如何修复 TypeError: can only concatenate list (not "str") to list？

Question

我正在使用以下函数来存储对目标列的预测。

#define target and Id column
target = 'is_promoted'
IDcol = 'employee_id'

from sklearn.model_selection import cross_val_score
from sklearn.metrics import f1_score 

def modelfit(alg, dtrain, dtest, pred, target, IDcol, filename):
    alg.fit(dtrain[pred],dtrain[target])
    
    #predict training set
    dtrain_pred = alg.predict(dtrain[pred])
    
    #perform cross validation
    cv_score = cross_val_score(alg, dtrain[pred],dtrain[target],cv=20)
    cv_score = np.sqrt(np.abs(cv_score))
    
    print('\nmodel report')
    print(f1_score(dtrain[target],dtrain_pred))
    print("CV Score : Mean - %.4g | Std - %.4g | Min - %.4g | Max - %.4g" % (np.mean(cv_score),
                                                                             np.std(cv_score),
                                                                             np.min(cv_score),np.max(cv_score)))
    
    #predict on testing data                                                
    dtest[target] = alg.predict(dtest[pred])
    
                                                                             
    #export submission file                                
    IDcol.append(target)
    submission =pd.DataFrame({x: dtest[x] for x in IDcol})
    
    submission.to_csv(filename,index=False)

from sklearn.linear_model import LogisticRegression
pred = [x for x in train.columns if x not in [target]+IDcol]

alg1=LogisticRegression()
modelfit(alg1, train, test, pred, target, IDcol, 'sub_log.csv')

但得到 TypeError 消息，如下所示：

有人可以解释为什么会发生这种情况以及如何解决吗？