I am trying to do some data preprocessing before it feed into machine learning models.
However, I had error of this
'DataFrame' object has no attribute 'dtype'

Before I did this

    if data[i].dtype == 'object':
        data[i] = data[i].str.strip().str.capitalize()

And it didn’t show any error.

When I tried to do

label_encoder = LabelEncoder()
for i in data_test.columns:
    if data_test[i].dtype == 'category':
        data_test[i] = label_encoder.fit_transform(data_test[i])

It showed

AttributeError                            Traceback (most recent call last)
~AppDataLocalTempipykernel_25508395068224.py in ?()
      1 from sklearn.preprocessing import LabelEncoder
      2 label_encoder = LabelEncoder()
      3 for i in data_test.columns:
----> 4     if data_test[i].dtype == 'category':
      5         data_test[i] = label_encoder.fit_transform(data_test[i])

~AppDataLocalanaconda3Libsite-packagespandascoregeneric.py in ?(self, name)
   6200             and name not in self._accessors
   6201             and self._info_axis._can_hold_identifiers_and_holds_name(name)
   6202         ):
   6203             return self[name]
-> 6204         return object.__getattribute__(self, name)

AttributeError: 'DataFrame' object has no attribute 'dtype'

Any help or direction would be really helpful, thank you!