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c/python · by bterwijn@programming.dev · 297d

Python Mutability

An exercise to help build the right mental model for Python data. The “Solution” link uses memory_graph to visualize execution and reveals what’s actually happening:

21 points · 6 comments · view on lemmy.world

6 Comments

joyjoy@lemmy.zip · 3 pts · 297d (3 replies)

It's always fun to guess whether python will call __add__ or __iadd__.

bterwijn@programming.dev · 3 pts · 297d (2 replies)

__add__ is called with + and __iadd__ is called with +=, and there is a difference: https://www.reddit.com/r/PythonLearning/comments/1nw08wu/right_mental_model_for_python_data/

joyjoy@lemmy.zip · 3 pts · 297d (1 reply)

Except if __iadd__ doesn't exist on the type, then __add__ is called. The variable is always reassigned. Quick example:

x = (0, [1, 2])
try:
    x[1] += [3]  # calls list.__iadd__, then reassigns x[1]
except TypeError as e:
    print(e)

x += (4,)  # calls tuple.__add__, then reassigns x

print(x)

What is printed?

bterwijn@programming.dev · 1 pts · 297d

Thanks. I didn't know: https://memory-graph.com/#code=x+%3D+%280%2C+%5B1%2C+2%5D%29%0Atry%3A%0A++++x%5B1%5D+%2B%3D+%5B3%5D++%23+calls+list.__iadd__%2C+then+reassigns+x%5B1%5D%0Aexcept+TypeError+as+e%3A%0A++++print%28e%29%0A%0Ax+%2B%3D+%284%2C%29++%23+calls+tuple.__add__%2C+then+reassigns+x%0A%0Aprint%28x%29%0A%0A%0Aclass+MyClass%3A%0A++++%0A++++def+__add__%28self%2C+other%29%3A%0A++++++++print%28%27__add__%27%29%0A++++++++%0A++++%23def+__iadd__%28self%2C+other%29%3A%0A++++%23++++print%28%27__iadd__%27%29%0A%0Aa+%3D+MyClass%28%29%0Ab+%3D+MyClass%28%29%0Aa+%2B%3D+b++%23+calls+__add__+if+__iadd_+doesn%27t+exist%0A&play=

Valmond@lemmy.world · 1 pts · 297d (1 reply)

I so wonder what a will result in.

bterwijn@programming.dev · 1 pts · 297d

Yes a you get for free to set the tone, the others are more interesting.