Sometimes, before optimizing your code, you may end up to have very slow running code in list comprehension statement. Because you have not yet tested the code fully, it easy to loose patience and just interrupt the code.
If you are making function, you can insert print statements for verification. But how to put print in list comprehension. If you are using function to produce the result you can insert a print there or you could use some debugging tools. I however often keep it simple and I came up with this pass through trivial function which basically just prints it's arguments and passes them through as result. You can insert this function for part which expects a sequence to iterate or produces a sublist or tuple.