f-string formatting: display number sign?

Question

Basic question about python f-strings, but couldn't find out the answer: how to force sign display of a float or integer number? i.e. what f-string makes 3 displayed as +3?

Answer 1

From Docs:

• Format Specification Mini-Language(Emphasis mine):

Option	Meaning
'+'	indicates that a sign should be used for both positive as well as negative numbers.
'-'	indicates that a sign should be used only for negative numbers (this is the default behavior).

Example from docs:

>>> '{:+f}; {:+f}'.format(3.14, -3.14)  # show it always
'+3.140000; -3.140000'
>>> '{:-f}; {:-f}'.format(3.14, -3.14)  # show only the minus -- same as '{:f}; {:f}'
'3.140000; -3.140000'
>>> '{:+} {:+}'.format(10, -10)
'+10 -10'

Above examples using f-strings:

>>> f'{3.14:+f}; {-3.14:+f}'
'+3.140000; -3.140000'
>>> f'{3.14:-f}; {-3.14:-f}'
'3.140000; -3.140000'
>>> f'{10:+} {-10:+}'
'+10 -10'

One caveat while printing 0 as 0 is neither positive nor negative. In python, +0 = -0 = 0.

>>> f'{0:+} {-0:+}'
'+0 +0'
>>> f'{0.0:+} {-0.0:+}'
'+0.0 -0.0'

0.0 and -0.0 are different objects¹.

• 0 in Computer Science(Emphasis mine):

In some computer hardware signed number representations, zero has two distinct representations, a positive one grouped with the positive numbers and a negative one grouped with the negatives; this kind of dual representation is known as signed zero, with the latter form sometimes called negative zero.

---

_{1. Negative 0 in Python. Also check out Signed Zero (-0)}

Answer 2

You can use :+ in f-string

number=3
print(f"{number:+}")

Output +3

Answer 3

Fastest solution: f"{['', '+'][number>0]}{number}"

numbers = [+3, -3]

for number in numbers:
    print(f"{['', '+'][number>0]}{number}")

Result:

+3
-3

EDIT: Small time analysis:

import time

numbers = [+3, -3] * 1000000

t0 = time.perf_counter()
[print(f"{number:+}", end="") for number in numbers]
t1 = time.perf_counter()
[print(f"{number:+.2f}", end="") for number in numbers]
t2 = time.perf_counter()
[print(f"{['', '+'][number>0]}{number}", end="") for number in numbers]
t3 = time.perf_counter()
print("\n" * 50)
print("""number:+ : """ + str(round(t1-t0, 2)) + "s")
print("""number:+.2f : """ + str(round(t2-t1, 2)) + "s")
print("""['', '+'][number>0] : """ + str(round(t3-t2, 2)) + "s")

Result:

number:+ : 1.43s
number:+.2f : 1.98s
['', '+'][number>0] : 1.23s

It looks like I have the fastest solution for integers.

Answer 4

You can add a sign with an f-string using f"{x:+}", where x is the int/float variable you need to add the sign to. For more information about the syntax, you can refer to the documentation.

Answer 5

use if statement if x > 0: .. "" else: .