# MatchVariables¶

MatchVariables() ensures that the columns in the test set are identical to those in the train set.

If the test set contains additional columns, they are dropped. Alternatively, if the test set lacks columns that were present in the train set, they will be added with a value determined by the user, for example np.nan.

In [1]:
import numpy as np
import pandas as pd

from feature_engine.preprocessing import MatchVariables

In [2]:
# Load titanic dataset from OpenML

data = data.replace('?', np.nan)
data['cabin'] = data['cabin'].astype(str).str[0]
data['pclass'] = data['pclass'].astype('O')
data['age'] = data['age'].astype('float')
data['fare'] = data['fare'].astype('float')
data['embarked'].fillna('C', inplace=True)
data.drop(
labels=['name', 'ticket', 'boat', 'body', 'home.dest'],
axis=1, inplace=True,
)
return data

In [3]:
data = load_titanic()


Out[3]:
pclass survived sex age sibsp parch fare cabin embarked
0 1 1 female 29.0000 0 0 211.3375 B S
1 1 1 male 0.9167 1 2 151.5500 C S
2 1 0 female 2.0000 1 2 151.5500 C S
3 1 0 male 30.0000 1 2 151.5500 C S
4 1 0 female 25.0000 1 2 151.5500 C S
In [4]:
# separate the dataset into train and test

train = data.iloc[0:1000, :]
test = data.iloc[1000:, :]

train.shape, test.shape

Out[4]:
((1000, 9), (309, 9))
In [5]:
# set up the transformer
match_cols = MatchVariables(missing_values="ignore")

# learn the variables in the train set
match_cols.fit(train)

Out[5]:
MatchVariables(missing_values='ignore')
In [6]:
# the transformer stores the input variables

match_cols.input_features_

Out[6]:
['pclass',
'survived',
'sex',
'age',
'sibsp',
'parch',
'fare',
'cabin',
'embarked']

## 1 - Some columns are missing in the test set¶

In [7]:
# Let's drop some columns in the test set for the demo
test_t = test.drop(["sex", "age"], axis=1)


Out[7]:
pclass survived sibsp parch fare cabin embarked
1000 3 1 0 0 7.7500 n Q
1001 3 1 2 0 23.2500 n Q
1002 3 1 2 0 23.2500 n Q
1003 3 1 2 0 23.2500 n Q
1004 3 1 0 0 7.7875 n Q
In [8]:
# Let's drop some columns in the test set for the demo
test_t = test.drop(["sex", "age"], axis=1)


Out[8]:
pclass survived sibsp parch fare cabin embarked
1000 3 1 0 0 7.7500 n Q
1001 3 1 2 0 23.2500 n Q
1002 3 1 2 0 23.2500 n Q
1003 3 1 2 0 23.2500 n Q
1004 3 1 0 0 7.7875 n Q
In [9]:
# the transformer adds the columns back
test_tt = match_cols.transform(test_t)

print()

The following variables are added to the DataFrame: ['sex', 'age']


Out[9]:
pclass survived sex age sibsp parch fare cabin embarked
1000 3 1 NaN NaN 0 0 7.7500 n Q
1001 3 1 NaN NaN 2 0 23.2500 n Q
1002 3 1 NaN NaN 2 0 23.2500 n Q
1003 3 1 NaN NaN 2 0 23.2500 n Q
1004 3 1 NaN NaN 0 0 7.7875 n Q

Note how the missing columns were added back to the transformed test set, with missing values, in the position (i.e., order) in which they were in the train set.

Similarly, if the test set contained additional columns, those would be removed:

## Test set contains variables not present in train set¶

In [10]:
test_t.loc[:, "new_col1"] = 5
test_t.loc[:, "new_col2"] = "test"


Out[10]:
pclass survived sibsp parch fare cabin embarked new_col1 new_col2
1000 3 1 0 0 7.7500 n Q 5 test
1001 3 1 2 0 23.2500 n Q 5 test
1002 3 1 2 0 23.2500 n Q 5 test
1003 3 1 2 0 23.2500 n Q 5 test
1004 3 1 0 0 7.7875 n Q 5 test
In [11]:
# set up the transformer with different
# fill value
match_cols = MatchVariables(
fill_value=0, missing_values="ignore",
)

# learn the variables in the train set
match_cols.fit(train)

Out[11]:
MatchVariables(fill_value=0, missing_values='ignore')
In [12]:
test_tt = match_cols.transform(test_t)

print()

The following variables are added to the DataFrame: ['sex', 'age']
The following variables are dropped from the DataFrame: ['new_col2', 'new_col1']


Out[12]:
pclass survived sex age sibsp parch fare cabin embarked
1000 3 1 0 0 0 0 7.7500 n Q
1001 3 1 0 0 2 0 23.2500 n Q
1002 3 1 0 0 2 0 23.2500 n Q
1003 3 1 0 0 2 0 23.2500 n Q
1004 3 1 0 0 0 0 7.7875 n Q

Note how the columns that were present in the test set but not in train set were dropped. And now, the missing variables were added back into the dataset with the value 0.