Unique function in python - python-3.x

Question

for i in df["col"].unique():
...
Here unique function is called after each iteration of loop or is it just called once and stores the result in memory??
Asking this just to check if unique function is executed after every iteration then there is chance that even in next iteration i might be same as in previous iteration.
Ex first time unique function is called then df["col"].unique() gives [1,2] so i would be 1 for first iteration and in second iteration unique function is again called and i may again get 1 as value.

Answer 1

The construction you are using first calculates the .unique() function and uses the result of that function, if iterable, to loop over.
If you'd want the loop to evaluate a function every iteration, you could use structures like:
list = [x.function() for x in items]
check this link for more information:
https://opensource.com/article/18/3/loop-better-deeper-look-iteration-python

Answer 2

Here is a quick code example to see how it works :
df = pd.DataFrame(0, index=np.arange(2), columns=['1','2'])
df.iloc[0][0]=1
for i in df['1'].unique():
print(f'unique values : {i}')
print(df)
df.iloc[1][0]=2
First, we create a 2x2 Dataframe of zeroeswith a 1 at [1][1] position :
1 2
0 1 0
1 0 0
Then we call unique to get all the uniques values of column 1 (i.e 0 and 1).
During the loop, we change the value one of the cell on column 1 (the one we iterate over). But as you can see in the output, it does not add any iteration in the loop.
This means that df.unique() store the result before iterating over it just as MdBrainz said and that modifying the value during the loop ain't going to change how many time the loop iterate.
Output :
unique values : 1
1 2
0 1 0
1 0 0
unique values : 0
1 2
0 1 0
1 2 0