added .free_symbols to Variable.

.travis.yml

@@ -41,7 +41,7 @@ env:
 before_install:
   - travis_retry pip install twine wheel coveralls
   - travis_retry pip install check-manifest pydocstyle
-  - travis_retry pip install https://github.com/sympy/sympy/archive/1.1.zip
+  - travis_retry pip install https://github.com/sympy/sympy/zipball/master
 
 install:
   - pip install -e .[all]

essm/variables/units.py

@@ -19,8 +19,11 @@
 # MA 02111-1307, USA.
 """Define unit symbols."""
 
+import functools
+import operator
+
 import sympy.physics.units as u
-from sympy.physics.units import Quantity
+from sympy.physics.units import Dimension, Quantity, find_unit
 
 from .SI import SI
 
@@ -33,6 +36,8 @@
 second = u.second
 watt = u.watt
 
+SI_DIMENSIONS = {str(d._dimension.name): d for d in SI._base_units}
+
 
 def markdown(unit):
     """Return markdown representation of a unit."""
@@ -55,8 +60,13 @@ def derive_quantity(expr, name=None):
 
 
 def derive_unit(expr, name=None):
-    """Derive a unit from an expression."""
-    return SI.print_unit_base(derive_quantity(expr, name=name))
+    """Derive SI-unit from an expression, omitting scale factors."""
+    dim = Dimension(Quantity.get_dimensional_expr(expr))
+    return functools.reduce(
+        operator.mul, (
+            SI_DIMENSIONS[d] ** p
+            for d, p in Dimension.get_dimensional_dependencies(dim).items()),
+        1)
 
 
 __all__ = (

essm/variables/utils.py

@@ -20,6 +20,8 @@
 """Utility function for variables."""
 
 from .units import markdown
+from ._core import BaseVariable
+from sympy import preorder_traversal
 
 
 def generate_metadata_table(variables=None, include_header=True):
@@ -40,3 +42,12 @@ def generate_metadata_table(variables=None, include_header=True):
         val = str(Variable.__defaults__.get(variable, '-'))
 
         yield (symbol, name, doc, val, markdown(variable.short_unit))
+
+
+def get_variables(expr):
+    """Traverses through expression and returns set of variables as list."""
+    variables = []
+    for arg in preorder_traversal(expr):
+        if isinstance(arg, BaseVariable):
+            variables.append(arg)
+    return list(set(variables))

tests/test_equations.py

@@ -8,6 +8,7 @@
 from essm.equations import Equation
 from essm.variables import Variable
 from essm.variables.units import joule, kelvin, meter, mole, second
+from essm.variables.utils import get_variables
 
 
 class demo_g(Variable):
@@ -17,6 +18,14 @@ class demo_g(Variable):
     unit = meter / second ** 2
 
 
+class demo_v(Variable):
+    unit = meter/second
+
+
+class demo_t(Variable):
+    unit = second
+
+
 class demo_fall(Equation):
     """Test equation."""
 
@@ -29,6 +38,12 @@ class t(Variable):
     expr = Eq(d, 1 / 2 * demo_g * t ** 2)
 
 
+class demo_velocity(Equation):
+    """Test equation."""
+
+    expr = Eq(demo_v, demo_g * demo_t)
+
+
 def test_equation():
     """Test variable definition."""
     assert demo_fall.__doc__ == demo_fall.definition.__doc__
@@ -56,6 +71,12 @@ def test_args():
         demo_fall.definition.t.definition, }
 
 
+def test_get_variables():
+    """Test free variables."""
+    assert set(get_variables(demo_velocity)) == {
+        demo_v, demo_t, demo_g}
+
+
 def test_unit_check():
     """Check unit test involving temperature."""
 

tests/test_variables.py

@@ -4,7 +4,7 @@
 import pytest
 
 from essm.variables import Variable
-from essm.variables.units import meter, second
+from essm.variables.units import derive_unit, joule, kilogram, meter, second
 
 
 class demo_variable(Variable):
@@ -59,6 +59,25 @@ class invalid_unit(Variable):
             unit = second
 
 
+def test_derive_unit():
+    """Test derive_unit from expression."""
+
+    class lambda_E(Variable):
+        unit = joule / kilogram
+
+    class E_l(Variable):
+        unit = joule / (meter ** 2 * second)
+
+    assert derive_unit(2 * lambda_E * E_l) \
+        == kilogram * meter ** 2 / second ** 5
+    assert derive_unit(E_l/E_l) == 1
+
+    class dimensionless(Variable):
+        expr = demo_variable / demo_expression_variable
+
+    assert dimensionless.definition.unit == 1
+
+
 def test_remove_variable_from_registry():
     """Check is the variable is removed from registry."""