191. Python’s F-strings Internals

1

Basic f-string Syntax

• The simplest way to use an f-string.

name = "Alice"
age = 25
print(f"My name is {name} and I am {age} years old.")

🔍 Under the hood:

• At runtime, Python replaces {name} and {age} with their values and evaluates them dynamically.

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2

f-strings Use str.format() Internally

• f-strings are syntactic sugar for str.format().

name = "Bob"
age = 30

# Equivalent f-string and str.format()
f_string = f"My name is {name} and I am {age} years old."
format_string = "My name is {} and I am {} years old.".format(name, age)

print(f_string)  
print(format_string)

🔍 Under the hood:

• f"My name is {name} and I am {age} years old." is roughly compiled to:

  "My name is " + str(name) + " and I am " + str(age) + " years old."

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3. f-strings Evaluate Expressions

• f-strings can evaluate any expression inside {}.

x = 10
y = 5

print(f"The sum of {x} and {y} is {x + y}")
print(f"Half of {x} is {x / 2:.2f}")  # Formatting with 2 decimal places

🔍 Under the hood:

• The expressions {x + y} and {x / 2:.2f} are evaluated before string formatting.

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4. f-strings Use __format__() Internally

• Python calls the __format__() method on objects inside f-strings.

class Person:
    def __init__(self, name):
        self.name = name

    def __format__(self, format_spec):
        return self.name.upper() if format_spec == "upper" else self.name

p = Person("Charlie")

print(f"Hello, {p:upper}!")  # Uses custom __format__() method

🔍 Under the hood:

• f"Hello, {p:upper}!" calls p.__format__("upper"), which customizes the output.

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5. f-strings are Faster than str.format()

• Since f-strings are evaluated at compile-time, they are faster than format().

import timeit

name = "Daniel"
age = 40

f_time = timeit.timeit(lambda: f"My name is {name} and I am {age} years old.", number=1000000)
format_time = timeit.timeit(lambda: "My name is {} and I am {} years old.".format(name, age), number=1000000)

print(f"f-string time: {f_time}")
print(f"format() time: {format_time}")

🔍 Under the hood:

• f-strings are compiled to efficient bytecode, avoiding extra function calls like format().

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