overloading arbitrary operator in python

Question

Is it possible to overload arbitrary operators in Python? Or is one restricted to the list of operators which have associated magic methods as listed here: https://www.python-course.eu/python3_magic_methods.php ?

I'm asking because I noticed that Numpy uses the @ operator to perform matrix multiplication e.g. C=A@B where A,B are Numpy arrays, and I was wondering how they did it.

Edit: The @ operator is not in the list I linked to.

Could someone point me to the Numpy source where this is done?

Thanks, I knew I should have looked in the docs but didn't know where to start. — NNN
– NNN, Commented Jul 10, 2020 at 9:39

Ken Kinder · Accepted Answer · 2020-07-10 10:08:05Z

3

In Python, you cannot create new operators, no. By defining those "magic" functions, you can affect what happens when objects of your own definition are operated upon using the standard operators.

However, the list you linked to is not complete. In Python 3.5, they added special methods for the @ operator. Here's the rather terse listing in the Python operator module docs and here are the docs on operator overloading.

operator.matmul(a, b)

operator.__matmul__(a, b)

Return a @ b.

New in version 3.5.

I hadn't seen that operator personally, so I did a little more research. It's intended specifically for matrix multiplication. But, I was able to use it for other purposes, though I would argue against doing so as a matter of style:

In [1]: class RichGuyEmailAddress(str): 
   ...:     def __matmul__(self, domain_name): 
   ...:         return f'{self}@{domain_name}' 
   ...:                                                                                                                                                                                       

In [2]: my_email = RichGuyEmailAddress('billg') @ 'microsoft.com'                                                                                                                              

In [3]: print(my_email)                                                                                                                                                                       
[email protected]

So, no, you can't overload any random character, but you can overload the @ operator.

edited Jul 10, 2020 at 10:08

answered Jul 10, 2020 at 9:56

Ken Kinder

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3 Comments

user2357112 Over a year ago

Your doc quote is for a different thing - it's for a pair of functions that invoke the operator, not for the hook(s) to overload the operator. The docs for the __matmul__ hook are here, as well as __rmatmul__ and __imatmul__.

Ken Kinder Over a year ago

Hmmm, those docs are also pretty terse. I found the operator docs more useful, but I updated the answer to link to them both.

NNN Over a year ago

Thanks for that example. In addition to clarifying my question, it helped me understand inheritance and the str & repr functions better.

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