TypeError: unsupported operand type(s) for +: 'int' and 'str'

Python · updated 2026-11-03

Raised when an operator gets types it can't combine. The message names the operator and both types, in order: the left operand first. 'NoneType' in the message means one side was None.

Traceback (most recent call last):
  File "/app/cart.py", line 14, in total
    subtotal = subtotal + item["qty"] * item["price"]
               ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Quick fix

Read the two types in the message. 'str' means a number arrived as text (form data, CSV, JSON or env vars): convert it with int() or Decimal() where it enters the program. 'NoneType' means a value was missing: find out why, or give it a default before the math.

Note the operator and the two types in the message, left then right.
Print type() of each operand just before the line to see which one is wrong.
Convert input once at the boundary (request parsing, CSV read, settings load), not inside every calculation.
For money, use Decimal(str_value) instead of float.
For None, decide whether it is a bug upstream or a valid "missing" that should default to 0.

python

from decimal import Decimal

def parse_item(raw: dict) -> dict:
    """Convert once, where the data comes in."""
    return {
        "qty": int(raw["qty"]),
        "price": Decimal(str(raw["price"])),
    }

items = [parse_item(r) for r in rows]
subtotal = sum((i["qty"] * i["price"] for i in items), Decimal("0"))

Why it happens

Numbers that arrived as strings

Form fields, query strings, CSV cells, environment variables and some JSON APIs deliver numbers as text. "3" + 1 fails; the conversion was skipped somewhere upstream.

A None from a missing value

dict.get() without a default, a function that returns nothing on one branch, or an empty database column gives None, and None + 1 raises this error with 'NoneType' in the message.

Concatenating text with a number

Building a message with "Total: " + total fails when total is a number. Use an f-string instead: f"Total: {total}".

Mixing incompatible numeric types

Decimal + float raises this error, as does a datetime + an int. Keep money in Decimal throughout and use timedelta for date arithmetic.

Other fixes

Add type hints and run mypy or pyright: most of these errors show up as type mismatches before the code runs.
Validate request bodies with pydantic so numeric fields arrive as numbers.
sum() on a list of strings fails the same way: convert the items, or pass start='' to join text (better: ''.join()).

Same error, other wording

TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
TypeError: unsupported operand type(s) for -: 'str' and 'int'
TypeError: can only concatenate str (not "int") to str

Let your coding agent catch the next one

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> Find the 'unsupported operand type' errors in production, show me which values had the wrong type and where they entered the program, and convert them at that boundary instead of at the calculation.

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