Python Type Conversion: Why "5" + 5 Fails and How to Fix It

"5" + 5 fails because Python refuses to guess. One side is text, the other is a number, and Python won't decide for you whether you meant "55" or 10. That's not Python being difficult. It's Python protecting you from a wrong answer that looks right.

The error

age = "5"
print(age + 5)

Traceback (most recent call last):
  File "/home/you/project/fail.py", line 2, in <module>
    print(age + 5)
          ~~~~^~~
TypeError: can only concatenate str (not "int") to str

Read the last line. can only concatenate str (not "int") to str means: the left side is text, so + means "join text", and you gave it a number. Python could turn the 5 into "5" and give you "55". It could turn "5" into 5 and give you 10. Both are possible, so it does neither.

Some languages do guess. JavaScript gives you "55". I think Python's choice is better. A crash on line 2 is easier to fix than a wrong total discovered next month.

Where the text comes from

You rarely write "5" yourself. Text that looks like a number comes from:

• input(), which always returns text, even when the person types digits
• files, including CSV files, where every value is text
• command-line arguments in sys.argv
• web forms and many APIs

So the real rule is: anything that came from outside your program is text until you convert it.

The conversion functions

print(int("42") + 1)
print(float("3.5") * 2)
print(str(42) + " items")
print(int(7.9))
print(round(7.9))
print(int("  42\n"))

43
7.0
42 items
7
8
42

Three things to notice:

1. **int() and float() turn text into numbers.** str() turns anything into text.
2. **int(7.9) is 7, not 8.** int() cuts off the decimal part. It doesn't round. If you want rounding, use round().
3. Spaces and a newline at the ends are fine. int() strips them, which helps with values read from files, where each line ends with \n.

When conversion fails

int() only accepts text that really is a whole number:

for text in ["42", "4.5", "4,500", "", "five"]:
    try:
        print(repr(text), "->", int(text))
    except ValueError as error:
        print(repr(text), "-> ValueError:", error)

'42' -> 42
'4.5' -> ValueError: invalid literal for int() with base 10: '4.5'
'4,500' -> ValueError: invalid literal for int() with base 10: '4,500'
'' -> ValueError: invalid literal for int() with base 10: ''
'five' -> ValueError: invalid literal for int() with base 10: 'five'

"4.5" fails because it's not a whole number. If decimals are allowed, use float(). "4,500" fails because of the comma. Remove it first with .replace(",", ""). And "" fails, which is exactly what you get when someone presses Enter without typing anything.

The other trap: bool("False")

This one gives no error at all, which makes it worse:

print(bool("False"))
print(bool(""))
print(bool(0))

True
False
False

bool() doesn't read the word. Any non-empty text is True, including "False" and "0". If you read yes/no or True/False from a file or a person, compare the text instead:

answer = "False"
is_active = answer.strip().lower() in ("true", "yes", "1")
print(is_active)

False

Checking the type when you're not sure

If a value behaves strangely, ask Python what it is:

price = "95.00"
print(type(price))
print(type(float(price)))

<class 'str'>
<class 'float'>

type() is the quickest way to stop guessing. If it says str and you expected a number, you found the bug.

Common mistakes

• Converting too late. Convert once, right where the value comes in. Not in five different places later.
• **Using int() to round.** It cuts. Use round().
• **Using float() for money and expecting exact cents.** It's fine for practice. For real money totals, look up the decimal module.
• **Trusting bool() on text.** Compare the text instead.

What happens if you skip it

If you're lucky, you get the TypeError and fix it in a minute. If you're unlucky, the text never gets added to a number. It gets compared or sorted instead, and "10" < "9" is True because text sorts letter by letter. No crash. Just a wrong order nobody notices.

If a CSV file gives you "10", "9" and "100", and you sort them without converting, which one comes first?