What are f-strings? | Interview Question

Q: What are f-strings?

**Target Audience:** Beginners formatting their first output; candidates who want the format mini-language, the CPython detail and the version history. **Difficulty:** Easy **Python version:** f-strings exist since Python 3.6. Examples 1 and 2 checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); Example 3 needs 3.12+ and was checked on 3.12.3 and 3.13.16. > **How to read this doc:** Every idea is explained in plain words first, then tagged `📌 Interview term:` with the precise name. All output is from real runs and is quoted in full in the **Verified** section. ## 1. Why This Even Matters — A Story First Think of a form letter: "Dear ____, your order of ____ items will arrive on ____." Older Python made you write the letter with blanks, then hand over the values in a separate list, in the right order: `"Dear %s, your order of %d items" % (name, qty)`. Mix up the order and the letter is wrong. An f-string writes the values *into the blanks themselves*:

The problem

Question presented to candidate: "What are f-strings, and why do most Python teams prefer them over % formatting and str.format? Show me a few format specs you use day to day — and tell me one place where you would not use an f-string."

What a strong answer should cover:

An f-string (formatted string literal, PEP 498, Python 3.6) is a string literal with an f prefix whose {...} fields hold Python expressions, evaluated at run time where the literal is written.
Field anatomy: {expression!conversion:format_spec} — !r/!s/!a pick repr/str/ascii, the spec after : is the format mini-language (.2f, ,, >10, 08b, %), and = (3.8) prints the expression text for debugging.
Each field calls format(value, spec), i.e. type(value).__format__, so types like datetime and your own classes define their own specs; braces are escaped by doubling.
f-strings are eager: evaluated once, immediately — not a reusable template, and wasted work in logging calls that are filtered out (use %s arguments there).
Version facts: PEP 701 (3.12) allows reusing the same quotes, backslashes and comments inside the braces; t-strings (PEP 750, 3.14) are the template version. Never build SQL or shell commands with f-strings.

Code / implementation expected: Yes — a few lines showing expressions, a numeric spec, !r and the = debug form.

fstrings

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Solution

Target Audience: Beginners formatting their first output; candidates who want the format mini-language, the CPython detail and the version history. Difficulty: Easy Python version: f-strings exist since Python 3.6. Examples 1 and 2 checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); Example 3 needs 3.12+ and was checked on 3.12.3 and 3.13.16.

How to read this doc: Every idea is explained in plain words first, then tagged 📌 Interview term: with the precise name. All output is from real runs and is quoted in full in the Verified section.

1. Why This Even Matters — A Story First

Think of a form letter: "Dear ____, your order of ____ items will arrive on ____." Older Python made you write the letter with blanks, then hand over the values in a separate list, in the right order: "Dear %s, your order of %d items" % (name, qty). Mi

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11 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonExpressions, format specs, conversions, the = debug form and nested fields

Practice

PythonEager evaluation, bytecode, __format__, logging cost and the 3.11 limits

Practice

PythonPython 3.12+ f-strings (PEP 701): same quotes, backslashes, comments

# Python 3.12+ only (PEP 701). On 3.11 this whole file is a SyntaxError.
songs = ["Take Me Back", "Rain", "Lucy"]
user = {"name": "Ada", "langs": ["Python", "Go"]}

# 1) Reuse the same quote character inside the braces.
print(f"Hello {user["name"]}, you know {", ".join(user["langs"])}")

# 2) Backslashes are allowed in the expression part.
print(f"Playlist:\n{"\n".join(f"  - {s}" for s in songs)}")

# 3) An expression may span lines and hold comments.
total = f"{
    len(songs)  # how many songs
    * 3         # minutes each
} minutes"
print(total)

# 4) Nesting depth is no longer limited by the four quote styles.
print(f"{f"{f"{f"{f"{1 + 1}"}"}"}"}")

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