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

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    print("Age: " + age)
TypeError: can only concatenate str (not "int") to str

Quick fix: Use an f-string: f"Age: {age}", or convert explicitly: "Age: " + str(age). If you meant arithmetic, convert the string to a number instead: int(s) + 1.

What it means

+ between two strings concatenates them. If the left side is a str and the right side is an int, float, list, NoneType, etc., Python raises this error rather than guessing what you meant. The type in quotes tells you what the right-hand operand was:

can only concatenate str (not "int") to str
can only concatenate str (not "NoneType") to str
can only concatenate str (not "list") to str

If the number is on the left (5 + "a") you get unsupported operand type(s) for +: 'int' and 'str' instead — same problem.

Fix 1: use an f-string (recommended)

age = 30
print(f"Age: {age}")
msg = f"{name} has {count} items costing {price:.2f}"

f-strings call format() on each value, so any type works, and you get formatting options like :.2f, :, and :>10.

Fix 2: convert with str()

print("Age: " + str(age))

Or let print do it: print("Age:", age) (comma-separated arguments are converted and joined with spaces).

Fix 3: you wanted arithmetic, not text

Values from input(), files, sys.argv, environment variables and web forms are always strings:

n = input("Number: ")
print(n + 1)        # TypeError
print(int(n) + 1)   # correct

Fix 4: joining a list

parts = ["a", "b"]
"prefix: " + parts            # (not "list")
"prefix: " + ", ".join(parts)  # 'prefix: a, b'

nums = [1, 2, 3]
", ".join(nums)                     # TypeError: expected str instance, int found
", ".join(str(n) for n in nums)     # '1, 2, 3'

Fix 5: (not "NoneType")

A variable you thought was a string is None — often a function with no return, dict.get() on a missing key, or os.environ.get("VAR") when the variable isn't set:

home = os.environ.get("APP_HOME")
path = home + "/config"     # (not "NoneType")

Give a default or fail clearly:

home = os.environ.get("APP_HOME", "/opt/app")
# or
home = os.environ["APP_HOME"]   # KeyError names the missing variable

For filesystem paths, prefer pathlib: Path(home) / "config".

Fix 6: building strings in a loop

out = ""
for i in range(3):
    out += str(i)        # not out += i

For many pieces, collecting into a list and calling "".join(...) at the end is faster.

Last updated 2026-11-03.