Guess the Output
	Try to guess what the output is before moving to the next page.

This code will print: 12.0

The pandas.Series.fillna documentation says the following:

Returns: Series or None
    Object with missing values filled or None if inplace=True.

It’s always a good idea to not change (mutate) an object passed to a function. On the other hand, Pandas tries to be efficient and not copy data around a lot.

The design decision for fillna, both in pandas.Series and pandas.DataFrame, was not to change the original object and return a copy. But the user has an option to pass inplace=True, and then the original object is changed.

When a method changes an object, the common practice in Python is to return None. Other languages, such as JavaScript, prefer to return the object, allowing method chaining.

If you change line 5 to s.fillna(3, inplace=True), you’ll see 15.0 as the output.

fillna will work on anything that is considered a missing value: numpy.nan, pandas.NA, pandas.NaT, None …​

Empty strings or collections are not considered missing values.

Further Reading

pandas.Series.fillna Documentation

http://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.fillna.html

Working with Missing Data in the Pandas Documentation

http://pandas.pydata.org/pandas-docs/stable/user_guide/missing_data.html

Method Chaining on Wikipedia

http://en.wikipedia.org/wiki/Method_chaining