f-string

f-string is the latest Python syntax to perform string formatting - They are called f-strings because you need to prefix a string with the letter f in order to get an f-string.

“F-strings provide a way to embed expressions inside string literals, using a minimal syntax. It should be noted that an f-string is really an expression evaluated at run time, not a constant value. In Python source code, an f-string is a literal string, prefixed with f, which contains expressions inside braces. The expressions are replaced with their values.” Source

The letter f also indicates that these strings are used for formatting.

  • f-string has been available since Python 3.6.
  • It is more readable, concise, faster and error-free than traditional string formatting.

Jupyter Lab file

f-string.ipynb

  • https://docs.python.org/3/reference/lexical_analysis.html#f-strings
  • https://www.python.org/dev/peps/pep-0498/

Interpolation

String interpolation is the most commonly used f-string feature.

You just need to do enclose the value or variable in curly braces { } and start with f

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name = 'Eric'
age = 24

print(f'Hello, {name}. You are {age}')

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Hello, Eric. You are 24

Variable names

You can get get both a variable’s value and its name.

This can be very useful when debugging, and can be easily done by putting an equal sign {=} in curly braces after calling the variable.

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name = 'Eric'
age = 24

print(f'Hello, {name=}. You are {age=}')

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Hello, name='Eric'. You are age=24

Mathematical operations

It is also possible to preform math operations on f-strings.

You can put a mathematical expression inside the curly braces and add an equal sign, then you get the expression and its result.

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num_val = 10

print(f'{num_val * 2 = }')

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num_val * 2 = 20

Number formatting

f-strings are a simple and practical way for formatting a string.

You just add a : (colon) followed by a format specifier.

  • s - String format, this is the default type for strings
  • d - Decimal Integer, outputs the number in base 10
  • n - Number, the same as d except that it uses the current locale setting
  • e - Exponent notation, scientific notation using the letter ‘e’, default precision is 6
  • f - Fixed-point notation. Default precision is 6.
  • % - Percentage, multiplies the number by 100, displays in fixed (‘f’) format, followed by a percent sign
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# Excamples
price_val = 1366.1265
discount = 0.22

# .2f
print(f'{price_val:.2f}')

# ,.2f - 
print(f'{price_val:,.2f}')

# .2n
print(f'{price_val:.1n}')

# .0%
print(f'{discount:.0%}')


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1366.13
1,366.13
1e+03
22%

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# Format pi by changing precision
from math import pi

for i in range(1, 7):
    print(f'{pi:.{i}f}')

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3.1
3.14
3.142
3.1416
3.14159
3.141593

Padding numbers with zeros

You can pad any number with zeros by using f-strings. A well-known example is to append leading zeros to ID numbers. The purpose here is to have the numbers with the same length by using leading zeros.

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f'{variable:0{width}}'

width is used to specify the total number of digits of a number after leading zeros.

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# 10 step countdown
for i in range(10, -1, -1):
    print(f'{i:02}')

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# Find the longest id and pad with 0 in front of the numbers
product_ids = [93, 123456789, 5332493, 32641, 15279535]

longest_product_id = len(max(map(str, product_ids), key=len))

for product_id in product_ids:
    print(f'{product_id:0{longest_product_id}}')

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000000093
123456789
005332493
000032641
015279535

Date time formatting

f-strings also support the formatting of datetime.

Dates are formatted the same way as numbers, using format specifiers.

  • %d - day
  • %m - month
  • %Y - year
  • %H - hours
  • %M - minutes
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from datetime import datetime;

date_val = datetime.now()

print(date_val)

print(f'{date_val:%d-%m-%Y}')

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2021-12-25 19:26:09.276564
25-12-2021

Align Strings

With f-string you can control the alignment of a string.

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f'{variable:{aligment}{width}}'

A variable name is followed by : (colon). Then, alignment is specified by one of these symbols:

  • Right alignment: <
  • Center alignment: ^
  • Left alignment: >

Lastly, decide the total width of the line.

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align = ['left', 'center', 'right']

print(f'{align[0]:<25}')
print(f'{align[1]:^25}')
print(f'{align[2]:>25}')

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left                     
         center          
                    right

Formatet output

Loop and formating, tabs and calucation 

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print(f'Number\t\tSquare\t\tCube')
for x in range(1, 11):
    x = float(x)
    print(f'{x:5.2f}\t\t{x*x:6.2f}\t\t{x*x*x:8.2f}')

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Number		Square		Cube
 1.00		  1.00		    1.00
 2.00		  4.00		    8.00
 3.00		  9.00		   27.00
 4.00		 16.00		   64.00
 5.00		 25.00		  125.00
 6.00		 36.00		  216.00
 7.00		 49.00		  343.00
 8.00		 64.00		  512.00
 9.00		 81.00		  729.00
10.00		100.00		 1000.00

Use of strings, decimals, and floats, as well as tabs for a print report. 

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APPLES = .50
BREAD = 1.50
CHEESE = 2.25

numApples = 3
numBread = 4
numCheese = 2

prcApples = 3 * APPLES
prcBread = 4 * BREAD
prcCheese = 2 * CHEESE

strApples = 'Apples'
strBread = 'Bread'
strCheese = 'Cheese'

total = prcBread + prcBread + prcApples

print(f'{"My List"}')
print(f'{"="*40}')
print(f'{strApples}\t{numApples:10d}\t\tkr.{prcApples:>5.2f}')
print(f'{strBread}\t{numBread:10d}\t\tkr.{prcBread:>5.2f}')
print(f'{strCheese}\t{numCheese:10d}\t\tkr.{prcCheese:>5.2f}')
print(f'{"Total:"}\t\t\t\tkr.{total:>4.2f}')

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My List
========================================
Apples	         3		kr. 1.50
Bread	         4		kr. 6.00
Cheese	         2		kr. 4.50
Total:				kr.13.50

Example of how to draw stairs on your console using f-strings. 

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step = '|__'

for i in range(1, 10):
    print(f'{step:>{3*i}}')

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|__
   |__
      |__
         |__
            |__
               |__
                  |__
                     |__
                        |__