How to concatenate all (string) values in a given pandas dataframe row to one string?

IIUC, by using apply , join

df.apply(lambda x :' '.join(x.astype(str)),1)
Out[348]: 
0    I want to join strings
1         But only in row 1
dtype: object

Then you can assign them

df1=df.iloc[1:]
df1['5']=df.apply(lambda x :' '.join(x.astype(str)),1)[0]
df1
Out[361]: 
     0     1   2    3  4                       5
1  But  only  in  row  1  I want to join strings

For Timing :

%timeit df.apply(lambda x : x.str.cat(),1)
1 loop, best of 3: 759 ms per loop
%timeit df.apply(lambda x : ''.join(x),1)
1 loop, best of 3: 376 ms per loop


df.shape
Out[381]: (3000, 2000)

Use str.cat to join the first row, and assign to the second.

i = df.iloc[1:].copy()   # the copy is needed to prevent chained assignment
i[df.shape[1]] = df.iloc[0].str.cat(sep=' ')

i     
     0     1   2    3  4                       5
1  But  only  in  row  1  I want to join strings