Python TypeError: unsupported operation between types

What this error means

A TypeError says the operation you requested is not defined for the type of value you gave it — regardless of the value itself. 'Score: ' + 90 fails not because 90 is wrong but because str + int has no meaning in Python; the language refuses to guess whether you wanted 'Score: 90' or arithmetic.

This is the deliberate opposite of JavaScript's silent coercion. Python's philosophy: better a loud crash at the exact line than a quietly wrong value that travels through your program. Because TypeErrors cover every kind of type mismatch, the message wording varies a lot — but each variant names the offending types, and that is the clue that matters.

A real example

score = 90
print("Your score: " + score)

Traceback (most recent call last):
  File "app.py", line 2, in <module>
    print("Your score: " + score)
          ~~~~~~~~~~~~~~~^~~~~~~
TypeError: can only concatenate str (not "int") to str

The fix is stating your intent: print(f"Your score: {score}") converts for display, while int(text) + score would convert for arithmetic.

The message decoder

Five wordings account for most Python TypeErrors — each points at a different mistake:

• can only concatenate str (not "int") to str — mixing text and numbers with +. Use an f-string or convert one side.
• 'int' object is not callable — parentheses after a non-function; usually a variable that shadowed a function (sum = sum(nums)).
• 'int' object is not iterable — a for-loop over a number; for i in 5: should be for i in range(5):.
• 'tuple' object does not support item assignment — writing into an immutable container; you need a list, or a new tuple.
• missing 1 required positional argument — the call supplied fewer arguments than the def line requires; the message names the missing one.

How to debug it

1. Read the type names in the message — they identify both operands, which is half the diagnosis.
2. Find which operand has the "surprising" type, then trace where that value was produced. input() returning str is the classic upstream source.
3. Decide the intent before converting: display (f-string), arithmetic (int()/float()), or a genuinely wrong value that should never have arrived.
4. Fix at the source, not the crash site — converting at the point of creation prevents the same TypeError three lines later.
5. Use print(type(x), repr(x)) when unsure; repr distinguishes 5 from '5' instantly.

Fix it interactively

Each of these free problems is a real TypeError variant, runnable and fixable in your browser:

• str + int in a print statement easy
• list + int instead of append easy
• 'int' object is not callable easy
• 'int' object is not iterable easy
• '<' not supported between str and int medium
• missing required positional argument medium
• tuple item assignment easy

Related errors

• ValueError — right type, wrong value — int('abc') is a ValueError, int + str a TypeError.
• NameError — the name itself is missing, before types even enter the picture.

Full catalogue with AttributeError, ImportError and more: Python errors guide.