Bug: gather does not select anything if `*args` is unspecified

[sotte]

The docs for gather state for *args:

If unspecified, all columns are selected.

However, if not *args are specified, nothing is selected.

Code to reproduce this:

# %%
import pandas as pd
from siuba import gather

# %%
df = pd.DataFrame(
    {
        "id": [1, 2],
        "price_x": [0.1, 0.2],
        "price_y": [0.4, 0.5],
        "price_z": [0.7, 0.8],
    }
)
print(df)
#    id  price_x  price_y  price_z
# 0   1      0.1      0.4      0.7
# 1   2      0.2      0.5      0.8

# %%
res = df >> gather()
print(res)
# Empty DataFrame
# Columns: [id, price_x, price_y, price_z, key, value]
# Index: []

[sotte]

This would be the fix: var_list = var_create(*args or __data.columns).

And with some context and a doctest:

@singledispatch2(pd.DataFrame)
def gather(__data, key, value, *args, drop_na=False, convert=False):
    """Reshape table by gathering it in to long format.

    Examples
    --------

    >>> import pandas as pd
    >>> from siuba import _, gather

    >>> df = pd.DataFrame({"x": [1, 2], "y": [3, None]})
    >>> gather(df, key="key", value="value")
      key  value
    0   x    1.0
    1   x    2.0
    2   y    3.0
    3   y    NaN

    """
    # TODO: implement var selection over *args
    if convert:
        raise NotImplementedError("convert not yet implemented")

    # TODO: copied from nest and select
    var_list = var_create(*args or __data.columns)  # <-- this is the fix
    od = var_select(__data.columns, *var_list)

    value_vars = list(od) or None

    id_vars = [col for col in __data.columns if col not in od]
    long = pd.melt(__data, id_vars, value_vars, key, value)

    if drop_na:
        return long[~long[value].isna()].reset_index(drop=True)

    return long

However there is a TODO and maybe you want to implement a more systematic solution.

[machow]

Should be fixed in v0.4.0!