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

iPaul
page 37 ANTLRInputStream input = new ANTLRInputStream(is); as of ANTLR 4 .8 should be: CharStream stream = CharStreams.fromStream(i...
New
johnp
Running the examples in chapter 5 c under pytest 5.4.1 causes an AttributeError: ‘module’ object has no attribute ‘config’. In particula...
New
raul
Page 28: It implements io.ReaderAt on the store type. Sorry if it’s a dumb question but was the io.ReaderAt supposed to be io.ReadAt? ...
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
rmurray10127
Title: Intuitive Python: docker run… denied error (page 2) Attempted to run the docker command in both CLI and Powershell PS C:\Users\r...
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
brunogirin
When I run the coverage example to report on missing lines, I get: pytest --cov=cards --report=term-missing ch7 ERROR: usage: pytest [op...
New
tkhobbes
After some hassle, I was able to finally run bin/setup, now I have started the rails server but I get this error message right when I vis...
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
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
What chair do you have while working… and why? Is there a ‘best’ type of chair or working position for developers?
New
New
DevotionGeo
The V Programming Language Simple language for building maintainable programs V is already mentioned couple of times in the forum, but I...
New
New
New
PragmaticBookshelf
Author Spotlight: VM Brasseur @vmbrasseur We have a treat for you today! We turn the spotlight onto Open Source as we sit down with V...
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
New
CommunityNews
A Brief Review of the Minisforum V3 AMD Tablet. Update: I have created an awesome-minisforum-v3 GitHub repository to list information fo...
New
AstonJ
This is cool! DEEPSEEK-V3 ON M4 MAC: BLAZING FAST INFERENCE ON APPLE SILICON We just witnessed something incredible: the largest open-s...
New

Sub Categories: