easiest way to include square brackets in a parameter's name

[Kima wari]

The issue is the following:

```

from lmfit import Parameters

params = Parameters()
params.add(name="binwidth[s]")

---------------------------------------------------------------------------

KeyError Traceback (most recent call last)

/tmp/ipykernel_74236/312279628.py in <cell line: 2>() 

    1 params = Parameters()

    ----> 2 params.add(name="binwidth[s]")

/gpfs/exfel/sw/software/mambaforge/22.11/envs/202401/lib/python3.10/site-packages/lmfit/parameter.py in add(self, name, value, vary, min, max, expr, brute_step) 

    432 self.__setitem__(name.name, name) 

    433 else:

    --> 434 self.__setitem__(name, Parameter(value=value, name=name, vary=vary, 

    435 min=min, max=max, expr=expr, 

    436 brute_step=brute_step))

/gpfs/exfel/sw/software/mambaforge/22.11/envs/202401/lib/python3.10/site-packages/lmfit/parameter.py in __setitem__(self, key, par) 

    148 """Set items of Parameters object.""" 

    149 if key not in self and not valid_symbol_name(key):

    --> 150 raise KeyError(f"'{key}' is not a valid Parameters name") 

    151 if par is not None and not isinstance(par, Parameter): 

    152 raise ValueError(f"'{par}' is not a Parameter")

KeyError: "'binwidth[s]' is not a valid Parameters name"

```

As the `Parameters` class uses the asteval.Interpreter module by default, I was wondering how one can add square brackets to a parameter name, as it can be useful to put units on the name.

Cheers,
David

[Matt Newville]

> KeyError: "'binwidth[s]' is not a valid Parameters name"

That is the correct message:  "binwidth[s]" is not a valid Parameter name. 

Parameter names must match the requirements for Python variable names (without `.` or `[](){}` as those are really operations on the names).   It would be easy to state that as

   [_A-Za-z][_A-Za-z0-9]*

but many unicode symbols (say, greek letters) are also supported, as with Python.  And Python keywords are not allowed.

Basically: "valid Python symbol".

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--Matt Newville <newville at cars.uchicago.edu> 630-327-7411