Wrong value returned for new object of a class derived from list

[mhirasuna]

Bug report

Bug description:

# Add a code block here, if required
class L (list):
    def __new__ (cls,value):
        if isinstance(value,str):
            if ',' in value:
                collect = []
                for item in value.split(','):
                    collect.append(item)
                value = collect
        return super().__new__(cls,value)

print(L("1,2"))

Expected output: ['1', '2']
Got instead: ['1', ',', '2']

If I don't cast the return value, I get the correct value but the type would not be what I want.

CPython versions tested on:

3.13

Operating systems tested on:

macOS

[StanFromIreland]

Hello, in the future, please ask help questions in the Python Discourse Help category.

As for this question, per the docs:

If new() is invoked during object construction and it returns an instance of cls, then the new instance’s init() method will be invoked like init(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to the object constructor.

In this case, list.__init__ still receives the original string "a,b", which it iterates character by character, producing ['a', ',', 'b']. This has been asked on SO, where people have provided longer answers which you may find helpful.

This example may also help:

>>> class L(list):
...     def __new__(cls, value):
...         if isinstance(value, str):
...             if ',' in value:
...                 value = value.split(',')
...         return super().__new__(cls, value)
...         
...     def __init__(cls, value):
...         pass
...        
... print(L("a,b"))
... 
[]

[mhirasuna]

Thank-you for your help. I was able to get the code to work, but it required a modification to the suggested code. The suggested code, see class L1 below, returns "[]", which I can't explain. I was able to find a work around, see class L2 below, which returns the desired output, "['a', 'b']. class L1 (list): def __new__ (cls,value): if isinstance(value,str): if ',' in value: value = value.split(',') return super().__new__(cls, value) def __init__(cls, value): pass class L2 (list): def __new__ (cls,value): if isinstance(value,str): collect = cls([]) if ',' in value: collect.extend(value.split(',')) return collect return super().__new__(cls,value) def __init__(cls, value): pass print(L1("a,b")) print(L2("a,b"))

…

On Fri, Sep 19, 2025 at 5:49 PM Stan Ulbrych ***@***.***> wrote: Closed #139172 <#139172> as not planned. — Reply to this email directly, view it on GitHub <#139172 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/A7C3UYQRE3UAMJRDCR2CYHL3TR26VAVCNFSM6AAAAACG7ZN5TOVHI2DSMVQWIX3LMV45UABCJFZXG5LFIV3GK3TUJZXXI2LGNFRWC5DJN5XDWMJZHAYDSOBXHAYTSMQ> . You are receiving this because you authored the thread.Message ID: ***@***.***>