How can I append or concatenate two dataframes in python polars?

Question

I see it’s possible to append using the series namespace (https://stackoverflow.com/a/70599059/5363883). What I’m wondering is if there is a similar method for appending or concatenating DataFrames.

In pandas historically it could be done with df1.append(df2). However that method is being deprecated (if it hasn’t already been deprecated) for pd.concat([df1, df2]).

df1

a	b	c
1	2	3

df2

a	b	c
4	5	6

res

a	b	c
1	2	3
4	5	6

Asked By: cnpryer

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Source

Answer 1

Looks like .extend() mutates df1 to extend its memory to df2.

import polars as pl

df1 = pl.DataFrame({"a": [1], "b": [2], "c": [3]})
df2 = pl.DataFrame({"a": [4], "b": [5], "c": [6]})
df1.extend(df2)

┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ 1   ┆ 2   ┆ 3   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 4   ┆ 5   ┆ 6   │
└─────┴─────┴─────┘

This makes sense, but if I wanted to create a totally distinct (in memory too) df3 I’m guessing it’d be

import polars as pl

df1 = pl.DataFrame({"a": [1], "b": [2], "c": [3]})
df2 = pl.DataFrame({"a": [4], "b": [5], "c": [6]})

df3 = pl.from_records(df1.to_numpy(), columns=["a", "b", "c"])
df3.extend(df2)

┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ 1   ┆ 2   ┆ 3   │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 4   ┆ 5   ┆ 6   │
└─────┴─────┴─────┘

Any feedback on less verbose methods would be appreciated.

Answered By: cnpryer

Answer 2

There are different append strategies depending on your needs.

df1 = pl.DataFrame({"a": [1], "b": [2], "c": [3]})
df2 = pl.DataFrame({"a": [4], "b": [5], "c": [6]})


# new memory slab
new_df = pl.concat([df1, df2], rechunk=True)

# append free (no memory copy)
new_df = df1.vstack(df2)

# try to append in place
df1.extend(df2)

To understand the differences, it is important to understand polars memory is immutable iff it has any copy.

Copies in polars are free, because it only increments a reference count of the backing memory buffer instead of copying the data itself.

However, if a memory buffer has no copies yet, e.g. the refcount == 1, we can mutate polars memory.

Knowing this background there are the following ways to append data:

concat -> concatenate all given DataFrames. This is sort of a linked list of DataFrames. If you pass rechunk=True, all memory will be reallocated to contiguous chunks.
vstack -> Adds the data from other to DataFrame by incrementing a refcount. This is super cheap. It is recommended to call rechunk after many vstacks. Or simply use pl.concat.
extend This operation copies data. It tries to copy data from other to DataFrame. If however the refcount of DataFrame is larger than 1. A new buffer of memory is allocated to hold both DataFrames.

Answered By: ritchie46

How can I append or concatenate two dataframes in python polars?

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