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

belgoros
Following the steps described in Chapter 6 of the book, I’m stuck with running the migration as described on page 84: bundle exec sequel...
New
telemachus
Python Testing With Pytest - Chapter 2, warnings for “unregistered custom marks” While running the smoke tests in Chapter 2, I get these...
New
brianokken
Many tasks_proj/tests directories exist in chapters 2, 3, 5 that have tests that use the custom markers smoke and get, which are not decl...
New
ianwillie
Hello Brian, I have some problems with running the code in your book. I like the style of the book very much and I have learnt a lot as...
New
simonpeter
When I try the command to create a pair of migration files I get an error. user=> (create-migration "guestbook") Execution error (Ill...
New
lirux
Hi Jamis, I think there’s an issue with a test on chapter 6. I own the ebook, version P1.0 Feb. 2019. This test doesn’t pass for me: ...
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
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
dtonhofer
@parrt In the context of Chapter 4.3, the grammar Java.g4, meant to parse Java 6 compilation units, no longer passes ANTLR (currently 4....
New
gorkaio
root_layout: {PentoWeb.LayoutView, :root}, This results in the following following error: no “root” html template defined for PentoWeb...
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
Exadra37
I am thinking in building or buy a desktop computer for programing, both professionally and on my free time, and my choice of OS is Linux...
New
PragmaticBookshelf
Design and develop sophisticated 2D games that are as much fun to make as they are to play. From particle effects and pathfinding to soci...
New
AstonJ
There’s a whole world of custom keycaps out there that I didn’t know existed! Check out all of our Keycaps threads here: https://forum....
New
PragmaticBookshelf
Create efficient, elegant software tests in pytest, Python's most powerful testing framework. Brian Okken @brianokken Edited by Kat...
New
AstonJ
Biggest jackpot ever apparently! :upside_down_face: I don’t (usually) gamble/play the lottery, but working on a program to predict the...
New
PragmaticBookshelf
Author Spotlight Jamis Buck @jamis This month, we have the pleasure of spotlighting author Jamis Buck, who has written Mazes for Prog...
New
Help
I am trying to crate a game for the Nintendo switch, I wanted to use Java as I am comfortable with that programming language. Can you use...
New
AnfaengerAlex
Hello, I’m a beginner in Android development and I’m facing an issue with my project setup. In my build.gradle.kts file, I have the foll...
New
AstonJ
This is a very quick guide, you just need to: Download LM Studio: https://lmstudio.ai/ Click on search Type DeepSeek, then select the o...
New

Sub Categories: