bterwijn

bterwijn

Practical Programming, Third Edition: immutable values are not necessarily unhashable (209)

On page 209 it says: “mutable values are unhashable”
However, mutability and hash-ability are separate properties. A type is hash-able when its hash function is defined. A counter example that shows a mutable list that gets hashed and added as key to a dict:

class mylist(list): # mylist (mutable) is a list with a __hash__ function

    def __hash__(self):
        hash = 0
        for i in self:
            hash ^= i
        return hash

ml= mylist()
ml.append(1)
ml.append(2)
ml.append(3)
print( ml ) # [1, 2, 3]

mydict={ ml:100, mylist([4,5,6]):200}
print( mydict ) # {[1, 2, 3]: 100, [4, 5, 6]: 200}

print( mydict[ml] ) # 100
print( mydict[mylist([4,5,6])] ) # 200
print( mylist([6,7,8]) in mydict) # False

Also a typo:
TypeError: unhashable type: ‘set’ => TypeError: unhashable type: ‘list’

First Post!

emildepp

emildepp

@jmontojo

The “TypeError: unhashable type: ‘list’” error message in Python occurs when you try to use a mutable object (such as a list) as a key in a dictionary. Since dictionaries use keys to index values, keys must be hashable objects, meaning they must be immutable (i.e. their value cannot be changed). Lists are mutable, so they cannot be used as dictionary keys. To fix the error, use an immutable object like a tuple, string, or number as the dictionary key instead.

To resolve the TypeError: unhashable type: ‘list’ error, you need to use an immutable object as a key in a dictionary instead of a mutable one like a list. For example, you can use tuples, strings, or numbers as keys, which are all hashable objects.

d = {[1, 2, 3]: "list_key"} //Using a list as a key in a dictionary
//This will raise the TypeError: unhashable type: 'list' error
d = {(1, 2, 3): "tuple_key"} //Using a tuple as a key in a dictionary
//This will work fine as tuples are hashable

Where Next?

Popular Pragmatic Bookshelf topics Top

jon
Some minor things in the paper edition that says “3 2020” on the title page verso, not mentioned in the book’s errata online: p. 186 But...
New
Mmm
Hi, build fails on: bracket-lib = “~0.8.1” when running on Mac Mini M1 Rust version 1.5.0: Compiling winit v0.22.2 error[E0308]: mi...
New
alanq
This isn’t directly about the book contents so maybe not the right forum…but in some of the code apps (e.g. turbo/06) it sends a TURBO_ST...
New
leba0495
Hello! Thanks for the great book. I was attempting the Trie (chap 17) exercises and for number 4 the solution provided for the autocorre...
New
jskubick
I’m running Android Studio “Arctic Fox” 2020.3.1 Patch 2, and I’m embarrassed to admit that I only made it to page 8 before running into ...
New
nicoatridge
Hi, I have just acquired Michael Fazio’s “Kotlin and Android Development” to learn about game programming for Android. I have a game in p...
New
taguniversalmachine
Hi, I am getting an error I cannot figure out on my test. I have what I think is the exact code from the book, other than I changed “us...
New
bjnord
Hello @herbert ! Trying to get the very first “Hello, Bracket Terminal!" example to run (p. 53). I develop on an Amazon EC2 instance runn...
New
davetron5000
Hello faithful readers! If you have tried to follow along in the book, you are asked to start up the dev environment via dx/build and ar...
New
SlowburnAZ
Getting an error when installing the dependencies at the start of this chapter: could not compile dependency :exla, "mix compile" failed...
New

Other popular topics Top

AstonJ
A thread that every forum needs! Simply post a link to a track on YouTube (or SoundCloud or Vimeo amongst others!) on a separate line an...
New
Rainer
My first contact with Erlang was about 2 years ago when I used RabbitMQ, which is written in Erlang, for my job. This made me curious and...
New
AstonJ
I ended up cancelling my Moonlander order as I think it’s just going to be a bit too bulky for me. I think the Planck and the Preonic (o...
New
AstonJ
Do the test and post your score :nerd_face: :keyboard: If possible, please add info such as the keyboard you’re using, the layout (Qw...
New
Margaret
Hello everyone! This thread is to tell you about what authors from The Pragmatic Bookshelf are writing on Medium.
1147 29994 760
New
PragmaticBookshelf
Programming Ruby is the most complete book on Ruby, covering both the language itself and the standard library as well as commonly used t...
New
DevotionGeo
I have always used antique keyboards like Cherry MX 1800 or Cherry MX 8100 and almost always have modified the switches in some way, like...
New
First poster: bot
zig/http.zig at 7cf2cbb33ef34c1d211135f56d30fe23b6cacd42 · ziglang/zig. General-purpose programming language and toolchain for maintaini...
New
First poster: AstonJ
Jan | Rethink the Computer. Jan turns your computer into an AI machine by running LLMs locally on your computer. It’s a privacy-focus, l...
New
PragmaticBookshelf
Explore the power of Ash Framework by modeling and building the domain for a real-world web application. Rebecca Le @sevenseacat and ...
New

Sub Categories: