BUG: Losing the geometry column should convert to a DataFrame

Branched out from the discussion in #1653

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of geopandas.

• (optional) I have confirmed this bug exists on the master branch of geopandas.

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Code Sample, a copy-pastable example

From @jorisvandenbossche 's comment in #1653

df = geopandas.read_file(geopandas.datasets.get_path("naturalearth_cities"))

type(df.drop("geometry", axis=1)) # geopandas.geodataframe.GeoDataFrame
type(df.reindex(columns=["name"])) # geopandas.geodataframe.GeoDataFrame

Problem description

When a GeoDataFrame loses its geometry column (via .drop() or .reindex(), for instance), the result should be a regular pandas DataFrame. This makes intuitive sense and would allow easier compatibility with the pandas API, as well as preventing issues such as the one found in #1653.

Right now, these methods are simply inherited from the DataFrame class but we can overwrite them in the GeoDataFrame to automate this downcasting when possible.

Expected Output

type(df.drop("geometry", axis=1)) # pandas.core.frame.DataFrame
type(df.reindex(columns=["name"])) # pandas.core.frame.DataFrame

[m-richards]

Hi @arredond is this still something you are planning on working on? This is something I’d quite like to see in geopandas, and if you’re not, I’d like to make a start on it.

[jorisvandenbossche]

For a possible solution, I think we should also look into the suggestion I wrote down in https://github.com/geopandas/geopandas/issues/1153, having a GeoDataFrame._constructor that holds this logic of returning a GeoDataFrame or a DataFrame.