TypeError

Q: What does TypeError: can only concatenate str (not "int") to str mean?

You used + between a str and a number, e.g. 'Total: ' + 5. Python never converts automatically. Use an f-string (f'Total: {n}'), str(n), or print('Total:', n) — print converts each argument for you.

Q: How do I fix 'NoneType' object is not subscriptable (or not iterable / not callable)?

A variable you index, loop over or call is None. The usual sources: a function without a return statement, an in-place method whose result you assigned (x = x.sort()), dict.get() or re.match() finding nothing. Print or check the variable just before the failing line and trace back where it was set to None.

Q: Should I raise TypeError or ValueError?

TypeError when the argument is the wrong kind of object (a str where a number is needed, None, a list where a hashable is needed). ValueError when the type is acceptable but this particular value is not (int('abc'), a negative length, an unknown mode name). int() itself shows the rule: int(None) is a TypeError, int('abc') is a ValueError.

Q: Why does 1 + 'a' give a different message than 'a' + 1?

'a' + 1 is handled by str.__add__, which raises can only concatenate str (not "int") to str. For 1 + 'a', int.__add__ returns NotImplemented, Python then tries str.__radd__, which does not exist, so it reports unsupported operand type(s) for +: 'int' and 'str'.

Q: What does "missing 1 required positional argument" mean?

The function was called with fewer arguments than it has parameters without defaults. When the missing one is 'self', you called a method on the class (Cart.total()) instead of on an instance (Cart().total()), or forgot the parentheses when creating the instance.

The operation does not work on that type at all — the message names the types involved, so read it for the one you did not expect (very often NoneType).

InheritsBaseException›Exception›TypeError

Type / value exceptionPython 3 (all)Live demo

TypeError(*args)

Raised by

'a' + 1, None[0], f(1) for f(a, b), len(5), {[1]: 2}

Message

names the types: unsupported operand type(s) for +: 'int' and 'str'

Quick fix

convert explicitly (str(n), int(s)) or check for None

Watch out

'NoneType' object … = a variable is None, often a method that returns nothing

Demo

Live evaluation

Add two values. Numbers mix freely, str only joins str — and which side the str is on changes the message.

Try:

Inputs

aint | float | strnumbers stay numbers

bint | float | strnumbers stay numbers

Code

'Total: ' + 'ten'

Result

'Total: ten'

Compare str + int with int + str: the same mistake gives two different messages. With the str on the left, str.__add__ refuses with can only concatenate str (not "int") to str; with the int on the left, int.__add__ and str.__radd__ both decline and Python reports unsupported operand type(s). In Handle, None is the wrong type (TypeError) while '4.5' is a str with a bad value (ValueError).

Constructor

Name	Type	Required	Description
*args	object	no	Usually one message string. str(e) is that string; with several arguments it is the repr of the args tuple.

Attributes

Attribute	Type	Meaning
args	tuple	The constructor arguments — normally a 1-tuple holding the message.
__traceback__	traceback	Where it was raised. The last frame is usually the line with the operator or call that got the wrong type.
__notes__	list[str]	Extra lines added with e.add_note(), printed after the message (Python 3.11+).

Common patterns

Convert at the boundary

Text from input(), files, env vars and query strings is always str. Convert it once, where it enters, not at every use.

port = int(os.environ.get('PORT', '8000'))
print('listening on ' + str(port))

Fail early with a clear TypeError

In your own API, check the type up front and name what you got. Accept the widest type that works (numbers.Real, collections.abc.Iterable) instead of one concrete class.

def scale(values, factor):
    if not isinstance(factor, (int, float)):
        raise TypeError(f'factor must be int or float, not {type(factor).__name__}')
    return [v * factor for v in values]

Guard against None

Most 'NoneType' object … errors come from a lookup or function that returned None. Check at the source instead of wrapping every use in try/except.

match = re.search(r'\d+', text)
if match is None:
    raise ValueError(f'no number in {text!r}')
number = int(match.group())

Return NotImplemented from operators

In __add__ and friends, return NotImplemented for types you do not handle; Python then tries the other operand and raises the standard TypeError only if both decline.

class Money:
    def __init__(self, cents):
        self.cents = cents
    def __add__(self, other):
        if not isinstance(other, Money):
            return NotImplemented
        return Money(self.cents + other.cents)

Examples

1. str + int

'Age: ' + 30

Returns

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

2. Fix: f-string or str()

age = 30 f'Age: {age}'

Returns

'Age: 30'

3. Method that returns None

nums = [3, 1, 2].sort() nums[0]

Returns

TypeError: 'NoneType' object is not subscriptable

4. Looping over None

def find_users(): pass # forgot to return for user in find_users(): pass

Returns

TypeError: 'NoneType' object is not iterable

5. Missing argument

def greet(name, greeting): return f'{greeting}, {name}' greet('Ann')

Returns

TypeError: greet() missing 1 required positional argument: 'greeting'

6. Too many arguments

def greet(name): return 'Hi ' + name greet('Ann', 'Bob')

Returns

TypeError: greet() takes 1 positional argument but 2 were given

7. Unhashable dict key

{[1, 2]: 'pair'}

Returns

TypeError: unhashable type: 'list'

8. Sorting mixed types

sorted([3, '10', 2])

Returns

TypeError: '<' not supported between instances of 'str' and 'int'

Pitfalls

1. list.sort() returns None

In-place methods (sort, append, reverse, update) return None. Assigning their result replaces your data with None, and the TypeError appears one line later.

nums = nums.sort()

nums = [3, 1, 2]
nums = nums.sort()
nums[0]

TypeError: 'NoneType' object is not subscriptable

sorted() returns a list

nums = [3, 1, 2]
nums = sorted(nums)
nums[0]

2. Shadowing a built-in

A variable named list, str, dict, sum or input hides the built-in for the rest of the module; calling it later gives 'X' object is not callable.

str = …

str = 'hello'
str(42)

TypeError: 'str' object is not callable

Different name

text = 'hello'
str(42)

'42'

3. input() always returns str

Numbers typed by the user, read from a file or from os.environ arrive as text. Convert before doing arithmetic.

Arithmetic on text

age = '30'  # from input()
age + 1

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

int() first

age = '30'  # from input()
int(age) + 1

4. Calling a method on the class

Calling through the class instead of an instance means nothing is passed for self — Python reports it as a missing argument.

Cart.total()

class Cart:
    def total(self):
        return 0

Cart.total()

TypeError: Cart.total() missing 1 required positional argument: 'self'

Cart().total()

class Cart:
    def total(self):
        return 0

Cart().total()

When to use

Use it

Raising it when a caller passes an argument of the wrong type to your function
Catching it around duck-typed code that may receive unsupported objects
Returning NotImplemented from operator methods so Python raises it for you

Reach for something else

The type is right but the value is bad (negative size, bad string) → ValueError
A required attribute or method is missing on an object → AttributeError
Wrapping big blocks in except TypeError — it also hides real bugs inside

Notes

CPython impl

Objects/abstract.c — binary_op1() tries a.__add__ then b.__radd__; if both return NotImplemented, the "unsupported operand type(s)" message is built from both type names

str on the left

str.__add__ raises its own message first: can only concatenate str (not "int") to str

Catch via

except TypeError, or except Exception. It has no built-in subclasses.

Rule of thumb

Wrong kind of object → TypeError. Right kind, unacceptable value → ValueError.

FAQ

What does TypeError: can only concatenate str (not "int") to str mean? ▾

You used + between a str and a number, e.g. 'Total: ' + 5. Python never converts automatically. Use an f-string (f'Total: {n}'), str(n), or print('Total:', n) — print converts each argument for you.

How do I fix 'NoneType' object is not subscriptable (or not iterable / not callable)? ▾

Should I raise TypeError or ValueError? ▾

Why does 1 + 'a' give a different message than 'a' + 1? ▾

What does "missing 1 required positional argument" mean? ▾

TypeError

Demo

Constructor

Attributes

Common patterns

Examples

Pitfalls

When to use

Notes

Related

FAQ