Pandas: concise way of finding and writing unique occurrences

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For a dataframe such as this:

Col1 Col2
1 A D
2 B A
3 C B

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

pandas dataframe compare python
1 Answers

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']
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