Match one column's dataframe to another dataframe with a series of columns and extracting the columns header - Python

Use melt to flatten your second dataframe then merge it with your first dataframe:

>>> df1.merge(df2.melt(var_name='label', value_name='numbers'), on='numbers')
    numbers         label
0       100  construction
1       101  construction
2       102  construction
3       103  construction
4       200        fields
5       201        fields
6       202        fields
7       203        fields
8       300        plates
9       301        plates
10      302        plates
11      303        plates

After melt, your second dataframe looks like:

>>> df2.melt(var_name='label', value_name='numbers')
           label  numbers
0   construction      100
1   construction      101
2   construction      102
3   construction      103
4         fields      200
5         fields      201
6         fields      202
7         fields      203
8         plates      300
9         plates      301
10        plates      302
11        plates      303

Another way with map:

df1['label'] = df1['numbers'].map(df2.melt().set_index('value').squeeze())
print(df)

# Output
    numbers         label
0       100  construction
1       101  construction
2       102  construction
3       103  construction
4       200        fields
5       201        fields
6       202        fields
7       203        fields
8       300        plates
9       301        plates
10      302        plates
11      303        plates