TypeError after overriding the __add__ method

by

You need to define __radd__ as well to get this to work.

__radd__ is reverse add. When Python tries to evaluate x + y it first attempts to call x.__add__(y). If this fails then it falls back to y.__radd__(x).

This allows you to override addition by only touching one class. Consider for example how Python would have to evaluate 0 + x. A call to 0.__add__(x) is attempted but int knows nothing about your class. You can’t very well change the __add__ method in int, hence the need for __radd__. I suppose it is a form of dependency inversion.

As Steven pointed out, sum operates in place, but starts from 0. So the very first addition is the only one that would need to use __radd__. As a nice exercise you could check that this was the case!

More Related Contents:

  1. How to print instances of a class using print()?
  2. How can I remove the ANSI escape sequences from a string in python
  3. What are the ‘levels’, ‘keys’, and names arguments for in Pandas’ concat function?
  4. Count Vowels in String Python
  5. How to identify on which OS Python is running on?
  6. Plot a horizontal line on a given plot
  7. Using a dictionary to select function to execute
  8. Src layout to dispense .src prefix in imports? Activate venv in PyCharm terminal for development installs
  9. python matplotlib framework under macosx?
  10. How do you call Python code from C code?
  11. The main() function appears to not work [duplicate]
  12. What is the best way to copy a list? [duplicate]
  13. How does Django Know the Order to Render Form Fields?
  14. Django server killed frequently
  15. Click a Button in Scrapy
  16. check if a key exists in a bucket in s3 using boto3
  17. Difference between “grid” and “pack” geometry managers
  18. PyCharm: Unresolved reference with Scapy
  19. What’s the algorithm of ‘set.intersection()’ in python?
  20. In Python how should I test if a variable is None, True or False
  21. Create Django model or update if exists
  22. Using Matplotlib 3D axes, how to drag two axes at once
  23. Python Multiprocess Pool. How to exit the script when one of the worker process determines no more work needs to be done?
  24. How to call a script from another script?
  25. Plotting time in with Matplotlib
  26. How to get/set a pandas index column title or name?
  27. Is there a way to circumvent Python list.append() becoming progressively slower in a loop as the list grows?
  28. Vim autocomplete for Python
  29. How can I create the minimum size executable with pyinstaller?
  30. Python: Making a beep noise

Leave a Comment