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Learning pandas - Second Edition by Michael Heydt

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How to work with missing data

Data is missing in pandas when it has a value of NaN (also seen as np.nan - the form from NumPy). This NaN value means that there is no value specified for the particular index label in a particular Series.

How can data be missing? 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 ...

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