Python: Can not understand why I am getting the error: Can not concatenate 'int' and 'str' object

This error occurs when Python tries to concatenate (combine) an integer and a string using the + operator. Python cannot automatically convert between these data types during concatenation operations.

Common Cause of the Error

The error typically happens when using string formatting with %d placeholder. If you write %d % i + 1, Python interprets this as trying to add the integer 1 to the formatted string result.

Incorrect Code (Causes Error)

i = 5
# This causes the error - operator precedence issue
print("Num %d" % i + 1)

Solution: Use Parentheses

Wrap the arithmetic operation in parentheses to ensure it happens before string formatting ?

i = 5
print("Num %d" % (i + 1))

Num 6

Alternative Solutions

Using f-strings (Recommended)

i = 5
print(f"Num {i + 1}")

Num 6

Using .format() Method

i = 5
print("Num {}".format(i + 1))

Num 6

Manual Type Conversion

i = 5
print("Num " + str(i + 1))

Num 6

Understanding Operator Precedence

The root cause is operator precedence. The % operator has higher precedence than +, so Python evaluates "Num %d" % i first, then tries to add 1 to the resulting string.

Conclusion

Use parentheses around arithmetic operations in string formatting to avoid concatenation errors. Modern Python code should prefer f-strings for cleaner, more readable string formatting.