Master Python f-strings: Syntax, Tricks, and Real-World Examples

1. f‑string Introduction

Python 3.6 introduced a new string‑formatting method called f‑string . Compared with %s and str.format(), f‑strings are more intuitive, faster, and simpler to use.

f‑strings build on the concepts of format(), so learning %s and format() first is helpful.

2. Common f‑string Usage

2.1 Basic Usage

Expressions are placed inside curly braces {} and are evaluated at runtime.

name = "Huang Wei"
f"Hello, my name is {name}"  # 'Hello, my name is Huang Wei'
num = 2
f"I have {num} apples"          # 'I have 2 apples'
price = 95.5
f"He has {price}$"               # 'He has 95.5$'

2.2 Expression Evaluation & Function Calls

f"They have {2+5*2} apples"          # 'They have 12 apples'
name = "Huang Wei"
f"my name is {name.lower()}"      # 'my name is huang wei'
import math
f"Π的值为{math.pi}"                # 'Π的值为3.141592653589793'

2.3 Using Lambda in f‑strings

aa = 123.456
f"{(lambda x: x*5-2)(aa):.2f}"   # '615.28'
bb = 8
cc = 2
f"{(lambda x,y: x+y)(bb,cc)}"    # '10'

Note: the first parentheses enclose the lambda expression, the second passes arguments.

2.4 Quote Handling

Quotes inside the braces must not conflict with the outer string delimiters. Use different combinations of single, double, triple quotes as needed.

f'I am {"Huang Wei"}'   # 'I am Huang Wei'
f'''I am {'Huang Wei'}'''   # 'I am Huang Wei'
f"I am {'Huang Wei'}"   # 'I am Huang Wei'

Backslashes cannot appear inside the expression part.

2.5 Escaping Braces

To display a literal brace, double it: {{ or }}.

f"5{{apples}}"   # '5{apples}'
f"{{5}}{apples}" # '{5}apples'

2.6 Padding

Use >, <, or ^ for left, right, or center padding. A fill character can be specified before the alignment symbol.

name = "Huang Wei"
f"{name:>20}"   # '           Huang Wei'
f"{name:<20}"   # 'Huang Wei           '
f"{name:^20}"   # '   Huang Wei   '
f"{name:_>20}"  # '___________Huang Wei'

2.7 Numeric Sign Specifiers

a = 12
b = -25
f"{a:+}"   # '+12'
f"{b:+}"   # '-25'
f"{a:-}"   # '12'
f"{b:-}"   # '-25'
f"{a: }"   # ' 12'
f"{b: }"   # '-25'

2.8 Width & Precision

a = 123.456
f"{a:10}"      # '   123.456'
f"{a:010}"     # '000123.456'
f"{a:8.1f}"    # '   123.5'
f"{a:8.2f}"    # '  123.46'
f"{a:.2f}"     # '123.46'
f"{a:2f}"      # '123.456000'

2.9 Truncation & Padding Combination

a = "Hello"
f"{a:10.3}"    # 'Hel       '
f"{a:_>10.3}"   # '_______Hel'
f"{a:_<10.3}"   # 'Hel_______'

2.10 Date/Time Formatting

from datetime import *
a = date.today()
f"{a:%Y-%m-%d}"   # '2020-02-01'

3. Comprehensive Example: Multiplication Table

Printing the 9×9 multiplication table using three formatting methods.

3.1 Using % Operator

for i in range(1,10):
    for j in range(1,i+1):
        print("%s*%s=%s" % (j,i,j*i), end=" ")
    print("
")

3.2 Using format()

for i in range(1,10):
    for j in range(1,i+1):
        print("{0}*{1}={2}".format(j,i,j*i), end=" ")
    print("
")

3.3 Using f‑strings

for i in range(1,10):
    for j in range(1,i+1):
        print(f"{j}*{i}={j*i}", end=" ")
    print("
")

All three approaches produce the same formatted multiplication table.