Number pyramid pattern in Python (nested loops)

Row i of the pyramid holds the numbers 1 to i, with a space between them. Spaces in front of each row push it to the centre.

The program

python

rows = int(input("Number of rows: "))

for i in range(1, rows + 1):
    spaces = " " * (rows - i)
    numbers = " ".join(str(n) for n in range(1, i + 1))
    print(spaces + numbers)

Typed in this example: 5. Press Run to type your own.

output

Number of rows: 5
    1
   1 2
  1 2 3
 1 2 3 4
1 2 3 4 5

How it works

" ".join(...) puts one space between the numbers and none after the last one, so no line ends with a space.
Row i is 2 * i - 1 characters wide (i digits and i - 1 spaces). Each row is two characters wider than the one above, so one space less in front keeps it centred.
The top row needs rows - 1 spaces in front, and the bottom row needs none.
With 10 rows or more, the two-digit numbers make the rows wider and the shape is no longer even.

For the same shape with stars, see the star triangle pattern.

A palindrome pyramid

Each row counts up to i and back down again, so it reads the same from both ends:

python

rows = 5

for i in range(1, rows + 1):
    up = "".join(str(n) for n in range(1, i + 1))
    down = up[-2::-1]
    print(" " * (rows - i) + up + down)

output

up is the counting-up part, such as "123".
up[-2::-1] starts at the second-to-last character and steps backwards to the start, which gives "21". The middle digit is not repeated.
On row 1, up is "1" and the slice is empty, so the row is just 1.
This version works for up to 9 rows, because each number must be one digit.