I discussed this with @ritchie46 and we think it's a reasonable request.

Problem description

Is it possible to add a method to list class so we can justify (pad) list just like str? example:

ser = pl.Series([[1, 2, 3, 4], [2, 3, 4, 5, 6], [3, 2, 1]])
ser.list.pad(5, 0)
# output:
# pl.Series([[0, 1, 2, 3, 4], [2, 3, 4, 5, 6], [0, 0, 3, 2, 1]])

It could have more feature like clipping lists with greater length.

In the mean time you could do something like this:

df = pl.DataFrame(ser)
(df
 .with_columns(pl.lit(0).alias('pad_value'))
 .with_columns(
     pl.concat_list(
         pl.col('pad_value').repeat_by(5 - pl.col('column_0').list.lengths()), 
         'column_0')
         .alias('column_0'))
 .drop('pad_value'))

Related issue that was suggest by @ritchie46 (extend_constant): #2810

@reswqa as discussed. Free to pick up. :)

@ritchie46 I want to do it myself :) Can I?

@balakhaniyan Of course, feel free to pick this up. ritchie and I will help review the code then.

Would love this feature! I often need it for machine learning purpose

Can't believe string has pad_end and pad_start feature but list doesn't have it yet

I may be able to help, but I'm new to this kind of thing, not sure where to look the _pyexpr code

Searched py-polars/polars/expr/expr.py but there doesn't seem to be a function called str_pad_start

It is in rust, under crates/polars-python/src/expr

It calls .str().pad_start() on the rust side:

.list.lengths() has also since been renamed in the repeat_by approach mentioned above:

df = s.to_frame("x")

df.with_columns(
    pl.lit(0).repeat_by(5 - pl.col("x").list.len())
      .list.concat("x")
      .alias("list.pad_start")
)

# shape: (3, 2)
# ┌─────────────────┬─────────────────┐
# │ x               ┆ list.pad_start  │
# │ ---             ┆ ---             │
# │ list[i64]       ┆ list[i64]       │
# ╞═════════════════╪═════════════════╡
# │ [1, 2, 3, 4]    ┆ [0, 1, 2, 3, 4] │
# │ [2, 3, 4, 5, 6] ┆ [2, 3, 4, 5, 6] │
# │ [3, 2, 1]       ┆ [0, 0, 3, 2, 1] │
# └─────────────────┴─────────────────┘

Sorry, can't help with that, I'm not familiar enough with Rust 😅

I spouted my previous comment spontaneously, forgetting that Polars core is written in Rust

@AndhikaWB if you try, you will learn it ;)