How to Fix: TypeError: unsupported operand type(s) for -: ‘str’ and ‘int’

This TypeError is the cousin of TypeError: unsupported operand type(s) for +. It means: “You tried to use a math operator (like -, *, /) on data types that don’t support it.”

You cannot subtract a number from a word.

# WRONG — input() returns "10" as a string, subtraction needs an integer
total_score = 100
penalty = input("Enter penalty: ")   # user types 10 — penalty is "10"
final = total_score - penalty        # TypeError: unsupported operand type(s) for -: 'int' and 'str'

# WRONG — same error fires on *, /, ** between mismatched types
price = "25.99"
discount = 5
sale_price = price - discount        # TypeError: unsupported operand type(s) for -: 'str' and 'int'

# RIGHT — convert at the point of assignment, before math runs
penalty = int(input("Enter penalty: "))
final = total_score - penalty        # works — both are integers

# RIGHT — float() for decimal strings
price = float("25.99")
sale_price = price - discount        # works — both are numbers

# RIGHT — production pattern: wrap conversion in try/except
try:
    penalty = int(input("Enter penalty: "))
    final = total_score - penalty
    print(f"Final score: {final}")
except ValueError:
    print("Enter a whole number.")

The Cause

The most common cause is, again, input(). The input() function always gives you a string. Problem Code:

total_score = 100
penalty_str = input("Enter penalty points: ") # User types '10'

# 'total_score' is an INT (100)
# 'penalty_str' is a STR ("10")
final_score = total_score - penalty_str 
# CRASH! TypeError: unsupported operand type(s) for -: 'int' and 'str'

The Fix: Convert Your Types

You must make sure all your variables are the same, numeric type before you do math. Use int() or float() to convert the string.

total_score = 100
penalty_str = input("Enter penalty points: ")

try:
    # THE FIX: Convert the string "10" to the number 10
    penalty_int = int(penalty_str)
    
    final_score = total_score - penalty_int
    print(f"Final score: {final_score}")

except ValueError:
    print("That was not a valid number!")

This applies to all math operators: * (multiplication), / (division), and ** (exponent).

TypeError: unsupported operand type(s) for -: ‘str’ and ‘int’ — The Type Contract Every Math Operator Enforces

TypeError: unsupported operand type(s) for -: ‘str’ and ‘int’ fires on -, *, /, //, %, and ** equally. Every math operator in Python requires both operands to be numeric. No operator silently converts a string — Python never guesses your intent.

The + operator is the one exception that confuses this issue. “10” + 5 fires TypeError because + between str and int is ambiguous. But “10” + “5” produces “105” because string + string does concatenation. This asymmetry is why type-checking at the assignment point matters more than checking at the operator.

Two sources deliver strings where math expects numbers.

input() — unconditionally. Wrap every input() that feeds math with int() or float() at the exact line where the value enters your code. Never let a raw input() result reach a -, *, /, or ** operator.

External data. CSV files, JSON APIs, database results, and environment variables all return strings by default. A column named “price” with value “25.99” is the string “25.99” until you call float(). Convert at the load step — not at the calculation step three functions later when the crash is harder to trace.

The one pattern that handles both sources and both int and float inputs cleanly:

def to_number(value: str) -> float:
try:
return float(value)
except (ValueError, TypeError):
return 0.0 # or raise, or return None — choose based on your pipeline

Apply to_number() to any value coming from input(), a CSV cell, or a JSON field before it touches a math operator. One function, every source covered, no TypeError at runtime.