Select pandas Series elements based on condition

Given a dataframe, I know I can select rows by condition using below syntax:

df[df['colname'] == 'Target Value']

But what about a Series? Series does not have a column (axis 1) name, right?

My scenario is I have created a Series by through the nunique() function:

sr = df.nunique()

And I want to list out the index names of those rows with value 1.

Having failed to find a clear answer on the Net, I resorted to below solution:

for (colname, coldata) in sr.iteritems(): 
   if coldata == 1: 

Question: what is a better way to get my answer (i.e list out index names of Series (or column names of the original Dataframe) which has just a single value?)

The ultimate objective was to find which columns in a DF has one and only one unique value. Since I did not know how to do that direct from a DF, I first used nunique() and that gave me a Series. Thus i needed to process the Series with a “== 1” (i.e one and only one)

I hope my question isnt silly.


It is unclear what you want. Whether you want to work on the dataframe or on the Series ?

Case 1: Working on DataFrame

In case you want to work on the dataframe to to list out the index names of those rows with value 1, you can try:



data = {'Col1': [0, 1, 2, 2, 0], 'Col2': [0, 2, 2, 1, 2], 'Col3': [0, 0, 0, 0, 1]}
df = pd.DataFrame(data)

   Col1  Col2  Col3
0     0     0     0
1     1     2     0
2     2     2     0
3     2     1     0
4     0     2     1

Then, run the code, it gives:

[1, 3, 4]

Case 2: Working on Series

If you want to extract the index of a Series with value 1, you can extract it into a list, as follows:

sr.loc[sr == 1].index.tolist()

or use:

sr.index[sr == 1].tolist()