Strange behaviour using multiple tilelayers and control layer

Please add a code sample or a nbviewer link, copy-pastable if possible

# Executed within a Jupyter notebook cell
import folium
mymap = folium.Map(location=(39.472997, -6.370755), zoom_start=10,
                  tiles='OpenStreetMap', attr='Javi')

another_tile = folium.TileLayer('cartodbdark_matter', attr='Javi')
another_tile.layer_name = "darker"
another_tile.add_to(mymap)

folium.map.LayerControl().add_to(mymap)

# If one leaves the comments 
# If one uncomments the next line, the map will not have any tile layer active by default
# mymap.render()

mymap

# If one does a save to html. I will save it with no default active tile layer
# mypath = '~'
# mymap.save(mypath)

Problem description

Hi, I found this issue while I was working in my maps in a Jupyter notebook and then saving the result as html.

The behavior I think is wrong (or weird) is that after executing the render function of the map, when I have multiple tile layers, and I try to show them again in in html or within a jupyter cell (I think both make usage of the render), all the layers are unchecked in the control layer, so I have to click on one of them.

So I share two screenshots: https://drive.google.com/open?id=1qbtwMI2ZlwzcYlk0LEqvMvaB_MtUzFHa https://drive.google.com/open?id=1V-T079qvUOeeol9IuZ4u9chAwWcSLqWr

Expected Output

So, the expected behavior is like the first time that render is called i.e. that one layer is checked by default in the control layer.

Output of folium.__version__

Working with: Python 3.6.7

appdirs==1.4.3 attrs==19.1.0 backcall==0.1.0 bleach==3.1.0 boto==2.49.0 boto3==1.9.205 botocore==1.12.205 branca==0.3.1 cached-property==1.5.1 cachetools==3.1.0 certifi==2019.3.9 chardet==3.0.4 Click==7.0 click-plugins==1.1.1 cligj==0.5.0 colour==0.1.5 cycler==0.10.0 decorator==4.4.0 defusedxml==0.5.0 dill==0.2.9 docutils==0.14 entrypoints==0.3 Fiona==1.8.6 folium==0.10.0 geopandas==0.4.1 google-auth==1.6.3 google-auth-oauthlib==0.3.0 googleads==18.1.0 googlemaps==3.0.2 gspread==3.1.0 gspread-dataframe==3.0.2 holoviews==1.12.1 httplib2==0.13.0 idna==2.8 ipykernel==5.1.0 ipython==7.4.0 ipython-genutils==0.2.0 ipywidgets==7.4.2 isodate==0.6.0 jedi==0.13.3 Jinja2==2.10.1 jmespath==0.9.4 joblib==0.13.2 jsonschema==3.0.1 jupyter==1.0.0 jupyter-client==5.2.4 jupyter-console==6.0.0 jupyter-core==4.4.0 jupyterlab==0.35.4 jupyterlab-server==0.2.0 kiwisolver==1.0.1 lxml==4.3.3 MarkupSafe==1.1.1 matplotlib==3.0.3 mistune==0.8.4 multiprocess==0.70.7 munch==2.3.2 mysql==0.0.2 mysqlclient==1.4.2.post1 nbconvert==5.4.1 nbformat==4.4.0 notebook==5.7.8 numpy==1.16.2 oauth2client==4.1.3 oauthlib==3.0.1 pandas==0.24.2 pandocfilters==1.4.2 param==1.9.0 parso==0.4.0 patsy==0.5.1 pexpect==4.7.0 pickleshare==0.7.5 polyline==1.3.2 prometheus-client==0.6.0 prompt-toolkit==2.0.9 psycopg2-binary==2.8.2 ptyprocess==0.6.0 pyasn1==0.4.5 pyasn1-modules==0.2.4 pycodestyle==2.5.0 Pygments==2.3.1 PyMySQL==0.9.3 pyparsing==2.4.0 pyproj==2.1.3 pyrsistent==0.14.11 python-dateutil==2.8.0 python-Levenshtein==0.12.0 pytz==2019.1 pyviz-comms==0.7.2 PyYAML==5.1 pyzmq==18.0.1 qtconsole==4.4.3 requests==2.21.0 requests-oauthlib==1.2.0 requests-toolbelt==0.9.1 rsa==4.0 Rtree==0.8.3 s3transfer==0.2.1 scikit-learn==0.21.2 scipy==1.2.1 seaborn==0.9.0 Send2Trash==1.5.0 Shapely==1.6.4.post2 six==1.12.0 SQLAlchemy==1.3.3 statsmodels==0.9.0 suds-jurko==0.6 terminado==0.8.2 testpath==0.4.2 tornado==6.0.2 tqdm==4.31.1 traitlets==4.3.2 uk-postcode-utils==1.0 urllib3==1.24.1 wcwidth==0.1.7 webencodings==0.5.1 widgetsnbextension==3.4.2 xlrd==1.2.0 xlwt==1.3.0 xmltodict==0.12.0 zeep==3.3.

[Hanneska]

I have found a workaround that is not 100% perfect, but still ok. Eventhough, no base layer is selected by default (no visible tick in the LayerControl), you can still see a TileLayer after adding the LayerControl:

1. Create a new TileLayer.
2. Make it an exact duplicate (same parameters) of the one you want to be displayed after rendering.
3. Set the parameter control=False.

This means that this TileLayer is not unchecked by the render method of LayerControl, see:

https://github.com/python-visualization/folium/blob/60ae79cd1f4f16fc48e206947c7cef870ea28b8e/folium/map.py#L156

A disadvantage is that no layer is selected in the LayerControl. Therefore, the user might not know at the outset which layer she/he is actually looking at.

[Conengmo]

It seems like this is indeed a bug. When we render the layer control object we try to make sure we only check the first base layer. We do that by unchecking the others. Because the object has already been rendered it unchecks all base layers I suspect.

https://github.com/python-visualization/folium/blob/60ae79cd1f4f16fc48e206947c7cef870ea28b8e/folium/map.py#L161

We have to look into this and see how we can solve this. The render function of each class should be idempotent.