TypeError: bad operand type for unary +:’str'” [Solved]

Introduction

The error, “TypeError: bad operand type for unary +: ‘str'” is common in Python when you try to use the unary plus operator (`+`) directly with a string (`str`) operand, which is not allowed in Python. In this case, (`+`) cannot be used with non-numeric value, such as a string.

To fix the error, you should convert the value to numeric representation, such as an integer or float using the respective `int()` or `float()` functions. Or if you want to concatenate two strings, you may use `+` on two strings or use `f-strings` (formatted string literals) for a more elegant solution.

Example Code 1

Here’s is a simple example of using `+` directly with a string.

a = "18";
b = +a;
print(b);

Output:

In this example, we use the unary plus operator (`+`) with a variable, `a` with has a string value “18”. This will prompt the error, “TypeError: bad operand type for unary +: ‘str'” because Python doesn’t allow using unary plus operator with a string.

Now, we use `int()` function and try again.

a = "18";
b = +int(a); #Use `int()` to parse `a` as an integer
print(b);

Output:

18

Here, we did use `int()` function to parse the string value, “18” of variable `a` into an integer value. As a result, the value assignment to variable `b` is success because using unary plus operator (`+`) with a numeric value is valid.

Example Code 2

In this example, we’ll try using two string values.

firstName = "Sally";
lastName = "Ling";
print(firstName + +lastName) # Use `+` twice

Output:

Here, the error occurs because the unary plus operator (`+`) is mistakenly used twice in the expression `print(firstName + +lastName)`

Now, we remove one of the `+`:

firstName = "Sally";
lastName = "Ling";
print(firstName + lastName) # Use `+` as normal string concatenation operator

Output:

SallyLing

As a result, we should always be aware of the use of `+` when dealing with string values.

Solution

There’re several ways to solve the error, “TypeError: bad operand type for unary +: ‘str'”. Let’s look at each of them:

1. Convert `string` value to numeric value using `int()` or `float()` functions

You can convert the string to a numeric data type using the `int()` or `float()` functions. This ensures that the unary plus operator (`+`) can work with the numeric value as expected:

# Convert a string to an `integer` before using the unary plus operator
num_str = "42"
num = +int(num_str)
print(num)

# Convert a string to an `float` before using the unary plus operator
float_str = "1.4352"
_float = +float(float_str)
print(_float)

Output:

Here, we have two string variables, `num_str` containing the value “42” and `float_str` containing the value “1.4352”. We use the `int()` and `float()` function to convert “42” and “1.4352” to an integer and a float and assign them to the variables, `num` and `_float` respectively. Then, we use the unary plus operator (`+`) on the numeric variables, `num` and `_float`, which is valid.

2. Check the operand before performing the operation

Besides, you can use conditional statements to ensure that the operand is of a numeric data type before applying the unary plus operator (`+`):

# Check the data type before using the `+`
value = "John Dong"
if isinstance(value, (int, float)):
    result = +value
    print(result)
else:
    print("Error: Operand is not a numeric type.")

Output:

In this example, we have a variable `value` containing the value “John Dong”, which is a string. We use the `isinstance() function to check if the value is of type `int` or `float`. Since the value is not a numeric type, we display a customized error message. Hence, we can effectively avoid using the unary plus operator (`+`) on a string.

3. Check the assignment for string concatenation

name = "John"
#! Error: TypeError: bad operand type for unary +: 'str'
message = "Hello, " +  +name
print(message)

If the intention is to concatenate strings, make sure you are using the correct assignment for string concatenation. The unary plus operator (`+`) is not the correct choice for this operation.

# Correctly concatenate strings using the `+` operator
name = "John"
message = "Hello, " + name
print(message)

Output:

Here, we want to create a greeting message with the variable `name`. We use the `+` operator to concatenate the string “Hello, ” and the value of the `name` variable, resulting in the desired output.

4. Use `f-strings`

Another way of string concatenation to avoid this error is using `f-strings` (formatted string literals) to embed expressions within the string. This is to ensure a more concise and readable way to format strings:

# Using `f-strings`` for string formatting
name = "Andrew Lim"
age = 22
greeting = f"Hello, my name is {name} and I am {age} years old."
print(greeting)

Output:

In this example, we use an f-string to embed the values of the `name` and `age` variables within the `greeting` message. The expressions `{name}` and `{age}` are replaced with their respective values, resulting in the correctly formatted output.

5. Use `print()` correctly

name = "John"
#! Error: TypeError: bad operand type for unary +: 'str'
print("Hello, " + +name)

Ensuring that you’re not mistakenly using the unary plus operator (`+`) before the `print()` function can avoid the error as well.

# Correct usage of print() function
name = "John"
# Option 1
print("Hello, " + name)
# Option 2
print("Hello,", name)

Output:

In this example, we use the `print()` function with the correct string concatenation using the `+` operator. The `print()` function displays the greeting message without any issues.

Practices to Avoid the Error in the Future

1. Validate Input Types: Implement checks to confirm that the input values are of the expected types (e.g., string, integer, or float) before performing any operations on them. This can be achieved using `isinstance()` function or other validation techniques.
2. Type Conversion: When dealing with user inputs or data that might be of different types, use proper type conversion methods like `int()` or `float()` to explicitly convert strings to integers or floats, respectively, before using them in arithmetic or mathematical operations.
3. Code Review: Conduct thoroughly code reviews through your codespace constantly to realise any potential errors and fix them at earlier stage.

Conclusion

Understanding and avoiding the “TypeError: bad operand type for unary +: ‘str'” error is crucial when working with a mix of string, integer, and double data types in Python. By following the solutions provided in this article, including converting string values to numeric types, checking operands before performing operations, using proper assignment for string concatenation, correctly using f-strings, and being cautious with print() statements, you can handle the error effectively.

Additionally, adopting best practices such as validating input types, performing type conversions, and conducting code reviews will not only help you overcome this specific error but also improve the overall robustness and reliability of your Python code. By applying these practices, you can write cleaner, more efficient code, and reduce the likelihood of encountering type-related errors in your future Python projects. Happy coding!