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

What this message means

The + operator does one thing per type pair. String + string concatenates. Number + number adds. String + number is not defined for any combination — Python refuses to guess whether you wanted "Age: 25" or 7, and raises this TypeError instead.

Read the message as a sentence about the *left* operand: the left side was a str, Python tried to concatenate onto it, and the right side (not "int") was the wrong type. When the left side is the number, the same bug produces a different wording — unsupported operand type(s) for +.

Traceback (most recent call last):
  File "main.py", line 1, in <module>
    print("Age: " + 25)
          ~~~~~~~~^^~~^
TypeError: can only concatenate str (not "int") to str

A real example

name = "Aarav"
age = 21

greeting = "Hi " + name + ", you are " + age + " today"
print(greeting)

Four + signs, one of them mixing a string with age. The fix that keeps this style is str(age) — but notice how noisy the corrected line already looks. That noise is the real argument for f-strings, below.

greeting = "Hi " + name + ", you are " + str(age) + " today"   # works, but loud

The fixes, best first

1. Use an f-string. f"Hi {name}, you are {age} today" — any type interpolates, the code reads like the output, and there is no conversion to forget.
2. Wrap the number in str(). str(age) when you must build the string with +.
3. Pass them to print separately. print("Age:", age) — the comma inserts a space and prints any type.
4. For lists of mixed items, " ".join(str(x) for x in items) converts every element first, because join refuses non-strings too.

age = 21
print(f"Age: {age}")        # f-string — best
print("Age: " + str(age))   # str() — fine
print("Age:", age)          # comma — also fine

The input() trap

input() always returns a string, even when the user types digits. So weight + 5 — where weight came from input() — is string + int and raises this exact error. The reverse mistake, int(input()) + " kg", produces the sibling message instead.

weight = input("Weight: ")   # user types 70 — weight is "70"
total = weight + 5           # TypeError: can only concatenate str (not "int") to str

total = int(weight) + 5      # fix: convert once, at the boundary

The habit that prevents the whole family of bugs: convert at the boundary — the moment data enters your program — and keep numbers as numbers inside it.

Why Python refuses to auto-convert

Languages like JavaScript pick a rule and live with it: there "5" + 3 becomes "53". Python chose the opposite trade-off: explicit is better than implicit. A silent conversion would turn typos like zip_code + 1 into "3020" + "1" and hide the bug. The TypeError is annoying for one afternoon and protective for the rest of your career.

Related messages worth knowing

• TypeError: the parent guide — all five famous wordings decoded on one page.
• TypeError: unsupported operand types for + — the same bug when the number is on the left of the +.
• ValueError: invalid literal for int() — what happens when the str you convert is not a valid number.

Fix it interactively

These free problems are all this exact bug wearing different clothes — fix them in the browser and the reflex becomes automatic:

• Mixing text and numbers in print easy
• String concatenation with + easy
• Converting text that isn't a number medium