Edge cases for string expression evaluation

Due to the way we evaluate expressions and look up columns we have issues with column names containing spaces (or presumably other characters which might make them invalid python variables names):

>>> m = gp.Model()
>>> df = pd.DataFrame({"a": [1, 2, 3], "b": [4,5,6]})
>>> df = df.grb.pd_add_vars(m, name="ab cd")
>>> m.update()
>>> df
   a  b                  ab cd
0  1  4  <gurobi.Var ab cd[0]>
1  2  5  <gurobi.Var ab cd[1]>
2  3  6  <gurobi.Var ab cd[2]>
>>> df.grb.pd_add_constrs(m, "ab cd", gp.GRB.EQUAL, 1, name="c1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/bowly/workspaces/gurobipy-pandas/src/gurobipy_pandas/accessors.py", line 173, in pd_add_constrs
    return self._add_constrs_by_args(model, lhs, sense, rhs, name=name)
  File "/Users/bowly/workspaces/gurobipy-pandas/src/gurobipy_pandas/accessors.py", line 177, in _add_constrs_by_args
    c = [
  File "/Users/bowly/workspaces/gurobipy-pandas/src/gurobipy_pandas/accessors.py", line 180, in <listcomp>
    lhs=getattr(row, lhs) if lhs in self._obj.columns else lhs,
AttributeError: 'Pandas' object has no attribute 'ab cd'
>>> df.grb.pd_add_constrs(m, "`ab cd` == 1", name="c2")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/bowly/workspaces/gurobipy-pandas/src/gurobipy_pandas/accessors.py", line 171, in pd_add_constrs
    return self._add_constrs_by_expression(model, lhs, name=name)
  File "/Users/bowly/workspaces/gurobipy-pandas/src/gurobipy_pandas/accessors.py", line 207, in _add_constrs_by_expression
    "lhs": eval(lhs, None, self._obj),
  File "<string>", line 1
    `ab cd` 
    ^
SyntaxError: invalid syntax

The first case should definitely work and is easy to fix. The second case should work in theory since df.eval("ab cd", engine='python') works, but we can't use pandas's eval method as it doesn't play well with operators on object dtype arrays.

Gurobi / gurobipy-pandas

Edge cases for string expression evaluation #43