raise KeyError(key) from err

This exception is raised when you try to access a key in a dictionary that does not exist. The "from err" part of the exception message indicates that the KeyError was raised due to another error that occurred earlier in the program.

For example, consider the following code snippet:

my_dict = {'a': 1, 'b': 2, 'c': 3}
print(my_dict['d'])

The output will be:

KeyError: 'd'

This is because 'd' is not a key in the dictionary `my_dict`. The `raise KeyError(key) from err` part of the exception message indicates that the KeyError was raised due to another error that occurred earlier in the program.

In general, it is a good practice to catch and handle exceptions like KeyError to prevent your program from crashing unexpectedly.