how custom sort of rows in polars

Question

How to sort row with spesific order

df = pl.DataFrame({"currency": ["EUR","EUR","EUR","USD","USD","USD"], "alphabet": ["A","B","C","A","B","C"]})

i need to descending the currency and custom sort of alphabet

expected to be like this

currency	alphabet
USD	C
USD	A
USD	B
EUR	C
EUR	A
EUR	B

Answer 1

For example you can make your own order of pl.Categorical data using pl.StringCache.

df = pl.DataFrame({
    "currency": ["EUR","EUR","EUR","USD","USD","USD","USD"],
    "alphabet": ["A","B","C","A","B","C","A"]
})

with pl.StringCache():
    currency = sorted(["EUR", "USD"], reverse=True)
    pl.Series(["C", "A", "B", *currency]).cast(pl.Categorical)
    
    df = df.with_columns(
        pl.col(pl.Utf8).cast(pl.Categorical),
    ).sort(
        pl.col(pl.Categorical).to_physical()
    )
    
    print(df)

┌──────────┬──────────┐
│ currency ┆ alphabet │
│ ---      ┆ ---      │
│ cat      ┆ cat      │
╞══════════╪══════════╡
│ USD      ┆ C        │
│ USD      ┆ A        │
│ USD      ┆ A        │
│ USD      ┆ B        │
│ EUR      ┆ C        │
│ EUR      ┆ A        │
│ EUR      ┆ B        │
└──────────┴──────────┘

Answer 2

Create a polars expression that maps the "alphabet" values to numbers that respect the desired order of the column values using Expr.map_dict. Use the DataFrame.sort method to sort the rows first by "currency" value in descending order, and second by the previous expression value (in ascending order).

df = pl.DataFrame({
    "currency": ["EUR","EUR","EUR","USD","USD","USD"], 
    "alphabet": ["A","B","C","A","B","C"]
})

abc_order = {val: idx for idx, val in enumerate(["C", "A", "B"])}

res = df.sort(pl.col("currency"), 
              pl.col("alphabet").map_dict(abc_order),
              descending=[True, False])

Output:

>>> res

shape: (6, 2)
┌──────────┬──────────┐
│ currency ┆ alphabet │
│ ---      ┆ ---      │
│ str      ┆ str      │
╞══════════╪══════════╡
│ USD      ┆ C        │
│ USD      ┆ A        │
│ USD      ┆ B        │
│ EUR      ┆ C        │
│ EUR      ┆ A        │
│ EUR      ┆ B        │
└──────────┴──────────┘