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The method to choose a value from a specific column in a dataframe when a different column in the same dataframe matches a certain value found in another dataframe is to use the pandas merge method.
For example, if we have two dataframes df1 and df2 with columns "ID" and "Value" in both dataframes, we can merge them on the "ID" column using the following code:
merged_df = pd.merge(df1, df2, on='ID')
This will create a new dataframe merged_df that contains all rows from both dataframes where the "ID" column matches. We can then select a value from a specific column, say "Value" in df1 where the "Value" column matches a certain value in df2 using the following code:
selected_value = merged_df.loc[merged_df['Value_y'] == 'certain_value', 'Value_x'].iloc[0]
This will select the first row in mergeddf where the "Valuey" column matches the certain value, and then select the corresponding value from the "Value_x" column in that row.