In sympy, I want to create a constant e.g. to represent the speed of light.
Thus I initialize c with the keyword constant=True.
Later I want to check, if a symbol is a constant or a variable.
Unexpectedly the command c.is_constant() outputs False.
c = Symbol( 'c' , constant=True, real=True )
c.is_real # outputs True as expected.
c.is_constant() # outputs False. This is unexpected !
How can I check, if a symbol is a constant or a variable ?
Instead of creating a Symbol, creating a Quantity object should do the job.
The code example below does what you are trying to do.
from sympy.physics.units.quantities import Quantity
c = Quantity('c')
print(c.is_constant())
Output:
True
In case you need something else, or more customized, you may try doing something similar to what Sympy does for constant values such as e and pi, you can find the source code for that here:
And for more information on why the is_constant method was returning False, check out this
https://docs.sympy.org/latest/modules/core.html#sympy.core.expr.Expr.is_constant
Look what happens when I construct a symbol with various keywords:
In [93]: c = Symbol( 'c' , constant=True, real=True, foobar=True )
In [94]: c._assumptions
Out[94]:
{'constant': True,
'real': True,
'extended_real': True,
'hermitian': True,
'commutative': True,
'imaginary': False,
'finite': True,
'complex': True,
'infinite': False,
'foobar': True}
Those keywords, plus some defaults, are put in a dict. I included your constant, and random choice of my own foobar. Neither has special meaning.
The is_constant method does not look for this assumption value:
def is_constant(self, *wrt, **flags):
if not wrt:
return False
return not self in wrt
We have to study its docs to see what a wrt is.
Specifying an assumption is one thing, actually using it is quite another.