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?
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Are you looking for a solution like a kind of row display? (Without any statement?)Vincent Bénet– Vincent Bénet2021-12-17 10:56:22 +00:00Commented Dec 17, 2021 at 10:56
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5 Answers
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.
Update: From Python 3.11 and above, allows negative floating point zero as positive zero.
The
'z'option coerces negative zero floating-point values to positive zero after rounding to the format precision. This option is only valid for floating-point presentation types.
Example from PEP682:
>>> x = -.00001 >>> f'{x:z.1f}' '0.0' >>> x = decimal.Decimal('-.00001') >>> '{:+z.1f}'.format(x) '+0.0'
1. Negative 0 in Python. Also check out Signed Zero (-0)
Comments
You can use :+ in f-string
number=3
print(f"{number:+}")
Output
+3
Comments
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.
Comments
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.
3 Comments
Christian
A bit cryptic, but I like this solution, using the result of number>0 as the index! Very clever.
ShpielMeister
clever and indeed cryptic. number>0 will evaluate to True or False. Python conveniently hard wires True to 1, and False to 0.
['', '+'][number>0] is thus a subscripted list, and the surrounding {} force evaluation.
Rolf of Saxony
Of note, this can also be used to place parenthesis (or anything else) around a negative number (financial etc) e.g.
f"{['(', ''][n>=0]}{n:3.4}{[')', ''][n>=0]}" -> (-2.455) - I couldn't find any other way to do that.use if statement if x > 0: .. "" else: .
