Default arguments are only evaluated once: At the time the function is created. If you provide a mutable default keyword argument and then change it in the function, the next time the function is called without that keyword, the default will point to the same address as in the first call; but the argument will have already changed, so the default in the first call and the default in the second call are different.
Solution: Only provide non-mutable default arguments. See the example.
A source of errors can be naming a module the same as another module that is imported, in this example the module is named math.py but also imports math from the standard Python library; and function calls using methods from the math module will fail, as Python will look for those in the math.py file.
Solution: Name your module file different than the modules that you are importing. See the example.
Iterators and generators can be exhausted, meaning you can only use them once.
Solution: If you create an iterator or a generator and you need it more than once you need to save it first. As in the example provided, the iterator is created using zip, and can be saved in a list.
Assigning a variable within a function shadows any assignment that may have happened in an outer scope.
Solution: Pass the variable as an argument into the inner scope or use the return value of a new assignment. See the example.
Python uses late binding, resulting that in closures variables are only looked up once the inner function is called.
Solution: Make sure the referenced variables are either passed to the inner function or are set correctly in the surrounding scope. See the example.
Task 1: Experiment with the different modules in this section and reproduce the error and the solution.