add default arguments

Readme.md

@@ -30,6 +30,26 @@ result = x(a=(1, 20), b=(2,  30), c=2)
 print(result)
 # >> array([ 1.41421356, 35.35533906])
 
+# As a goodie, you can print out the gaussian error distribution in
+# symbolic form. (Works best in Jupyter Notebooks)
+x.pretty_gauss_propagation('a', 'b', 'c')
+```
+
+### Default Values
+To reduce boylerplate one can set default substitutions for symbols.
+This especially usefull for constants.
+
+```python
+from SecondaryValue import SecondaryValue
+
+# create a secondary value with default arguments
+x = SecondaryValue("a + b", defaults=dict(b=1/2))
+
+# this works because `b` is substituted from the defaults
+result = x(b=1/2)
+print(result)
+# >> 1.0
+
 # As a goodie, you can print out the gaussian error distribution in
 # symbolic form. (Works best in Jupyter Notebooks)
 x.pretty_gauss_propagation('a', 'b', 'c')

SecondaryValue/SecondaryValue.py

@@ -14,28 +14,32 @@ class SecondaryValue:
     """
     A value computed from a formular, optionally with error propagation.
     """
-    def __init__(self, expr, dependencies=False, dtype=np.float64):
+    def __init__(self, expr, dependencies=False, defaults=False,
+                 dtype=np.float64):
         """Creates a new Secondary Value.
 
         :param str expr: the expression from which to calculate the
             value, may be a sympy expression or a string.
         :param dict dependencies: secondary values to calculate as
             dependencies (in order)
+        :param dict defaults: Default arguments for the call.
         :param np.dtype dtype: the numpy datatype for the resulting
             value
+
         """
 
         self._expr = expr
         self._parsed = sympify(self._expr) if isinstance(self._expr, str) \
             else self._expr
 
-        self._symbols = set([symbol.__str__() \
-                         for symbol in self._parsed.free_symbols])
+        self._symbols = {symbol.__str__() \
+                         for symbol in self._parsed.free_symbols}
 
         self._deps = {name: dependency \
                       for name, dependency in dependencies.items() \
                       if name in self._symbols} if dependencies else {}
 
+        self._defaults = defaults
         self._dtype = dtype
 
         # with per-instance cache
@@ -64,6 +68,22 @@ class SecondaryValue:
 
         return kwargs, calc_deps
 
+    def _inject_defaults(self, **kwargs):
+        """Injects the default value for arguments.
+
+        :returns: the modified argument dictionary
+        :rtype: dict
+        """
+
+        if not self._defaults:
+            return kwargs
+
+        for name, value in self._defaults.items():
+            if name not in kwargs:
+                kwargs[name] = value
+
+        return kwargs
+
     def __call__(self, *args, **kwargs):
         """Calculates a value from the expression by substituting
         variables by the values of the given keyword arguments.  If an
@@ -78,6 +98,7 @@ class SecondaryValue:
         """
 
         kwargs, dep_values = self._calc_deps(**kwargs)
+        kwargs = self._inject_defaults(**kwargs)
 
         # check for missing symbols
         if not self._symbols <= set(kwargs.keys()):

setup.py

@@ -6,7 +6,7 @@ def readme():
         return f.read()
 
 setup(name='SecondaryValue',
-      version='0.0.5',
+      version='0.0.6',
       description='A helper to calculate the gaussian error propagation.',
       long_description=readme(),
       long_description_content_type='text/markdown',