Ability to do a repr on a CRS.

[pelson]

Currently there is not enought infrastructure in place to be able to do a code generating repr on a CRS. This would be a very useful feature and is something that we should certainly strive for.

[DPeterK]

I looked into this and made the following changes to add a basic __repr__ (and _repr_html_ for a visual representation of a Projection in a jupyter notebook):

diff --git a/lib/cartopy/_crs.pyx b/lib/cartopy/_crs.pyx
index 81a321c..b4e3ae0 100644
--- a/lib/cartopy/_crs.pyx
+++ b/lib/cartopy/_crs.pyx
@@ -202,8 +202,23 @@ cdef class CRS:
         """
         self.__init__(self, **state)
 
+    def __repr__(self):
+        name = self.__class__.__name__
+        str_params = {'lat_0': 'central_latitude',
+                      'lon_0': 'central_longitude',
+                      'h': 'height',
+                      'ellps': 'ellipse',
+                      'proj': 'projection'}
+        str_items_dict = {}
+        for param_name, param_desc in str_params.items():
+            proj4_param = self.proj4_params.get(param_name, None)
+            if proj4_param is not None:
+                str_items_dict[param_desc] = proj4_param
+        params_str = ', '.join(['{}={}'.format(k, v)
+                                for k, v in str_items_dict.items()])
+        return '<{}: {}>'.format(name, params_str)
+
     # TODO
-    #def __str__
     #def _geod(self): # to return the pyproj.Geod
 
     def _as_mpl_transform(self, axes=None):
diff --git a/lib/cartopy/crs.py b/lib/cartopy/crs.py
index 46d088e..1ba3187 100644
--- a/lib/cartopy/crs.py
+++ b/lib/cartopy/crs.py
@@ -146,6 +146,13 @@ class Projection(six.with_metaclass(ABCMeta, CRS)):
             domain = self._domain = sgeom.Polygon(self.boundary)
         return domain
 
+    def _repr_html_(self):
+        import matplotlib.pyplot as plt
+        ax = plt.axes(projection=self)
+        ax.set_global()
+        ax.coastlines('110m')
+        plt.show()
+
     def _as_mpl_axes(self):
         import cartopy.mpl.geoaxes as geoaxes
         return geoaxes.GeoAxes, {'map_projection': self}

This exposes the following problems:

• what information do we want to include in the repr of a CRS? If we miss key information from the repr it may make the repr less worthwhile than not having a repr at all.
• This is endemic of a larger problem with CRS: it has an inconsistent API (see Variations in Projection subclasses public API  #950).
• The html repr of a projection renders immediately – instead we should be able to store it locally and display it at our choice in the future.