Python is an inherently flexible programming language. It’s filled with easy ways to get around limitations typically seen in other languages. This is especially true with Python’s relationship to variables.
Python’s data types can be easily assigned, manipulated, and converted. But this flexibility can lead to issues when variables don’t meet the requirements for a particular operation. And the “not all arguments converted during string formatting” Python error offers a perfect demonstration of that principle. But thankfully the solution to this error is fairly simple once you understand the underlying issues.
A Summery of the String Conversion Error
The “not all arguments converted during string formatting” error occurs when Python isn’t able to properly form a string from all of the components passed to it. This typically occurs when people misuse the string formatting syntax. But it can also happen when the Python interpreter runs into a conflict between the modulo operator and string formatting rules. In short, the error essentially just means that your code is trying to create a string out of incompatible pieces.
A More In-Depth Look at the String Conversion Error
Python’s interpreter is usually relatively descriptive when pointing out errors in string formation. For example, x = 5+”6″ would result in an error of “unsupported operand type(s) for +: ‘int’ and ‘str'”. This error points out that the operand types aren’t appropriate for an integer and string. So why aren’t we seeing an equally descriptive explanation with the string conversion error?
The underlying issue with this error comes down to points of confusion with Python’s interpreter. With the previous example, we see a fairly clear-cut issue of two incompatible types. The interpreter is able to determine exactly what’s going on with the attempted operation and raises an appropriate error. But there are situations where this won’t be the case.
One of the most common examples stems from some redundancy in Python’s string formatting syntax. As the language evolved, different formatting methods came into play and rose in popularity. This has led to a situation where the % and {} operators can provide similar functionality when formatting strings. But the two options can only be used in isolation from each other. The use of both in the same operation will raise a conversion error due to confusion over the operator conflict.
We also find issues with the % operator when working with both strings and integers. This is due to the fact that % performs different functions when paired with different data types. If the % is used with a string then Python’s interpreter will use it as a formatting command. But if % is used with a number then it will be interpreted as a mathematical operation.
To be more precise, the % is used to calculate something called modulo numbers. This is the remainder value that can result from division operations. Python can essentially become confused about intent when % is used in potentially ambiguous situations. Fixing both of these issues simply calls for some more precise formatting.
How To Fix the String Conversion Error
We can begin fixing the error message by looking at an example of % being incorrectly used to perform a modulo calculation. Consider the following Python code.
firstNum = “1”
secondNum = firstNum % 2
print(str(secondNum))
We begin by assigning “1” to the firstNum variable. Note that this is a string value rather than a true number. Next, we try to perform a modulo calculation on the result of firstNum and 2. This should be assigned to secondNum and converted to a string which will be printed to the screen on the final line. However, we instead see a “not all arguments converted during string formatting” error. Now, consider the following variation on this code.
firstNum = “1”
secondNum = int(firstNum) % 2
print(str(secondNum))
This code is almost the same as our original. The only difference is that we’ve used int to change firstNum into an integer. This is compatible with a modulo operation so the assignment goes through cleanly. And we then see the final value printed to screen with no error message.
Of course, modulo is only one explanation for the error. This problem can also occur from a conflict in string formatting. Consider the following Python code.
firstVal = “one”
secondVal = 2
print(“We’ll print the first {} and second {}. ” % firstVal, str(secondVal))
We begin by assigning “one” to firstVal and 2 to secondVal. We then try to print them both to the screen using two types of string formatting. This raises a “not all arguments converted during string” error. But we can fix it quite easily by simply settling on one formatting option. Take a look at the following code.
firstVal = “one”
secondVal = 2
print(“We’ll print the first {} and second {}.”.format(firstVal, str(secondVal)))
In this example, we forgo the use of the % operator. We instead solely use {} with the format method. This strategy results in code that runs cleanly and prints a fully formed string to the screen.