Allow physical constants in extract expressions.

doc/examples.rst

@@ -464,9 +464,9 @@ to date.
         gain2 = 0.5            -- gain of second integrator
         Vmax = 1               -- quantizer maximum input voltage
         Vmin = -1              -- quantizer minimum input voltage
-        -- levels = 16            -- quantizer output levels
+        # levels = 16            -- quantizer output levels
         levels = 4             -- quantizer output levels
-        Tstop = 1/Fin "s"      -- how many input periods to simulate
+        Tstop = 1/Fin "s"      -- simulation stop time
         Tstart = -0.5/Fin "s"  -- initial transient interval (discarded)
 
     The values given above are used in the simulation, no further modification 
@@ -477,12 +477,6 @@ to date.
     from math import sin, tau
     from inform import display, error, os_error
 
-    parameters = Quantity.extract(__doc__, ignore_nonconforming_lines=True)
-    globals().update(parameters)
-    display('Simulation parameters:')
-    for v in parameters.values():
-        display('   ', v.render(show_label='f'))
-
     class Integrator:
         def __init__(self, gain=1):
             self.state = 0
@@ -515,6 +509,15 @@ to date.
             return self.amp*sin(self.omega*t)
 
 
+    # read simulation parameters and load into module namespace
+    parameters = Quantity.extract(__doc__)
+    globals().update(parameters)
+
+    # display the simulation parameters
+    display('Simulation parameters:')
+    for v in parameters.values():
+        display('   ', v.render(show_label='f'))
+
     # instantiate components
     integrator1 = Integrator(gain1)
     integrator2 = Integrator(gain2)

doc/user.rst

@@ -1380,8 +1380,9 @@ a version that displays the name and description by default.
     Kvco = 9.07 GHz/V  -- Gain of VCO
 
 With :meth:`quantiphy.Quantity.extract()`  the values of quantities can be given 
-as a expression that contains previously defined quantities. This requires that 
-you set the *evaluate* argument to True. For example:
+as a expression that contains previously defined quantities (or :ref:`physical 
+constants <physical-constants> or select mathematical constants (pi, tau, π, or 
+τ).  This feature is disabled if the *evaluate* argument is False.  For example:
 
 .. code-block:: python
 

quantiphy.py

@@ -20,7 +20,7 @@
 """
 
 # License {{{1
-# Copyright (C) 2016 Kenneth S. Kundert
+# Copyright (C) 2016-2018 Kenneth S. Kundert
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -524,13 +524,11 @@ def add_constant(value, alias=None, unit_systems=None):
 }
 
 # These constants are available to expressions in extract strings.
-# Be aware that this dictionary is passed as globals to eval, so it gets
-# polluted with things like __builtins__.
 CONSTANTS = {
     'pi': math.pi,
     'π': math.pi,
-    'tau': 2*math.pi,
-    'τ': 2*math.pi,
+    'tau': getattr(math, 'tau', 2*math.pi),
+    'τ': getattr(math, 'tau', 2*math.pi),
 }
 
 
@@ -1835,13 +1833,14 @@ def extract(cls, text, evaluate=True, ignore_nonconforming_lines=True):
                 // comment
 
         :arg str evaluate:
-            When true extract attempt to evaluate a value it does not recognize
+            When true extract attempts to evaluate a value it does not recognize
             as a number. In this case the expression for the value may be
             followed by a string surrounded by double quotes, which is taken as
             the units.  For example: Tstop = 5/Fin "s". The expressions may only
-            contain number literals, quantities defined previously in the same set of
-            definitions, and the constants pi and tau (2*pi), which may be named
-            π or τ. The units should not include a scale factor.
+            contain number literals, quantities defined previously in the same
+            set of definitions, physical quantities, and the constants pi and
+            tau (2*pi), which may be named π or τ. The units should not include
+            a scale factor.
 
         :arg str ignore_nonconforming_lines:
             When true extract will ignore all lines that are not matched by
@@ -1908,7 +1907,8 @@ def extract(cls, text, evaluate=True, ignore_nonconforming_lines=True):
                             value = ' '.join(components[:-1])
                         else:
                             units = ''
-                        value = eval(value, CONSTANTS, quantities)
+                        predefined = ChainMap(quantities, _active_constants, CONSTANTS)
+                        value = eval(value, None, predefined)
                         quantity = cls(value, units=units, name=qname, desc=desc)
                     else:
                         raise