Local variable referenced before - arrays

Question

I have an array of data which in the first column has years, and the other 3 columns has data for 3 different groups, the last of which is carrots.
I am trying to find the year in which the carrot value is the highest, by comparing the carrot value each year to the current highest, and then finding the year that the value takes place on.
I have used identical code for the other 2 columns with just the word carrot replaced and i in year[i] changed appropriately and the code works, but for this it throws up the error "local variable 'carrot_maxyear' referenced before assignment"
def carrot(data):
year0 = data[0]
carrotmax = year0[3]
for year in data:
if year[3] > carrotmax:
carrotmax = year[3]
carrot_maxyear = year[0]
return carrot_maxyear

Answer 1

Python's builtin max will make this easier:
def carrot(data):
maxyear = max(data, key=lambda year: year[3])
return maxyear[0]
This way you don't need the year0 and carrotmax initialization. We need to use the key argument to max because it looks like you want to return year[0] instead of the year[3] value used for the max calculation.
Your original version with a fix would look like:
def carrot(data):
year0 = data[0]
carrotmax = year0[3]
carrot_maxyear = 0 # initialize carrot_maxyear outside of loop to avoid error
for year in data:
if year[3] > carrotmax:
carrotmax = year[3]
carrot_maxyear = year[0]
return carrot_maxyear
but IMO the version utilizing max is more clear and Pythonic.