call site 0 for code.Source.__len__
code/testing/test_source.py - line 67
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   def test_source_strips(): 
       source = Source("")
       assert source == Source()
       assert str(source) == ''
->     assert source.strip() == source 
code/source.py - line 63
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   def strip(self):
       """ return new source object with trailing
               and leading blank lines removed.
           """
->     start, end = 0, len(self)
       while start < end and not self.lines[start].strip():
           start += 1
       while end > start and not self.lines[end-1].strip():
           end -= 1
       source = Source()
       source.lines[:] = self.lines[start:end]
       return source