Frequently Asked Questions¶
Why is pandas not enough?¶
pandas, thanks to its unrivaled speed and flexibility, has emerged as the premier python package for working with labeled arrays. So why are we contributing to further fragmentation in the ecosystem for working with data arrays in Python?
Sometimes, we really want to work with collections of higher dimensional arrays (ndim > 2), or arrays for which the order of dimensions (e.g., columns vs rows) shouldn’t really matter. For example, climate and weather data is often natively expressed in 4 or more dimensions: time, x, y and z.
Pandas does support N-dimensional panels, but the implementation is very limited:
- You need to create a new factory type for each dimensionality.
- You can’t do math between NDPanels with different dimensionality.
- Each dimension in a NDPanel has a name (e.g., ‘labels’, ‘items’, ‘major_axis’, etc.) but the dimension names refer to order, not their meaning. You can’t specify an operation as to be applied along the “time” axis.
Fundamentally, the N-dimensional panel is limited by its context in pandas’s tabular model, which treats a 2D DataFrame as a collections of 1D Series, a 3D Panel as a collection of 2D DataFrame, and so on. In my experience, it usually easier to work with a DataFrame with a hierarchical index rather than to use higher dimensional (N > 3) data structures in pandas.
Another use case is handling collections of arrays with different numbers of dimensions. For example, suppose you have a 2D array and a handful of associated 1D arrays that share one of the same axes. Storing these in one pandas object is possible but awkward – you can either upcast all the 1D arrays to 2D and store everything in a Panel, or put everything in a DataFrame, where the first few columns have a different meaning than the other columns. In contrast, this sort of data structure fits very naturally in an xarray Dataset.
Pandas gets a lot of things right, but scientific users need fully multi- dimensional data structures.
How do xarray data structures differ from those found in pandas?¶
The main distinguishing feature of xarray’s DataArray over labeled arrays in pandas is that dimensions can have names (e.g., “time”, “latitude”, “longitude”). Names are much easier to keep track of than axis numbers, and xarray uses dimension names for indexing, aggregation and broadcasting. Not only can you write x.sel(time='2000-01-01') and x.mean(dim='time'), but operations like x - x.mean(dim='time') always work, no matter the order of the “time” dimension. You never need to reshape arrays (e.g., with np.newaxis) to align them for arithmetic operations in xarray.
Should I use xarray instead of pandas?¶
It’s not an either/or choice! xarray provides robust support for converting back and forth between the tabular data-structures of pandas and its own multi-dimensional data-structures.
That said, you should only bother with xarray if some aspect of data is fundamentally multi-dimensional. If your data is unstructured or one-dimensional, stick with pandas, which is a more developed toolkit for doing data analysis in Python.
What is your approach to metadata?¶
We are firm believers in the power of labeled data! In addition to dimensions and coordinates, xarray supports arbitrary metadata in the form of global (Dataset) and variable specific (DataArray) attributes (attrs).
Automatic interpretation of labels is powerful but also reduces flexibility. With xarray, we draw a firm line between labels that the library understands (dims and coords) and labels for users and user code (attrs). For example, we do not automatically interpret and enforce units or CF conventions. (An exception is serialization to and from netCDF files.)
An implication of this choice is that we do not propagate attrs through most operations unless explicitly flagged (some methods have a keep_attrs option). Similarly, xarray does not check for conflicts between attrs when combining arrays and datasets, unless explicitly requested with the option compat='identical'. The guiding principle is that metadata should not be allowed to get in the way.