How do I extract data from a Bokeh ColumnDatasource

If the source input is a Pandas DataFrame, you can use the Standard method:

source = ColumnDataSource(
    data= pd.DataFrame( dict(
        x=[1, 2, 3, 4, 5],
        y=[2, 5, 8, 2, 7],
        desc=['A', 'b', 'C', 'd', 'E'],
    ))
)
print( source.data['x'].max() )

A ColumnDataSource object has an attribute data which will return the python dictionary used to create the object in the first place.

from bokeh.plotting import ColumnDataSource

# define ColumnDataSource
source = ColumnDataSource(
    data=dict(
        x=[1, 2, 3, 4, 5],
        y=[2, 5, 8, 2, 7],
        desc=['A', 'b', 'C', 'd', 'E'],
    )
)

# find max for variable 'x' from 'source'
print( max( source.data['x'] ))