Calling epsilon throws invalid name error

[mmikhasenko]

Just started with the packages (thanks for that), and quickly stumbled upon several issues.
Let me drop it here. Maybe I can cover the issues to tests once it is resolved

lorenz = TensorIndexType("lorenz","L",4)
@indices lorenz μ ν σ ρ
ϵ = lorenz.epsilon
ϵ(μ,ν,σ,ρ)

gives the error:

PyError ($(Expr(:escape, :(ccall(#= C:\Users\Mikhasenko\.julia\packages\PyCall\L0fLP\src\pyfncall.jl:43 =# @pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, pyargsptr, kw))))) <class 'ValueError'>

ValueError('invalid name')
File "\.julia\conda\3\lib\site-packages\sympy\core\basic.py", line 953, in subs
rv = rv._subs(old, new, **kwargs)
File "\.julia\conda\3\lib\site-packages\sympy\core\cache.py", line 72, in wrapper
retval = cfunc(*args, **kwargs)
File "\.julia\conda\3\lib\site-packages\sympy\core\basic.py", line 1067, in _subs
rv = fallback(self, old, new)
File "C:\Users\Mikhasenko\.julia\conda\3\lib\site-packages\sympy\core\basic.py", line 1044, in fallback
rv = self.func(*args)
File "\.julia\conda\3\lib\site-packages\sympy\tensor\tensor.py", line 1735, in __new__
raise ValueError("invalid name")

pyerr_check@exception.jl:62[inlined]
pyerr_check@exception.jl:66[inlined]
_handle_error(::String)@exception.jl:83
macro expansion@exception.jl:97[inlined]
#107@pyfncall.jl:43[inlined]
disable_sigint@c.jl:458[inlined]
__pycall!@pyfncall.jl:42[inlined]
_pycall!(::PyCall.PyObject, ::PyCall.PyObject, ::Tuple{SymPy.Sym, SymbolicTensors.TensorIndex}, ::Int64, ::Ptr{Nothing})@pyfncall.jl:29
_pycall!(::PyCall.PyObject, ::PyCall.PyObject, ::Tuple{SymPy.Sym, SymbolicTensors.TensorIndex}, ::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})@pyfncall.jl:11
var"#_#114"(::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, ::PyCall.PyObject, ::SymPy.Sym, ::Vararg{Any})@pyfncall.jl:86
(::PyCall.PyObject)(::SymPy.Sym, ::Vararg{Any})@pyfncall.jl:86
(::SymPy.Sym)(::SymbolicTensors.TensorIndex, ::Vararg{SymbolicTensors.TensorIndex})@utils.jl:54
top-level scope@Local: 1[inlined]

[rjrosati]

Thanks for opening all of these. The package is not very user friendly yet, sorry about that.
I never implemented epsilon from sympy.tensor.tensor, mostly because I don't like how it treats the indices of epsilon and delta (the Kronecker delta) (see #4). They're not real tensors, but raising and lowering indices with the metric can produce objects like this:

julia> contract_metric(lorenz.delta(-μ,-ν) * lorenz.metric(ν,σ), lorenz.metric)
lorenz.delta(-μ,σ)

I usually think of a delta as always representing an identity matrix, but these delta objects with mixed indices don't. The same issue affects epsilon.
Anyway, I could still implement epsilon for now, keeping the same behavior.

[mmikhasenko]

Thanks for pointing where the problem might be. My project with tensor is paused now, but I hope to come back to it in the future.