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ENH: Support dissolve(by=None) as unary_union of all geoms

See original GitHub issue

In order to have one unique geometry to deal with, I’m trying to dissolve all many edges that compose a route using the .dissolve() method on a GeoDataFrame instance but it throws me an error (while by=None is the default value);

gdf_unique_route = gdf_route_edges.dissolve() # or gdf_route_edges.dissolve(by=None) 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-160-808abd5a70f0> in <module>
----> 1 gdf_unique_route = gdf_route_edges.dissolve(by=None)

/usr/local/lib/python3.6/dist-packages/geopandas/geodataframe.py in dissolve(self, by, aggfunc, as_index)
    684         # Process non-spatial component
    685         data = self.drop(labels=self.geometry.name, axis=1)
--> 686         aggregated_data = data.groupby(by=by).agg(aggfunc)
    687 
    688         # Process spatial component

/usr/local/lib/python3.6/dist-packages/pandas/core/frame.py in groupby(self, by, axis, level, as_index, sort, group_keys, squeeze, observed)
   5796 
   5797         if level is None and by is None:
-> 5798             raise TypeError("You have to supply one of 'by' and 'level'")
   5799         axis = self._get_axis_number(axis)
   5800 

TypeError: You have to supply one of 'by' and 'level'

If I use by='attribute1 it works, but I end up with a few geometries instead of only one.
I really want to dissolve them all, regardless of their attributes, so I used the default argument values.

Help on the `.dissolve()` method is as follows:

Help on method dissolve in module geopandas.geodataframe:

dissolve(by=None, aggfunc='first', as_index=True) method of geopandas.geodataframe.GeoDataFrame instance
    Dissolve geometries within `groupby` into single observation.
    This is accomplished by applying the `unary_union` method
    to all geometries within a groupself.
    
    Observations associated with each `groupby` group will be aggregated
    using the `aggfunc`.
    
    Parameters
    ----------
    by : string, default None
        Column whose values define groups to be dissolved
    aggfunc : function or string, default "first"
        Aggregation function for manipulation of data associated
        with each group. Passed to pandas `groupby.agg` method.
    as_index : boolean, default True
        If true, groupby columns become index of result.
    
    Returns
    -------
    GeoDataFrame

There is no such level arg in there, it’s something used by pandas DataFrame .groupy() method actually (which you probably rely on).

As I was facing this error using GeoPandas so I’m reporting it here, but let me know if it should be reported to Pandas developers.

Thanks a lot!