How to Resolve "TypeError: must be str, not int" in Python

When working in Python, you might encounter a TypeError: must be str, not int or the very similar TypeError: can only concatenate str (not "int") to str. Both errors occur when you try to perform an operation that exclusively requires a string, but you provide an integer instead. The most common cause is attempting to join a string and an integer using the + operator.

Because Python is a strongly-typed language, it does not automatically convert the integer into a string for you in this context. This guide will explain why this error happens and walk you through the standard and modern ways to fix it, with a strong recommendation for using f-strings.

Understanding the Error: Strong Typing and Concatenation

In Python, the + operator has two different meanings depending on the context:

• For numbers, it performs addition.
• For strings, it performs concatenation (joining them together).

When you try to use + with a mix of a string and an integer, Python doesn't know which operation you intend. Should it try to add them (which is nonsensical) or join them? Rather than guessing, it raises a TypeError. You must be explicit about your intent by converting the integer to a string first.

Reproducing the TypeError

The error is most commonly triggered when trying to build a string by concatenating a string literal with an integer variable.

Example of code causing the error:

temp_f = 42

# Incorrect: Trying to concatenate a string with an integer.
message = "Today's temperature is: " + temp_f + " F"
print(message)

Output:

Traceback (most recent call last):
  File "main.py", line 4, in <module>
    message = "Today's temperature is: " + temp_f + " F"
TypeError: can only concatenate str (not "int") to str

note

While the error message here is can only concatenate str (not "int") to str, other operations like writing to a file (file.write(42)) will produce the must be str, not int message. The solution for both is the same: convert the integer to a string.

Solution 1: Explicit Conversion with str()

The most direct fix is to wrap your integer variable with the built-in str() function. This explicitly converts the integer to its string representation before the concatenation occurs.

Solution:

temp_f = 42

# ✅ Correct: Convert the integer to a string before concatenating.
message = "Today's temperature is: " + str(temp_f) + " F"
print(message)

Output:

Today's temperature is: 42 F

Solution 2: Using f-Strings (Recommended Best Practice)

Introduced in Python 3.6, f-strings (formatted string literals) are the modern, clean, and highly readable way to embed expressions inside string literals. Python handles the conversion for you automatically and efficiently.

Solution:

temp_f = 42

# ✅ Correct: Use an f-string to embed the variable directly.
# The 'f' prefix tells Python to format the string.
message = f"Today's temperature is: {temp_f} F"
print(message)

Output:

Today's temperature is: 42 F

note

This is the preferred method in modern Python for its clarity and performance.

Solution 3: Using the str.format() Method

Before f-strings, the .format() method was the standard way to format strings. It is still widely used and is more powerful than simple + concatenation.

Solution:

temp_f = 42

# ✅ Correct: Use the .format() method with a placeholder {}.
message = "Today's temperature is: {} F".format(temp_f)
print(message)

Output:

Today's temperature is: 42 F

A Note on the print() Function

You might have noticed that you can pass integers and strings to the print() function separated by commas without an error.

temp_f = 42
print("Today's temperature is:", temp_f, "F")

Output:

Today's temperature is: 42 F

This works because you are passing multiple, separate arguments to print(). The print() function is designed to automatically convert each argument to a string and join them with a space by default. This is different from creating a single string through concatenation with + before passing it to print.

Conclusion

If you need to...	The best solution is...	Example
Combine strings and numbers (Modern Python 3.6+)	Use an f-string.	f"Value: {my_int}"
Combine strings and numbers (Older Python)	Use the str.format() method.	"Value: {}".format(my_int)
Perform a quick fix for concatenation	Explicitly convert with str().	"Value: " + str(my_int)

The TypeError: must be str, not int is a fundamental error related to Python's strong typing system. By using modern string formatting tools like f-strings, you can write code that is not only correct but also significantly more readable and maintainable.