GeoPandas: Get the maximum length inside a polygon

An approximation since it will only measure distances between existing vertices:

import geopandas as gpd
from shapely.geometry import LineString, mapping
from itertools import combinations
import re

df = gpd.read_file('/home/bera/GIS/Data/testdata/longest_line2.shp')

longest_lines = []
for g in list(df.geometry):
    all_coords = str(mapping(g)["coordinates"]) #https://gis.stackexchange.com/questions/287306/list-all-polygon-vertices-coordinates-using-geopandas
    all_xys = re.findall("d+.d+", all_coords) #I know this is ugly, but it works in extracting floats from nested tuples
    all_xys = zip(all_xys[::2],all_xys[1::2])
    lines = []
    for p1,p2 in combinations(all_xys,2): #For all combinations of polygon vertices create a line
        line = LineString([(float(p1[0]),float(p1[1])), (float(p2[0]),float(p2[1]))])
        line = line.intersection(g) #To only include the line inside polygon
        lines.append(line)
    longest_lines.append(max(lines, key=lambda x: x.length)) #Find longest line

df.geometry=longest_lines #Replace polygon geometris with lines
df.to_file('/home/bera/GIS/Data/testdata/longest_lines2.shp') #Save

For a polygon looking like this it will not find the longest line since there is no vertice top mid: