What is really the difference between += operation and append function in python lists? inside functions definations

Question

So till now the only real difference that I knew about the difference in the append and += operation was that of speed but recently I stumbled upon a case in which += would throw an error while append would not can someone help out with what has been happening?

This code would not run and throw an error namely
UnboundLocalError: local variable ‘travel_log’ referenced before assignment

travel_log = [
{
  "country": "France",
  "visits": 12,
  "cities": ["Paris", "Lille", "Dijon"]
},
{
  "country": "Germany",
  "visits": 5,
  "cities": ["Berlin", "Hamburg", "Stuttgart"]
},
]

def add_new_country(Country,Visits,Cities):
    travel_log+={"country":Country,"visits":Visits,"cities":Cities}

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

while this code would run

travel_log = [
{
  "country": "France",
  "visits": 12,
  "cities": ["Paris", "Lille", "Dijon"]
},
{
  "country": "Germany",
  "visits": 5,
  "cities": ["Berlin", "Hamburg", "Stuttgart"]
},
]

def add_new_country(Country,Visits,Cities):
    travel_log.append({"country":Country,"visits":Visits,"cities":Cities})

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

Asked By: Priyanshu Upadhyay

||

Source

Answer 1

While using the operator ‘+’, you should create the list

travel_log = []

before using it.

def add_new_country(Country,Visits,Cities):
travel_log = []
travel_log+={"country":Country,"visits":Visits,"cities":Cities}

Answered By: baovolamada

Answer 2

this is because you used the travel_log variable in a function and not mentioning that you mean the global variable
this will fix the issue:

travel_log = [
{
  "country": "France",
  "visits": 12,
  "cities": ["Paris", "Lille", "Dijon"]
},
{
  "country": "Germany",
  "visits": 5,
  "cities": ["Berlin", "Hamburg", "Stuttgart"]
},
]

def add_new_country(Country,Visits,Cities):
    global travel_log
    travel_log += [{"country":Country,"visits":Visits,"cities":Cities}]

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

the difference is when you call a method from travel_log, it grabs the global variable and calls the method, but when you use assignment operators python thinks you meant to create a local variable and therefore it says referenced before assignment

Answered By: No.BoD

What is really the difference between += operation and append function in python lists? inside functions definations

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