The f-string is the workhorse

Look at any twenty lines of Python Cursor wrote you this week. Count the f-strings. You'll find one every three or four lines. They're how AI builds log messages, error reports, return values, prompt strings, and LLM inputs. They are everywhere.

The syntax is small, but it has four moving parts and AI uses all four. Run the editor on the right and watch all four come out. Then read on.

Part 1: the prefix f

f"hi, {name}"

The f before the opening quote is the entire feature switch. Drop the f and the curly braces stop being magic — they print as literal { and }. Forgetting the f is the single most common f-string typo Cursor makes when it's tired:

print(f"score: {score}")    # → score: 87
print("score: {score}")     # → score: {score}        ← bug

If your output has literal curly braces in it, you forgot the f. Look at the start of the string.

Part 2: the curly braces

Anything inside { } gets evaluated as Python and the result gets substituted into the string. Variables work:

print(f"{name} scored {score}")    # → maya scored 87

But it's not just variables — any expression works:

print(f"next: {score + 1}")        # → next: 88
print(f"upper: {name.upper()}")    # → upper: MAYA

This is huge for reading AI code. When you see f"{user.profile.email}" that's three attribute accesses and a method call all happening inside the braces. Treat the braces as a tiny window into Python.

Part 3: the format spec (the colon trick)

After a colon inside the braces, you can shape how the value renders:

ratio = 0.8333333
print(f"win rate: {ratio:.2f}")    # → win rate: 0.83

The :.2f means "format as a float with 2 decimals." This is the most common spec by far. Two more you'll see:

• :, — comma thousands separator. f"{1000000:,}" → 1,000,000
• :>10 — right-align in a 10-character column.

You don't have to memorize these. When you see a colon inside a brace, recognize it as "render rule here" and look it up if you need to.

Part 4: the = debug shortcut (Python 3.8+)

This one is small but AI uses it constantly when it's debugging:

print(f"{score=}")     # → score=87

The = after the variable name prints both the name and the value. It's a shortcut for print(f"score={score}"). When you see it in generated code, that's a sign Cursor was debugging and forgot to delete the line.

What this means for reading AI code

When you're reading a generated function and you hit a complex f-string, walk it left to right:

1. Confirm the f is there.
2. For each { ... }, identify what expression runs inside.
3. For each :..., identify how the value renders.

Three sub-skills. Same string. That's the whole pattern, repeated hundreds of times across any codebase.

Where AI specifically gets f-strings wrong

• Quotes inside quotes. f"name: {data["name"]}" looks fine but crashes — the inner double quotes close the f-string early. Use data['name'] instead. Cursor has gotten better at this, but it still trips occasionally.
• Nested braces. Two literal braces in a row become one in the output: f"{{not a variable}}" prints {not a variable}. Useful when you're generating code, confusing when you're not.
• Expressions with side effects. Don't put a function call that changes state inside an f-string. AI mostly avoids this, but when it doesn't, the bug is silent and weird.