Percent formatting is the oldest style and is still common in logging. str.format() is more powerful and supports positional and keyword fields, but it is more verbose. f-strings evaluate expressions at runtime, are compiled into efficient bytecode, and are usually the fastest and most readable for local formatting. The main trade-off is that f-strings are eager: the expression is evaluated immediately, which matters for logging. For dynamic format strings from config or user input, str.format() or a templating engine is often safer.

1
Use f-strings for local, fixed format strings and debugging.
2
Use str.format() when the format template is dynamic or you need positional/keyword substitution.
3
Use %-formatting mainly for compatibility or logging's lazy formatting style.
4
Trade-off: f-strings are eager; logging with f-strings can waste work if the log level is disabled.
5
Common mistake: using f-strings for user-controlled templates, which can be an injection risk if combined with eval-like behavior.
6
Version note: f-strings landed in 3.6; = debug specifier in 3.8; nested quotes improved in 3.12.

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Scenario Questions

0-2 years experience

1You need to format a user message with name and age. Which method?
2Why are f-strings often preferred?

2-5 years experience

1You need dynamic format strings from config. f-string vs str.format?
2You need logging with lazy evaluation. Which formatting?

5-8 years experience

1You need locale-aware formatting. Which method and why?
2You need to format many strings in a hot loop. Which is fastest and why?

8+ years experience

1Design a safe templating system that avoids injection and supports dynamic formats.
2You need to support Python 3.5 and 3.12. Which formatting features are available?

Follow-up Questions

Why can f-strings be worse than lazy logging?
How would you safely format a user-provided template?

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Questions

4 of 14

1What are the common built-in data types in Python?

2what is an array

3Since Python 3.7, dictionaries preserve insertion order. How is this guaranteed internally, and what changed from earlier versions?

4What is the difference between str.format(), %-formatting, and f-strings? What are the performance and readability tradeoffs?

5When would you use collections.deque instead of a list, and why?

6Why are strings immutable in Python, and what performance implications does this have for repeated concatenation in a loop?

7What problem does collections.defaultdict solve, and how does it differ from using dict.setdefault?

8How would you efficiently remove duplicates from a list while preserving order?

9How would you design a Least Recently Used (LRU) cache using Python's built-in data structures?

10How does a Python dictionary achieve average O(1) lookup time internally?

11What is the difference between a list and a tuple, and when would you choose one over the other?

12What are the time complexities of common list operations (indexing, append, insert, pop, search)?

13What is the difference between a set and a frozenset?

14How does Python handle Unicode internally, and what is the difference between str and bytes?

Python

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Control Flow and Functions

Data Structures

Comprehensions & Functional Programming

Iterators, Generators & Decorators

Object-Oriented Programming

Exception Handling & Debugging

Concurrency & Parallelism

Performance & Optimization

Testing

Security

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Type Hinting & Modern Python

System Design & Architecture with Python

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Edge Cases & Tricky Interview Questions

04 / 14

What is the difference between str.format(), %-formatting, and f-strings? What are the performance and readability tradeoffs?

Difficulty: 3/10

Strings, Formatting, Performance

f-strings are fastest and most readable; format and % remain useful for dynamic templates

1
Use f-strings for local, fixed format strings and debugging.
2
Use str.format() when the format template is dynamic or you need positional/keyword substitution.
3
Use %-formatting mainly for compatibility or logging's lazy formatting style.
4
Trade-off: f-strings are eager; logging with f-strings can waste work if the log level is disabled.
5
Common mistake: using f-strings for user-controlled templates, which can be an injection risk if combined with eval-like behavior.
6
Version note: f-strings landed in 3.6; = debug specifier in 3.8; nested quotes improved in 3.12.