Python TypeError: What It Means and How to Fix the Common Ones

Python stops with a TypeError:

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

A TypeError means an operation received a value of a type it can't work with. Python is telling you the what precisely — the trick is knowing the handful of shapes these take. Here are the ones you'll actually hit.

'can only concatenate str (not "int") to str'

You tried to + a string and a number:

age = 25
print("Age: " + age)          # ❌ str + int

+ won't mix types. Convert, or use an f-string:

print("Age: " + str(age))     # explicit conversion
print(f"Age: {age}")          # cleaner — f-string handles it

The mirror image, TypeError: unsupported operand type(s) for +: 'int' and 'str', is the same problem from the other side — usually reading a number from input (which is always a str) without int(...).

'object is not callable'

TypeError: 'list' object is not callable

You put () after something that isn't a function — often because a variable shadowed a built-in:

list = [1, 2, 3]
nums = list(range(5))         # ❌ 'list' is now your variable, not the type

Rename the variable (items, not list). The same happens if you name a variable str, dict, sum, etc. — never shadow built-ins.

'missing N required positional arguments' / 'takes N but M given'

The function call doesn't match its signature:

def greet(name, greeting): ...
greet("Ada")                  # ❌ missing 'greeting'

Pass the right number of arguments. A very common variant in classes is forgetting self:

class Dog:
    def bark():                # ❌ no self
        print("woof")
Dog().bark()                   # TypeError: bark() takes 0 args but 1 given

Add self: def bark(self):.

'NoneType object is not subscriptable' / 'not iterable'

You indexed or looped over None — usually a function that returned nothing:

data = my_dict.get("key")     # returns None if key missing
first = data[0]               # ❌ None is not subscriptable

A function with no return gives back None. Check the value before using it, and make sure functions return what you think they do.

How to read any TypeError

The last line names the operation and the offending type. Work backwards from the traceback's bottom frame — that's your line — and ask: "what type did this actually get, versus what it needed?" A quick print(type(x)) above the failing line settles it instantly.

The checklist

1. can only concatenate str → convert with str() or use an f-string.
2. not callable → you shadowed a built-in or added () to a non-function.
3. missing/takes N arguments → fix the call; in methods, don't forget self.
4. NoneType is not subscriptable/iterable → something returned None; guard it.
5. Print type(x) above the failing line to confirm the real type.

Frequently Asked Questions

What does 'can only concatenate str (not "int") to str' mean?

You tried to join a string and an integer with +. Python won't auto-convert. Wrap the number in str() ("Age: " + str(age)) or use an f-string (f"Age: {age}").

Why do I get 'object is not callable'?

You put parentheses after something that isn't a function — commonly a variable that shadows a built-in like list or str. Rename the variable so the built-in name is free again.

What causes 'NoneType' object errors?

A value is None when you used it as if it weren't — indexing it, iterating it, or accessing an attribute. Usually a function returned None (no return, or a .get() miss). Check the value before you use it.