I defined a class like below:

from sklearn.base import BaseEstimator, TransformerMixin
from sklearn.preprocessing import StandardScaler

class CustomScaler(BaseEstimator, TransformerMixin):
    def __init__(self, columns, copy=True, with_mean=True, with_std=True):
        self.scaler = StandardScaler(copy, with_mean, with_std)
        self.columns = columns
        self.mean_ = None
        self.var_ = None
    def fit(self, X, y = None):
        self.scaler.fit(X[self.columns], y)
        self.mean_ = np.mean(X[self.columns])
        self.var_ = np.var(X[self.columns])
        return self
    def transform(self, X, y=None, copy=None):
        init_col_order = X.columns
        X_scaled = pd.DataFrame(self.scaler.transform(X[self.columns]), columns=self.columns)
        X_not_scaled = X.loc[:, ~X.columns.isin(self.columns)]
        return pd.concat([X_not_scaled, X_scaled], axis = 1)[init_col_order]

When I try to create an instance from it:

columns_to_scale = ['col_A', 'col_B']
scaler = CustomScaler(columns_to_scale)

I got this error:

init() takes 1 positional argument but 4 were given

What's the problem? And how to solve it?


  • Python: 3.9.1
  • Scikit-learn: 1.0.2

1 Answer 1


As the error mentions, you are passing multiple positional arguments whereas the __init__ method of StandardScaler only takes in one. The arguments you are trying to pass to the scaler should instead be passed as keywords arguments:

self.scaler = StandardScaler(copy=copy, with_mean=with_mean, with_std=with_std)

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