I'm reading temperature entries stored in a file.
Each entry is generated when the temperature value changes, so it is not stored in regular intervals.

An example of the data could be as follows:

```
timestamp  | temperature
-----------+------------
1477400000 | 31
1477400001 | 31.5
1477400003 | 32
1477400010 | 31.5
1477400200 | 32
1477400201 | 32.5

```

I would need a fast way to get the temperature at any timestamp, **even if it is not in the index**. For instance, temperature at `1477400002` would be `31.5`, but `1477400002` is not in the index.

For sake of easier reproducibility, the same dataframe may be generated as follows: 

```
df = pd.DataFrame(data={'temperature': [31, 31.5, 32, 31.5, 32, 32.5]},
                  index=[1477400000, 1477400001, 1477400003, 1477400010, 1477400200, 1477400201])

```