and issubclass(getattr(globals(), testname), TestCase)) unittest.main() </code></pre> <p>The problem is, that all tests succeed even though I do have untested functions in my unit test module.<br> After a little trying around I figured out that the variable <code>testname</code> has the same value every time the <code>lambda</code> is called.<br> I could minimize the problem to this piece of code for reproducability:</p> <pre><code>lambdas = [] for i in range(5): lambdas.append(lambda: str(i)) print ", ".join(f() for f in lambdas) </code></pre> <p>I'd expect this output:</p> <pre><code>0, 1, 2, 3, 4 </code></pre> <p>but instead I get:</p> <pre><code>4, 4, 4, 4, 4 </code></pre> <p>It seems that the lambdas are initialized lazily.<br> Could anyone please explain this behaviour or give me a hint on how I would accomplish my goal properly?</p> <p>Thanks in advance</p>
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    1. COpossible duplicate of [Python Lambdas and Variable Bindings](http://stackoverflow.com/questions/2731111/python-lambdas-and-variable-bindings)
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      1. USinterjay
    2. COpossible duplicate of [Local variables in Python nested functions](http://stackoverflow.com/questions/12423614/local-variables-in-python-nested-functions)
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      1. USMartijn Pieters
    3. COThat goes in the same direction and solves my reproducer, but my initial problem is something else. Providing a default value wouldn't and couldn't fix my problem, because it's not the argument passed to the lambda function that always stays the same, but it is `testname`, a simple variable that is not passed as an argument
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