Polars

Misc

• Notes from Comparing dplyr with polars

• Read csv:

  import polars as pl
  
  df = pl.read_csv("https://gist.githubusercontent.com/slopp/ce3b90b9168f2f921784de84fa445651/raw/4ecf3041f0ed4913e7c230758733948bc561f434/penguins.csv", null_values="NA")

• Chaining operations:

  df.filter(pl.col("species") == "Gentoo").select(pl.col("^bill.*$"))

Columns

• Quick Reference

  	dplyr	polars
  Select column	select(df, x)	df.select(pl.col("x"))
  Select multiple columns	select(df, x, y)	df.select(pl.col("x", "y"))
  Rename column	rename(df, new_x = x)	df.rename({"x": "new_x"})
  Select all columns except x	select(df, -x)	df.select(pl.exclude("x"))
  Select all columns that start with “str”	select(df, starts_with("str"))	df.select(pl.col("^bill.*$")) or df.select(cs.starts_with("str"))[1]
  Select numeric columns	select(df, where(is.numeric))	df.select(cs.float(), cs.integer())[1,2]

Rows

• Quick Reference

  	dplyr	polars
  First n rows	head(df, n)	df.head(n)
  Filter range of values	filter(df, between(x, lo, hi))	df.filter(pl.col("x").is_between(lo, hi))
  Conditional	filter(df, x > 4)	df.filter(pl.col("x") > 4 )
  Sort rows	arrange(df, x)	df.sort("x")

Mutate

• Quick Reference

  	dplyr	polars
  Create new column	mutate(df, x_mean = mean(x))	df.with_columns(pl.col("x").mean().alias("x_mean"))

• Example: Ratio variables

  {{polars}}

  df.with_columns((pl.col("bill_length_mm") / pl.col("flipper_length_mm")).alias("bill_flipper_ratio"))
  
  #> shape: (344, 10)
  #> ┌───────┬───────────┬───────────┬────────────────┬───┬─────────────┬────────┬──────┬────────────────────┐
  #> │ rowid ┆ species   ┆ island    ┆ bill_length_mm ┆ … ┆ body_mass_g ┆ sex    ┆ year ┆ bill_flipper_ratio │
  #> │ ---   ┆ ---       ┆ ---       ┆ ---            ┆   ┆ ---         ┆ ---    ┆ ---  ┆ ---                │
  #> │ i64   ┆ str       ┆ str       ┆ f64            ┆   ┆ i64         ┆ str    ┆ i64  ┆ f64                │
  #> ╞═══════╪═══════════╪═══════════╪════════════════╪═══╪═════════════╪════════╪══════╪════════════════════╡
  #> │ 1     ┆ Adelie    ┆ Torgersen ┆ 39.1           ┆ … ┆ 3750        ┆ male   ┆ 2007 ┆ 0.216022           │
  #> │ 2     ┆ Adelie    ┆ Torgersen ┆ 39.5           ┆ … ┆ 3800        ┆ female ┆ 2007 ┆ 0.212366           │
  #> │ 3     ┆ Adelie    ┆ Torgersen ┆ 40.3           ┆ … ┆ 3250        ┆ female ┆ 2007 ┆ 0.206667           │
  #> │ 4     ┆ Adelie    ┆ Torgersen ┆ null           ┆ … ┆ null        ┆ null   ┆ 2007 ┆ null               │
  #> │ …     ┆ …         ┆ …         ┆ …              ┆ … ┆ …           ┆ …      ┆ …    ┆ …                  │
  #> │ 341   ┆ Chinstrap ┆ Dream     ┆ 43.5           ┆ … ┆ 3400        ┆ female ┆ 2009 ┆ 0.215347           │
  #> │ 342   ┆ Chinstrap ┆ Dream     ┆ 49.6           ┆ … ┆ 3775        ┆ male   ┆ 2009 ┆ 0.256995           │
  #> │ 343   ┆ Chinstrap ┆ Dream     ┆ 50.8           ┆ … ┆ 4100        ┆ male   ┆ 2009 ┆ 0.241905           │
  #> │ 344   ┆ Chinstrap ┆ Dream     ┆ 50.2           ┆ … ┆ 3775        ┆ female ┆ 2009 ┆ 0.253535           │
  #> └───────┴───────────┴───────────┴────────────────┴───┴─────────────┴────────┴──────┴────────────────────┘

  {dplyr}

  df |> 
    mutate(bill_flipper_ratio = bill_length_mm / flipper_length_mm)

Summarize

• Quick Reference

  	dplyr	polars
  Group	group_by(df, x)	df.group_by("x")
  Summarize	summarize(df, x_n = n())	df.agg(pl.count().alias("x_n"))

• Example: Group_By, Count, Sort

  df.group_by("species").agg(pl.count().alias("counts")).sort("counts", descending=True)
  
  #> shape: (3, 2)
  #> ┌───────────┬────────┐
  #> │ species   ┆ counts │
  #> │ ---       ┆ ---    │
  #> │ str       ┆ u32    │
  #> ╞═══════════╪════════╡
  #> │ Adelie    ┆ 152    │
  #> │ Gentoo    ┆ 124    │
  #> │ Chinstrap ┆ 68     │
  #> └───────────┴────────┘

• Example: For each species, find the penguin with the lowest body mass

  {{polars}}

  df.group_by("species").agg(pl.all().sort_by("body_mass_g").drop_nulls().first())
  
  #> shape: (3, 9)
  #> ┌───────────┬───────┬───────────┬────────────────┬───┬───────────────────┬─────────────┬────────┬──────┐
  #> │ species   ┆ rowid ┆ island    ┆ bill_length_mm ┆ … ┆ flipper_length_mm ┆ body_mass_g ┆ sex    ┆ year │
  #> │ ---       ┆ ---   ┆ ---       ┆ ---            ┆   ┆ ---               ┆ ---         ┆ ---    ┆ ---  │
  #> │ str       ┆ i64   ┆ str       ┆ f64            ┆   ┆ i64               ┆ i64         ┆ str    ┆ i64  │
  #> ╞═══════════╪═══════╪═══════════╪════════════════╪═══╪═══════════════════╪═════════════╪════════╪══════╡
  #> │ Adelie    ┆ 4     ┆ Torgersen ┆ 36.5           ┆ … ┆ 181               ┆ 2850        ┆ female ┆ 2007 │
  #> │ Chinstrap ┆ 315   ┆ Dream     ┆ 46.9           ┆ … ┆ 192               ┆ 2700        ┆ female ┆ 2008 │
  #> │ Gentoo    ┆ 272   ┆ Biscoe    ┆ 42.7           ┆ … ┆ 208               ┆ 3950        ┆ female ┆ 2009 │
  #> └───────────┴───────┴───────────┴────────────────┴───┴───────────────────┴─────────────┴────────┴──────┘

  {dplyr}

  df |>
    group_by(species) |>
    arrange(body_mass_g) |>
    summarize(body_mass_g = first(body_mass_g))
  
  #> # A tibble: 3 × 2
  #>   species   body_mass_g
  #>   <fct>           <int>
  #> 1 Adelie           2850
  #> 2 Chinstrap        2700
  #> 3 Gentoo           3950

Joins

• Quick Reference

  	dplyr	polars
  Join dataframes	left_join(df1, df2, by=x)	df1.join(df2, on="x", how="left")