Unbound Local Error: local variable referenced before assignment

Question

Why does this code doesn't give any error?

travel_log = [
{
  "country": "France",
  "visits": 12,
  "cities": ["Paris", "Lille", "Dijon"]
},
{
  "country": "Germany",
  "visits": 5,
  "cities": ["Berlin", "Hamburg", "Stuttgart"]
},
]
# compare this **code block 1** with code block 2 and 3
def add_new_country(country, visits, cities) :
   new_country = {}
   new_country["country"] = country
   new_country["visits"] = visits
   new_country["cities"] = cities
   travel_log.append(new_country)


add_new_country("Russia", 2, ["Moscow", "Saint Petersburg"])
print(travel_log)

But this one does? Error: Unbound Local Error: local variable referenced before assignment

#code block 2 (Replace code block 1 with this and run the code)
def add_new_country(country, visits, cities) :
  new_country = {
    "country" : country,
    "visits" : visits,
    "cities" : cities,
  }
  travel_log += new_country

why is the error gone after making travel_log variable global? and why did the code work even if we didn't make the variable global in code block 1

#code block 3 (Replace code block 1 with this and run the code)
def add_new_country(country, visits, cities) :
  global travel_log
  new_country = {
    "country" : country,
    "visits" : visits,
    "cities" : cities,
  }
  travel_log += new_country