Python pandas: concat vertical and horizontal

Question

I am trying to concat two dataframes, horizontally. df2 contains 2 result variables for every observation in df1.

df1.shape 
(242583, 172)
df2.shape
(242583, 2)

My code is:

Fin = pd.concat([df1, df2], axis= 1)

But somehow the result is stacked in 2 dimensions:

Fin.shape
(485166, 174)

What am I missing here?

Answer 1

There are different index values, so indexes are not aligned and get NaNs:

df1 = pd.DataFrame({
    'A': ['a','a','a'],
    'B': range(3)
})
print (df1)
   A  B
0  a  0
1  a  1
2  a  2

df2 = pd.DataFrame({
    'C': ['b','b','b'],
    'D': range(4,7)
}, index=[5,7,8])
print (df2)
   C  D
5  b  4
7  b  5
8  b  6

---

Fin = pd.concat([df1, df2], axis= 1)
print (Fin)
     A    B    C    D
0    a  0.0  NaN  NaN
1    a  1.0  NaN  NaN
2    a  2.0  NaN  NaN
5  NaN  NaN    b  4.0
7  NaN  NaN    b  5.0
8  NaN  NaN    b  6.0

One possible solution is create default indexes:

Fin = pd.concat([df1.reset_index(drop=True), df2.reset_index(drop=True)], axis= 1)
print (Fin)
   A  B  C  D
0  a  0  b  4
1  a  1  b  5
2  a  2  b  6

Or assign:

df2.index = df1.index
Fin = pd.concat([df1, df2], axis= 1)
print (Fin)
   A  B  C  D
0  a  0  b  4
1  a  1  b  5
2  a  2  b  6

df1.index = df2.index
Fin = pd.concat([df1, df2], axis= 1)
print (Fin)
   A  B  C  D
5  a  0  b  4
7  a  1  b  5
8  a  2  b  6