narwhals 0.1.13

Extremely lightweight compatibility layer between pandas, Polars, cuDF, and Modin

Narwhals

Extremely lightweight compatibility layer between Polars, pandas, cuDF, and Modin.

Seamlessly support all four, without depending on any of them!

• ✅ Just use a subset of the Polars API, no need to learn anything new
• ✅ No dependencies (not even Polars), keep your library lightweight
• ✅ Separate Lazy and Eager APIs
• ✅ Use Polars Expressions

Note: this is work-in-progress, and a bit of an experiment, don't take it too seriously.

Installation

pip install narwhals

Or just vendor it, it's only a bunch of pure-Python files.

Usage

There are three steps to writing dataframe-agnostic code using Narwhals:

1. use narwhals.to_polars_api to wrap a pandas, Polars, cuDF, or Modin dataframe in the Polars API

2. use the subset of the Polars API defined in https://github.com/MarcoGorelli/narwhals/blob/main/narwhals/spec/__init__.py.

3. use narwhals.to_original_object to return an object to the user in their original dataframe flavour. For example:

   • if you started with pandas, you'll get pandas back
   • if you started with Polars, you'll get Polars back
   • if you started with Modin, you'll get Modin back
   • if you started with cuDF, you'll get cuDF back (and computation will happen natively on the GPU!)

Example

Here's an example of a dataframe agnostic function:

from typing import TypeVar
import pandas as pd
import polars as pl

from narwhals import translate_frame

AnyDataFrame = TypeVar("AnyDataFrame")


def my_agnostic_function(
    suppliers_native: AnyDataFrame,
    parts_native: AnyDataFrame,
) -> AnyDataFrame:
    suppliers, pl = translate_frame(suppliers_native, lazy_only=True)
    parts, _ = translate_frame(parts_native, lazy_only=True)
    result = (
        suppliers.join(parts, left_on="city", right_on="city")
        .filter(
            pl.col("color").is_in(["Red", "Green"]),
            pl.col("weight") > 14,
        )
        .group_by("s", "p")
        .agg(
            weight_mean=pl.col("weight").mean(),
            weight_max=pl.col("weight").max(),
        )
    )
    return result.collect().to_native()

You can pass in a pandas, Polars, cuDF, or Modin dataframe, the output will be the same! Let's try it out:

suppliers = {
    "s": ["S1", "S2", "S3", "S4", "S5"],
    "sname": ["Smith", "Jones", "Blake", "Clark", "Adams"],
    "status": [20, 10, 30, 20, 30],
    "city": ["London", "Paris", "Paris", "London", "Athens"],
}
parts = {
    "p": ["P1", "P2", "P3", "P4", "P5", "P6"],
    "pname": ["Nut", "Bolt", "Screw", "Screw", "Cam", "Cog"],
    "color": ["Red", "Green", "Blue", "Red", "Blue", "Red"],
    "weight": [12.0, 17.0, 17.0, 14.0, 12.0, 19.0],
    "city": ["London", "Paris", "Oslo", "London", "Paris", "London"],
}

print("pandas output:")
print(
    my_agnostic_function(
        pd.DataFrame(suppliers),
        pd.DataFrame(parts),
    )
)
print("\nPolars output:")
print(
    my_agnostic_function(
        pl.LazyFrame(suppliers),
        pl.LazyFrame(parts),
    )
)

pandas output:
    s   p  weight_mean
0  S1  P6         19.0
1  S2  P2         17.0
2  S3  P2         17.0
3  S4  P6         19.0

Polars output:
shape: (4, 3)
┌─────┬─────┬─────────────┐
│ s   ┆ p   ┆ weight_mean │
│ --- ┆ --- ┆ ---         │
│ str ┆ str ┆ f64         │
╞═════╪═════╪═════════════╡
│ S1  ┆ P6  ┆ 19.0        │
│ S3  ┆ P2  ┆ 17.0        │
│ S4  ┆ P6  ┆ 19.0        │
│ S2  ┆ P2  ┆ 17.0        │
└─────┴─────┴─────────────┘

Magic! 🪄

Scope

• Do you maintain a dataframe-consuming library?
• Is there a Polars function which you'd like Narwhals to have, which would make your job easier?

If, I'd love to hear from you!

Note: this is not a "Dataframe Standard" project. It just translates a subset of the Polars API to pandas-like libraries.

Why "Narwhals"?

Because they are so awesome.