What is the ‘TypeError: can only concatenate str (not “int”) to str’ Error?
This TypeError is a very common error in Python that occurs when you try to concatenate a string with a non-string data type (e.g., an integer, a list) using the + operator.
Python performs strong type checking, so it does not allow implicit concatenation of different types.
The error message clearly states, “You can only concatenate a string with another string (not an ‘integer’ type).”
Main Cause
The cause of this error is singular: attempting to directly append a variable or value of a different type to a string using the + operator.
name = "User"
age = 25
# Error: Directly concatenating a string (str) and an integer (int)
message = "Hello, " + name + ". Your age is " + age + "."
# TypeError: can only concatenate str (not "int") to str
In the code above, the age variable is an integer (int) type.
Python cannot directly add the integer 25 to the string "Hello, User. Your age is ", so it raises a TypeError.
How to Fix It
There are several simple and effective ways to solve this problem.
1. Explicit Type Conversion with str()
The most basic solution is to explicitly convert the non-string data to a string using the str() function.
name = "User"
age = 25
# Convert the integer age to a string using str()
message = "Hello, " + name + ". Your age is " + str(age) + "."
print(message)
# Output: Hello, User. Your age is 25.
str(age) converts the integer 25 to the string "25", so all elements become strings and can be concatenated correctly.
2. Using f-strings (Formatted String Literals)
If you are using Python 3.6 or higher, f-strings are the most modern and recommended method.
By prefixing the string with an f and placing variables directly inside curly braces {}, they are automatically converted to strings, which is very convenient.
name = "User"
age = 25
# Conc cisely format the string using an f-string
message = f"Hello, {name}. Your age is {age}."
print(message)
# Output: Hello, {name}. Your age is {age}.
The code becomes much more concise and readable.
3. Using the str.format() Method
Before the introduction of f-strings, the str.format() method was commonly used.
This method involves placing {} placeholders in the string and passing the variables as arguments to the .format() method.
name = "User"
age = 25
# Using the str.format() method
message = "Hello, {}. Your age is {}.".format(name, age)
print(message)
# Output: Hello, User. Your age is 25.
4. Using a Comma in the print Function
If the goal is simply to print multiple values to the console, you can pass them separated by commas in the print function.
The print function automatically separates each argument with a space and prints them.
name = "User"
age = 25
# Using a comma in the print function automatically converts each type to a string for output
print("Hello,", name, ". Your age is", age, ".")
# Output: Hello, User . Your age is 25 .
However, this method is useful only for printing multiple values, not for creating a single string that includes the variables.
Conclusion
TypeError: can only concatenate str (not "int") to str occurs when you try to concatenate non-string data without converting it to a string.
To solve this problem, it is best to either directly convert the type with the str() function or use string formatting features like f-strings or str.format().
F-strings, in particular, are the most concise and efficient method in modern Python, so it is highly recommended to use them actively.
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