How to pivot pandas dataframe?

Question

[kristopherfernandez]

Given a dataframe:

 Sample_Name SNP1 SNP2 SNP3
 Sample 1 AA AC GC
 Sample 2 GA AC CG
 Sample 3 AA AA GG

How do I get:

 Sample_Name SNP Value
 Sample 1 SNP1 AA
 Sample 1 SNP2 AC
 Sample 1 SNP3 GC
 Sample 2 SNP1 GA
 Sample 2 SNP2 AC
 Sample 2 SNP3 CG
 ...

I'm a beginner and I've been trying to find a way to do this but none of the options I've tried have been successful. Thanks to anyone who could help!

[Ram]

Minor point: You're unpivoting the data (going from wide-form to long-form), not pivoting it.

Answer 1

[finswimmer]

Like this?

import pandas as pd

df = pd.DataFrame(
    [["AA", "AC", "GC"], ["GA", "AC", "CG"], ["AA", "AA", "GG"]],
    index=["Sample 1", "Sample 2", "Sample 3"],
    columns=["SNP1", "SNP2", "SNP3"],
)

print(df.stack())

Output:

Sample 1  SNP1    AA
          SNP2    AC
          SNP3    GC
Sample 2  SNP1    GA
          SNP2    AC
          SNP3    CG
Sample 3  SNP1    AA
          SNP2    AA
          SNP3    GG
dtype: object

[kristopherfernandez]

Once you have this object, how can you subset / filter to find, say, all samples with SNP1 'AA'?

[finswimmer]

For this task there is no need to convert it. You can use:

df.index[df['SNP1'].str.match('AA')].tolist()