In a pandas data frame, I have stanzas where the values for two specific columns are exactly the same. How do I add a new column for the rows that assign a unique integer, starting with integer 1 (not integer 0)? All completely unique lines also receive an int.

This is a sample data frame in which the second and third columns use identical values for groups of rows

```
df = pd.DataFrame ([['plane1', '', 'az'] . ['plane2', '', 'az'] . ['plane3', 'az', ''] . ['plane4', 'az', ''] . ['plane5', 'ny', ''] . ['plane6', 'ny', '']. ['plane7', 'fl', 'fl']. ['plane8', 'fl', 'fl']. ['plane10', '', 'de']. ['plane11', '', 'de']. ['plane12', '', 'mo']. ['plane13', '', 'mo']])
```

output

```
0 1 2
0 plane1 az
1 plane2 az
2 plane3 az
3 plane4 az
4 plane5 ny
5 plane6 ny
6 plane fl
7 plane fl
8 plane10 de
9 plane11 de
10 plane12 mo
11 plane13 mo
```

And this is the desired output using a designed sample data frame

```
df = pd.DataFrame ([['plane1', '', 'az', 1] . ['plane2', '', 'az', 1] . ['plane3', 'az', '', 2] . ['plane4', 'az', '', 2] . ['plane41', 'az', '', 2]. ['plane5', 'ny', '', 3] . ['plane6', 'ny', '', 3]. ['plane7', 'fl', 'fl', 4]. ['plane8', 'fl', 'fl', 4]. ['plane10', '', 'de', 5]. ['plane11', '', 'de', 5]. ['plane12', '', 'mo', 6]. ['plane13', '', 'mo', 6]])
```

output

```
0 1 2 3
0 plane1 az 1
1 plane2 az 1
2 plane3 az 2
3 plane4 az 2
4 plane41 az 2
5 plane5 ny 3
6 plane6 3
7 plane fl 4
8 plane8 fl fl 4
9 plane10 DE 5
10 plane11 DE 5
11 plane12 mo 6
12 plane13 mo 6
```