1. Types of *operators*:
    1. Mapping Operators to Functions. This is the operator, which can be assigned to a variable and taken as a function. e.g. `ans = add(x, y)`
    2. In-place Operators. This is the operator, which can add to an function for edition. e.g. `str.strip()` <div id="call-operators">[operator  in the `operator` module](https://docs.python.org/3/library/operator.html#mapping-operators-to-functions)</div>  This work is concluded when doing [this](/cs61a/codes/letc01/operator_expressions.py) exercise.
2. Arithmetic
3. Bit
4. Logical
5. 
6. Identity
7. Index

in-place:
- https://en.wikipedia.org/wiki/In-place_algorithm - https://www.geeksforgeeks.org/inplace-vs-standard-operators-python/ - https://stackoverflow.com/questions/5317817/python-in-place-functions -