query関数の変数の代入法について

前提

pythonのquery関数について質問です。

以下のような出力を望んでいます。変数には@をつける方法が一般的と思っていましたが、これは数値のみ対応ということでしょうか。または他の問題が生じているのでしょうか、、、

出力において正解のコード

Python
1import pandas as pd
2
3df = pd.DataFrame({
4    'city': ['osaka', 'osaka', 'osaka', 'osaka', 'tokyo', 'tokyo', 'tokyo'],
5    'food': ['apple', 'orange', 'banana', 'banana', 'apple', 'apple', 'banana'],
6    'price': [100, 200, 250, 300, 150, 200, 400],
7    'quantity': [1, 2, 3, 4, 5, 6, 7]
8})
9
10df.query("price > 200")

該当のソースコード

Python
1import pandas as pd
2
3df = pd.DataFrame({
4    'city': ['osaka', 'osaka', 'osaka', 'osaka', 'tokyo', 'tokyo', 'tokyo'],
5    'food': ['apple', 'orange', 'banana', 'banana', 'apple', 'apple', 'banana'],
6    'price': [100, 200, 250, 300, 150, 200, 400],
7    'quantity': [1, 2, 3, 4, 5, 6, 7]
8})
9a = 'price'
10df.query("@a > 200")

発生しているエラー

TypeError                                 Traceback (most recent call last)
/var/folders/gr/t18r0f2934x_rfqfsphwz7180000gn/T/ipykernel_31526/1700137686.py in <module>
      6 })
      7 a = 'price'
----> 8 df.query("@a > 200")

~/opt/anaconda3/lib/python3.9/site-packages/pandas/core/frame.py in query(self, expr, inplace, **kwargs)
   4109         kwargs["level"] = kwargs.pop("level", 0) + 1
   4110         kwargs["target"] = None
-> 4111         res = self.eval(expr, **kwargs)
   4112 
   4113         try:

~/opt/anaconda3/lib/python3.9/site-packages/pandas/core/frame.py in eval(self, expr, inplace, **kwargs)
   4238         kwargs["resolvers"] = tuple(kwargs.get("resolvers", ())) + resolvers
   4239 
-> 4240         return _eval(expr, inplace=inplace, **kwargs)
   4241 
   4242     def select_dtypes(self, include=None, exclude=None) -> DataFrame:

~/opt/anaconda3/lib/python3.9/site-packages/pandas/core/computation/eval.py in eval(expr, parser, engine, truediv, local_dict, global_dict, resolvers, level, target, inplace)
    354         eng = ENGINES[engine]
    355         eng_inst = eng(parsed_expr)
--> 356         ret = eng_inst.evaluate()
    357 
    358         if parsed_expr.assigner is None:

~/opt/anaconda3/lib/python3.9/site-packages/pandas/core/computation/engines.py in evaluate(self)
     75         """
     76         if not self._is_aligned:
---> 77             self.result_type, self.aligned_axes = align_terms(self.expr.terms)
     78 
     79         # make sure no names in resolvers and locals/globals clash

~/opt/anaconda3/lib/python3.9/site-packages/pandas/core/computation/align.py in align_terms(terms)
    157     # if all resolved variables are numeric scalars
    158     if all(term.is_scalar for term in terms):
--> 159         return result_type_many(*(term.value for term in terms)).type, None
    160 
    161     # perform the main alignment

~/opt/anaconda3/lib/python3.9/site-packages/pandas/core/computation/common.py in result_type_many(*arrays_and_dtypes)
     21     """
     22     try:
---> 23         return np.result_type(*arrays_and_dtypes)
     24     except ValueError:
     25         # we have > NPY_MAXARGS terms in our expression

<__array_function__ internals> in result_type(*args, **kwargs)

TypeError: data type 'price' not understood

拙い文章ですいません！！よろしくお願いします。