My input:

df=(pd.DataFrame({'items':['foo','bar','xyz'],
                  'path':['c/folderOne/folderTwo/folderThree','c/folder1/folder2/folder3','c/folderO/folderT/folderTh'],
}))

I trying insert in column path some "fix" after 'c/... I want add new substring such as NEWPATH into list and re-join again, but I ran unexpected problem.
That my code:

df['path'] = df['path'].map(lambda x: x.split("/"))
df['path'] = df['path'].map(lambda x: x.insert(1,'NEWPATH'))
df['path'] = df['path'].map(lambda x: x.join('/'))

Code return None in column path, when I expected something like this, after second line of my code:
in column df['path'] : ['c','NEWPATH','folderOne'....] etc. for each row(cell) of my column.

>Solution :

Looks like you should better use a regex here:

NEWPATH = 'abc'
df['path2'] = df['path'].str.replace('^([^/]+/)', rf'\1{NEWPATH}/')

NB. assigning to a new column here for clarity

output:

  items                               path                                  path2
0   foo  c/folderOne/folderTwo/folderThree  c/abc/folderOne/folderTwo/folderThree
1   bar          c/folder1/folder2/folder3          c/abc/folder1/folder2/folder3
2   xyz         c/folderO/folderT/folderTh         c/abc/folderO/folderT/folderTh

inserting after the nth directory:

NEWPATH = 'abc'
df['path2'] = df['path'].str.replace('^(^(?:[^/]+/){3})', rf'\1{NEWPATH}/')

or

NEWPATH = 'abc'
N = 3
df['path2'] = df['path'].str.replace(f'^(^(?:[^/]+/){{{N}}})', rf'\1{NEWPATH}/')

output:

  items                               path                                  path2
0   foo  c/folderOne/folderTwo/folderThree  c/folderOne/folderTwo/abc/folderThree
1   bar          c/folder1/folder2/folder3          c/folder1/folder2/abc/folder3
2   xyz         c/folderO/folderT/folderTh         c/folderO/folderT/abc/folderTh