Here's as close as I've been able to get to the desired output (using a row for department number each time a new department's people are added to the table): NOTE: I was not able to find a way to change the column span of a cell, so I had to add blanks to fill up the expected number of cells or the row cannot be added.

+---------------------------+
|       Customer Info       |
+--------------+------+-----+
|      No      | Name | Age |
+--------------+------+-----+
| DEPARTMENT 1 |      |     |
|      1       | abc  |  41 |
|      2       | def  |  29 |
| DEPARTMENT 2 |      |     |
|      1       | pqr  |  21 |
|      2       | xyz  |  37 |
+--------------+------+-----+

With the following code:

from prettytable import PrettyTable

departments = {
    "1": {
        "abc": {
            "age": 41
        },
        "def": {
            "age": 29
        }
    },
    "2": {
        "pqr": {
            "age": 21
        },
        "xyz": {
            "age": 37
        }
    }
}

table_fields = ['No', 'Name', 'Age']
pt = PrettyTable(table_fields)
pt.padding_width = 1
pt.title = 'Customer Info'
person_no = 1
for department, people in departments.items():
    pt.add_row(["DEPARTMENT {}".format(department), "", ""])
    for name, person in people.items():
        pt.add_row([person_no, name, person["age"]])
        person_no += 1
    person_no = 1
print(pt)

I hope this is enough to meet your needs, and if you or anyone else knows how to specify column span for a cell, I would love to know!

Possible solution in case of number of rows in "y" should be less or equal to number of rows in "x". Or you can just switch the "x" "y".

z = PrettyTable()
z_rows = []
counter = 0
for i in x.rows:
    i.extend(y.rows[counter])
    counter += 1
    z_rows.append(i)

field = []
field.extend(x.field_names)
field.extend(y.field_names)

z.field_names = field
z.add_rows(z_rows)

Result would look like this:

• remember: y.rows <= x.rows