Pandas: concise way of finding and writing unique occurrences

For a dataframe such as this:

Desired outcome:

Unique occurrences of values in Col1 and Col2 in order of appearance by row

i.e. unique_list = [A, D, B, C]

Problem

Way to minimise iteration and processing due to number and size of dataframes

>Solution :

Use DataFrame.iloc for select first 2 columns, reshape by DataFrame.stack and get unique values in Series.unique:

unique_list = df.iloc[:, :2].stack().unique().tolist()
print (unique_list)
['A', 'D', 'B', 'C']