Pandas concat() Examples

Pandas concat() method is used to concatenate pandas objects such as DataFrames and Series. We can pass various parameters to change the behavior of the concatenation operation.

1. Pandas concat() Syntax

The concat() method syntax is:

concat(objs, axis=0, join='outer', join_axes=None, ignore_index=False,
           keys=None, levels=None, names=None, verify_integrity=False,
           sort=None, copy=True)

• objs: a sequence of pandas objects to concatenate.
• join: optional parameter to define how to handle the indexes on the other axis. The valid values are ‘inner’ and ‘outer’.
• join_axes: deprecated in version 0.25.0.
• ignore_index: if True, the indexes from the source objects will be ignored and a sequence of indexes from 0,1,2…n will be assigned to the result.
• keys: a sequence to add an identifier to the result indexes. It’s helpful in marking the source objects in the output.
• levels: a sequence to specify the unique levels to create multiindex.
• names: names for the levels in the resulting hierarchical index.
• verify_integrity: Check whether the new concatenated axis contains duplicates. It’s an expensive operation.
• sort: Sort non-concatenation axis if it is not already aligned when join is ‘outer’. Added in version 0.23.0
• copy: if False, don’t copy data unnecessarily.

Recommended Reading: Python Pandas Tutorial

2. Pandas concat() Example

Let’s look at a simple example to concatenate two DataFrame objects.

import pandas

d1 = {"Name": ["Pankaj", "Lisa"], "ID": [1, 2]}
d2 = {"Name": "David", "ID": 3}

df1 = pandas.DataFrame(d1, index={1, 2})
df2 = pandas.DataFrame(d2, index={3})

print('********\n', df1)
print('********\n', df2)

df3 = pandas.concat([df1, df2])

print('********\n', df3)

Output:

********
      Name  ID
1  Pankaj   1
2    Lisa   2
********
     Name  ID
3  David   3
********
      Name  ID
1  Pankaj   1
2    Lisa   2
3   David   3

Notice that the concatenation is performed row-wise i.e. 0-axis. Also, the indexes from the source DataFrame objects are preserved in the output.

3. Concatenating Along Column i.e. 1-axis

d1 = {"Name": ["Pankaj", "Lisa"], "ID": [1, 2]}
d2 = {"Role": ["Admin", "Editor"]}

df1 = pandas.DataFrame(d1, index={1, 2})
df2 = pandas.DataFrame(d2, index={1, 2})

df3 = pandas.concat([df1, df2], axis=1)
print('********\n', df3)

Output:

********
      Name  ID    Role
1  Pankaj   1   Admin
2    Lisa   2  Editor

The concatenation along column makes sense when the source objects contain different kinds of data of an object.

4. Assigning Keys to the Concatenated DataFrame Indexes

d1 = {"Name": ["Pankaj", "Lisa"], "ID": [1, 2]}
d2 = {"Name": "David", "ID": 3}

df1 = pandas.DataFrame(d1, index={1, 2})
df2 = pandas.DataFrame(d2, index={3})

df3 = pandas.concat([df1, df2], keys=["DF1", "DF2"])
print('********\n', df3)

Output:

********
          Name  ID
DF1 1  Pankaj   1
    2    Lisa   2
DF2 3   David   3

5. Ignore Source DataFrame Objects in Concatenation

d1 = {"Name": ["Pankaj", "Lisa"], "ID": [1, 2]}
d2 = {"Name": "David", "ID": 3}

df1 = pandas.DataFrame(d1, index={10, 20})
df2 = pandas.DataFrame(d2, index={30})

df3 = pandas.concat([df1, df2], ignore_index=True)
print('********\n', df3)

Output:

********
      Name  ID
0  Pankaj   1
1    Lisa   2
2   David   3

This is useful when the indexes in the source objects don’t make much sense. So we can ignore them and assign the default indexes to the output DataFrame.

6. References

• pandas.concat() API Doc