PythonのLambda関数を使ってソートする方法です。

サンプルコード

city_code = [(10, "Kanagawa"), (20, "Tokyo"), (50, "Hokkaido"), (15, "Shizuoka"), (17, "Chiba")]

print(city_code)

pref = city_code

pref.sort(key = lambda pref: pref[0])


print(pref)

出力結果

>>> 
*** Remote Interpreter Reinitialized ***
[(10, 'Kanagawa'), (20, 'Tokyo'), (50, 'Hokkaido'), (15, 'Shizuoka'), (17, 'Chiba')]
[(10, 'Kanagawa'), (15, 'Shizuoka'), (17, 'Chiba'), (20, 'Tokyo'), (50, 'Hokkaido')]

仕組みの説明