Creating a column based on strings from two columns and another boolean column

Question

I'm trying to create a column based on some specific "rules". I'd like to have a new column at the end "Result" with the following result based on the three first columns:

is_return	From	To	Result
True	Fir	Fem	FirFem
False	Tre	Syv	TreSyv
True	Syv	Tre	TreSyv_r
False	Tre	Syv	TreSyv2
True	Syv	Tre	TreSyv_r2
False	Snø	Van	SnøVan

Basically if there's a trip that is not a return then just combine from and to, and add a number starting from 2 to it if there are multiple (row 4). If it's tagged as a return trip then first check if it exists as a non-return trip, example given in row 2 and 3. But if it's tagged as a return without there being a non-return variant then keep this format as is (row 1).

Answer 1

IIUC, you need several steps (commented in the code):

import numpy as np

# compute the string for both directions
s1 = df['From']+df['To']
s2 = df['To']+df['From']

# compute the string is the correct order
# depending on the existence of the first trip
s = pd.Series(np.where(s2.isin(s1[~df['is_return']]), s2+'_r', s1),
              index=df.index)

# add number to duplicates
count = s.groupby(s).cumcount().add(1)
df['Result'] = s+np.where(count.gt(1), count.astype(str), '')

output:

   is_return From   To     Result
0       True  Fir  Fem     FirFem
1      False  Tre  Syv     TreSyv
2       True  Syv  Tre   TreSyv_r
3      False  Tre  Syv    TreSyv2
4       True  Syv  Tre  TreSyv_r2
5      False  Snø  Van     SnøVan