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Type equality checking raises for subclassed objects #22914

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Open
2 tasks done
nikaltipar opened this issue May 23, 2025 · 2 comments · May be fixed by #22915
Open
2 tasks done

Type equality checking raises for subclassed objects #22914

nikaltipar opened this issue May 23, 2025 · 2 comments · May be fixed by #22915
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python Related to Python Polars

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@nikaltipar
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Checks

  • I have checked that this issue has not already been reported.
  • I have confirmed this bug exists on the latest version of Polars.

Reproducible example

Example provided as-is from a comment made in #22802 by @ctdunc.

import polars as pl


class MyDf(pl.DataFrame):
    def has_column(self, col: str) -> bool:
        return col in self.columns


df = pl.DataFrame(
    {
        "foo": [1, 2, 3],
        "ham": ["a", "b", "c"],
    }
)
my_df = MyDf(
    {
        "foo": [1, 2, 3],
        "bar": [6, 7, 8],
    }
)

# works fine
df_join_mine = df.join(my_df, on="foo")

# type error
mine_join_df = my_df.join(df, on="foo")

Log output



Issue description


The type equality check is too restrictive, as sometimes subclasses are preferred in order to modify or add certain behaviours.


Expected behavior


Allow operations such as joins, that check for type equality, to go through, if the other argument is a subclass of self.


Installed versions




--------Version info---------
Polars:              1.30.0
Index type:          UInt32
Platform:            Windows-10-10.0.22631-SP0
Python:              3.11.7 (tags/v3.11.7:fa7a6f2, Dec  4 2023, 19:24:49) [MSC v.1937 64 bit (AMD64)]
LTS CPU:             False

----Optional dependencies----
Azure CLI            'az' is not recognized as an internal or external command,
operable program or batch file.
<not installed>
adbc_driver_manager  <not installed>
altair               <not installed>
azure.identity       <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               <not installed>
gevent               <not installed>
google.auth          <not installed>
great_tables         <not installed>
matplotlib           <not installed>
numpy                <not installed>
openpyxl             <not installed>
pandas               <not installed>
polars_cloud         <not installed>
pyarrow              <not installed>
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>




      		
    

      
  





        



            
            

              
            

        

      


      
    








      

    



      

  
            

    

      
    

    


      

          @nikaltipar
nikaltipar




            added
      

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      May 23, 2025

    

  



  

  

    
  


  

      

    @nikaltipar
nikaltipar



    linked a pull request
    
      May 23, 2025
    

    that will
    close
    this issue





  


  

      
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  @alexander-beedie





  


    

      

  

    

        
    
    

  


      
          
  
      
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            alexander-beedie
  

      

      

      commented


        May 24, 2025


      
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We don't actually support subclassing1, so this is expected/correct behaviour.


Some methods will return the subclassed type (when inheriting from DataFrame/etc), but many will not.

For example, with your MyDf class/example above:


type(my_df)
# __main__.MyDf

type(my_df.head(1))
# __main__.MyDf

type(my_df.select("bar"))
# polars.dataframe.frame.DataFrame

type(my_df.filter(pl.col("bar") >= 7))
# polars.dataframe.frame.DataFrame


Footnotes


    

  1. 

    https://github.com/pola-rs/polars/pull/7426#issuecomment-1475834972 ↩
    

    2. 




      		
    

  





        


            

            
            

              
            

        

      


      

            
  
  

    
  
    
    

  

  

  


    












      

  
            

    

      
    

    


      

          @alexander-beedie
alexander-beedie




            removed
      

  bug

  Something isn't working

      

  needs triage

  Awaiting prioritization by a maintainer

  labels


      May 24, 2025

    

  








      

  
      



  

  @nikaltipar





  


    

      

  

    

        
    
    

  


      
          
  
      
            Copy link

  

      
    

  


  

      

  
    Contributor


      

  Author


  


  

    

  





      


        


  
    

      
          


We don't actually support subclassing1, so this is expected/correct behaviour.


Some methods will return the subclassed type (when inheriting from DataFrame/etc), but many will not. For example, with your MyDf class/example above:


type(my_df)


main.MyDf


type(my_df.head(1))


main.MyDf


type(my_df.select("bar"))


polars.dataframe.frame.DataFrame


type(my_df.filter(pl.col("bar") >= 7))


polars.dataframe.frame.DataFrame


Footnotes


    

  1. feat(python): Easier subclassing #7426 (comment) ↩
    2. 





Added a comment on the PR, let me know if instead you'd like to continue the discussion here.

      		
    

  





        


            

            
            

              
            

        

      


      

            
  
  

    
  
    
    

  

  

  


    
















  
  

    

      

  





  




  
  

              

  
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      fix: Allow subclasses in type equality checking

    
      nikaltipar/polars
    





      
    

  




      

    

  
      

  

    

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          @alexander-beedie 
        
          @nikaltipar