ENH: Use left._constructor on pd.merge

[jwass]

Use the _constructor property when creating the merge result to
preserve the output type.

If a GeoPandas GeoDataFrame is merged with a DataFrame, the result is hard-coded to always be DataFrame GeoPandas Issue #118. We'd like it to return GeoDataFrame in these cases

>>> import geopandas as gpd
>>> import pandas as pd

>>> gdf = gpd.GeoDataFrame(...)
>>> df = pd.DataFrame(...)

>>> merged = pd.merge(gdf, df, on='column')
>>> type(merged)
GeoDataFrame

This PR uses left._constructor to generate the result type for merge operations.

[jreback]

sure

can u add a test pls (create an object that inherits from DataFrame and use it to confirm that this is correct)

[jwass]

@jreback Done.

[jankatins]

It would also be nice to mention this in the docs (i.e. that the output type is taken from left, so that pd.merge(df, gdf, on='column') still returns a DataFrame...)

[jreback]

@jwass this is fine, can you update the docs (and do-string). squash and looks ready.

[jwass]

@JanSchulz Missed your original comment from a few weeks ago - sorry for the delay here.

@jreback squashed and rebased. Let me know if the docs should change somewhere else or should be re-worded.

[jreback]

this should prob also have __finalize__ called on it? (I know wasn't in the original and doesn't actually do anything in pandas atm..but)

[jreback]

can you add a note in v0.15.0.txt in API section (and reference this PR number)

[jwass]

@jreback Yes - added the __finalize__ call in ordered_merge. While I was at it, added the test/doc for ordered_merge to reflect the new behavior here too.
Added the note for v0.15.0. Squashed, rebased.

[jreback]

merged via 123a555

thanks!