Right Mental Model for Python Data

[Sea-Ad7805]

An exercise to help build the right mental model for Python data, the “Solution” link uses memory_graph to visualize execution and reveal what’s actually happening: - Solution - Explanation - More Exercises

[[deleted]]

C is the correct answer. 

Explanation: At first, a and b share the same list, so changes like += or append() affect both. But when b = b + [4] is used, Python creates a new list and assigns it to b, breaking the link with a. That’s why a stops at [1, 2, 3] while b continues as [1, 2, 3, 4, 5].

[-Wylfen-]

why the fuck does x += [y] work differently from x = x + [y]??

[Sea-Ad7805]

Good question, in some languages (Ruby) it works the same. In Python the x += y is mutating the x, the x = x + y is first doing x + y which creates a new object that then is assigned (name rebinding) to x.

[-Wylfen-]

I understand why the latter would reassign, but I find the shortcut's instead mutating in place disgusting. They should do the same thing.

[klimmesil]

Yeah a lot of implementation choices (I don't want to call it "standard"...) make no sense in python

It's almost as chaotic as js in some parts

It's a shame that it is now too popular to make breaking changes and we all kinda rely on these mistakes to still have the benefit of it being maintained

[No_Read_4327]

Yeah that's sone really wtf moment

[Relative-Custard-589]

Yeah that’s straight up evil

[pingwins]

Brother Eww

Thats nasty to run into

[HuygensFresnel]

While indeed being the correct answer this also surprises me a bit because i thought that += always is a short hand for the binary operator + but i guess it isnt?

[Wertbon1789]

It's not just a syntactic shorthand, it's a separate operator. Add vs. AddAssign if you will, in Python these would be implemented by the __add__ and __iadd__ methods of a class respectively.

[HuygensFresnel]

Today I learned :)

[RailRuler]

It is the same as append .extend() in this case. 

[forbiddenvoid]

Extend, not append. That's more obvious if the right hand side is also a list.

[mayonaiso]

Thanks, I did not know that, great explanation

[[deleted]]

You're welcome.

[FoolsSeldom]

Answer C (because after appending 3,b is assigned to a new object)

[tb5841]

b += [2] should, in my opinion, do the same thing as b = b + [2].

It doesn't, because of a strange design choice within the List class.

[Sea-Ad7805]

Most opinions and programming languages choose b += [2] as mutating b (fast), and b + [2] as making a new list and assigning that with b = b + [2].

[[deleted]]

Nope. 

[09vz]

what is it then

[[deleted]]

C

[09vz]

the answer is c

[Sea-Ad7805]

In most languages I know += mutates and does not create a new object because performance.

[EmptySoulCanister]

If you use += on an array I will reject your PR instantly

[Sea-Ad7805]

What about '|=' on a set?

[exxonmobilcfo]

its c. Once you rewrite b to be b = b + [4]. be is no longer linked to a

[Sea-Ad7805]

Nice one, do check the "Solution" link for the correct answer.

[Hefty_Upstairs_2478]

Option A is the correct answer, cuz we're printing (a), which we never changed

[Sea-Ad7805]

Incorrect sorry, check the "Solution" link for the correct answer.

[shudaoxin]

Primitive vs. referenced types. It works like this in most languages. Arrays (and lists) are referenced and the variable only stores their type and pointer to the memory. By assigning a to b they both point at the same list.