partially fix documentation (#80)

pina/problem/spatial_problem.py

@@ -1,14 +1,53 @@
+"""Module for the SpatialProblem class"""
 from abc import abstractmethod
 
 from .abstract_problem import AbstractProblem
 
 
 class SpatialProblem(AbstractProblem):
+    """
+    The class for the definition of spatial problems, i.e., for problems
+    with spatial input variables.
+
+    Here's an example of a spatial 1-dimensional ODE problem.
+
+    :Example:
+        >>> from pina.problem import SpatialProblem
+        >>> from pina.operators import grad
+        >>> from pina import Condition, Span
+        >>> import torch
+
+        >>> class SimpleODE(SpatialProblem):
+
+        >>>     output_variables = ['u']
+        >>>     spatial_domain = Span({'x': [0, 1]})
+
+        >>>     def ode_equation(input_, output_):
+        >>>         u_x = grad(output_, input_, components=['u'], d=['x'])
+        >>>         u = output_.extract(['u'])
+        >>>         return u_x - u
+
+        >>>     def initial_condition(input_, output_):
+        >>>         value = 1.0
+        >>>         u = output_.extract(['u'])
+        >>>         return u - value
+
+        >>>     conditions = {
+        >>>         'x0': Condition(Span({'x': 0.}), initial_condition),
+        >>>         'D': Condition(Span({'x': [0, 1]}), ode_equation)}
+
+    """
 
     @abstractmethod
     def spatial_domain(self):
+        """
+        The spatial domain of the problem.
+        """
         pass
 
     @property
     def spatial_variables(self):
+        """
+        The spatial input variables of the problem.
+        """
         return self.spatial_domain.variables