How does `pd.date_range()` creates an object of `DatetimeIndex`, without calling any method of the class `DatetimeIndex`?

Question

This is the csv file I am using, which has day column as index

            temperature  windspeed   event
day                                       
2017-01-01         32.0        6.0    Rain
2017-01-04          NaN        9.0   Sunny
2017-01-05         28.0        NaN    Snow
2017-01-06          NaN        7.0     NaN
2017-01-07         32.0        NaN    Rain
2017-01-08          NaN        NaN   Sunny
2017-01-09          NaN        NaN     NaN
2017-01-10         34.0        8.0  Cloudy
2017-01-11         40.0       12.0   Sunny

I am running this code to insert missing date values in the day column which contains the date values.

dt = pd.date_range("01-01-2017","01-11-2017")
idx = pd.DatetimeIndex(dt)
df = df.reindex(idx)

The code is working fine but I want to understand how does this code snippet works behind the scene so I looked into documentation but couldn't find the exact explanation.

Then I asked ChatGPT, it says that the pd.date_range() creates an instance of the class DatetimeIndex. So I asked it, which method of the DatetimeIndex class is called when pd.date_range() function is used?

It answered that, pd.date_range() doesn't directly call the constructor of the DatetimeIndex class, but it accomplishes the same result.

I am not able to understand this. Can someone please explain me this?

Answer 1

You do not need to explicitly call DatetimeIndex, pandas.date_range already returns a DatetimeIndex.

# ensure index is datetime
df.index = pd.to_datetime(df.index)

dt = pd.date_range("01-01-2017","01-11-2017")
out = df.reindex(dt)

Why? Because it was written to do so:

pandas.date_range pandas.date_range(start=None, end=None, periods=None, freq=None, tz=None, normalize=False, name=None, inclusive='both', *, unit=None, **kwargs)

...

Returns: DatetimeIndex

Output:

            temperature  windspeed   event
2017-01-01         32.0        6.0    Rain
2017-01-02          NaN        NaN     NaN
2017-01-03          NaN        NaN     NaN
2017-01-04          NaN        9.0   Sunny
2017-01-05         28.0        NaN    Snow
2017-01-06          NaN        7.0     NaN
2017-01-07         32.0        NaN    Rain
2017-01-08          NaN        NaN   Sunny
2017-01-09          NaN        NaN     NaN
2017-01-10         34.0        8.0  Cloudy
2017-01-11         40.0       12.0   Sunny