Learning pandas

Working with missing data

Data is "missing" in pandas when it has a value of NaN (also seen as np.nan—the form from NumPy). The NaN value represents that in a particular Series that there is not a value specified for the particular index label.

In pandas, there are a number of reasons why a value can be NaN:

• A join of two sets of data does not have matched values
• Data that you retrieved from an external source is incomplete
• The NaN value is not known at a given point in time and will be filled in later
• There is a data collection error retrieving a value, but the event must still be recorded in the index
• Reindexing of data has resulted in an index that does not have a value
• The shape of data has changed and there are now additional rows or columns, which ...