Python SyntaxError: non-default argument follows default argument Solution

An argument can have a default value in a Python function. If you specify arguments with default values, they must come after arguments without default values. Otherwise, you encounter a “SyntaxError: non-default argument follows default argument” error.

In this guide, we talk about what this error means and why it is raised. We walk through an example of this error to help you understand how to solve it in your code.

SyntaxError: non-default argument follows default argument

Python assigns arguments to variables in the order in which they appear in a function call.

Default arguments must come before non-default arguments. This is because non-default arguments are mandatory.

Whereas default values have a value even if one is not specified, a non-default argument does not. Default arguments have no purpose if you have to state a value for each of them.

Consider the following code:

def test_function(score=25, class=6, name):
	pass

test_function("John", 22)

In this code, Python does not know whether “John” is assigned to “name” or “score”. We have to use explicitly the values which to be assigned to each argument in our function call:

test_function(name="John", score=22)

This syntax is more verbose than if you just state default arguments before non-default ones.

Alternatively, we can assign a value to each argument. This defeats the purpose of having arguments with default values.

In a function, the order in which arguments must appear is:

• Positional arguments (non-default)
• Keyword arguments (default)
• Keyword-only arguments (*args)
• Variable keyword arguments (**kwargs)

If you do not adhere to this order, Python returns an error message.

An Example Scenario

Write a program that calculates the percentage a student has earned on a test.

Start by defining a function that calculates the percentage a student earned on a test:

def calculate_percentage(out_of=100, score):
	return out_of / score * 100

Our function accepts two arguments: the score a student earned and how many marks were available. Next, ask a user to present their test score and the number of marks available in the test:

user_score = float(input("How many marks did you receive on the test? "))
total_available = float(input("How many marks were available on the test? "))

We convert both the values a user inserts to floating point numbers so we can perform a mathematical operation with them. Next, call our function and pass the values our user inserted through our function:

percentage = calculate_percentage(total_available, user_score)
print("You have received a {}% mark on your test.".format(round(percentage, 1)))

Once our program calculates the percentage a student earned on a test, we print out that value to the console. We round the percentage to two decimal places using the round() method. Run our code and see what happens:

  File "main.py", line 1
	def calculate_percentage(out_of=100, score):
                         	^
SyntaxError: non-default argument follows default argument

Our code returns an error. Let’s fix this problem.

The Solution

We have specified a default argument before a non-default argument in our calculate_percentage function:

def calculate_percentage(out_of=100, score):

This renders the purpose of having a default argument useless. How will Python know what value should be set for “score” if we only specify one value?

To solve this problem, we have to change the order of our arguments so that our default arguments appear last. Let’s change our code:

def calculate_percentage(score, out_of=100):

Try to run our code:

How many marks did you receive on the test? 67
How many marks were available on the test? 100
You have received a 67.0% mark on your test.

Our code successfully calculates the percentage a student has earned on their test.

Conclusion

The Python “SyntaxError: non-default argument follows default argument” error is raised when you specify a default argument before a non-default argument.

To solve this error, make sure that you arrange all the arguments in a function so that default arguments come after non-default arguments. The order in which arguments should appear is: non-default, default, keyword, variable keyword.

Now you’re ready to solve this common Python error like an expert coder!