The single most common type bug AI writes you

Run the code on the right. Read the error. We'll come back to it.

TypeError: can only concatenate str (not "int") to str

This is the type bug you'll see most often in AI-generated Python, and the reason is structural — Cursor pulls a value from a form, an env var, a JSON field, or a CLI argument, and it forgets that all of those things arrive as strings, not numbers. Then the next line tries to do math on the string, and Python refuses.

What just happened

Line 1: count = "0". The "0" has quotes. It's a string. The character zero, not the number zero.

Line 2: count + 1. Python sees a string on the left and a number on the right and stops. It will not silently guess what you meant. JavaScript would. PHP would. Python won't. That's a feature.

The mental model

Python types do not auto-convert across the divide between text and numbers. A few examples of what Python will and won't do:

"7" + "3"       # works → "73" — string concatenation
7 + 3           # works → 10
"7" + 3         # TypeError — won't mix
"7" * 3         # works → "777" — string repetition (yes, really)
True + 1        # works → 2 — bool secretly counts as int (don't lean on it)
None + 1        # TypeError — None has no math at all

If you remember nothing else: the boundary between str and int is a wall, and AI keeps walking into it.

How to fix it

Two functions, both built into Python, both used constantly:

• int("7") → the integer 7.
• str(7) → the string "7".

Pick the side you want to be on, cast the other one to match. The fixed version of the broken code is:

count = "0"
count = int(count) + 1
print(count)

Now both operands are ints. Math works. The script prints 1.

Where this hits real AI code

Three places, ranked by how often you'll see them:

1. Reading from input() or CLI args. Both return str. Cursor sometimes remembers to cast, sometimes forgets. When it forgets, your program crashes on the first arithmetic line.
2. Reading from os.environ. Same deal — env vars are always strings, even when they look like numbers. os.environ["PORT"] is "8080", not 8080.
3. F-strings hide it. f"count: {count}" works whether count is a string or an int — Python casts to str automatically inside the braces. So a buggy variable can travel through ten print statements undetected, and then crash on the first line that does math with it.

The None side of the same coin

None is its own trap. AI writes:

score = lookup_user("maya")    # might return None
total = score + 10             # crashes if score is None

The first line might return a real score, but if the user isn't found, it returns None. The second line then tries None + 10 and crashes with the same family of TypeError. We'll fix one of these in step 6.

What to take into the rest of the lesson

When you read AI code and see a variable being used in two ways — once as text, once as a number — pause. That's the spot where the bug lives. Trace back to the assignment and ask, what type did this start as? Cast at the boundary, not in the middle.