Difference Between Python Function Arguments and Parameters

When you start working with functions in Python you will likely ask yourself what is the difference between parameters and arguments.

This is a very common question and it often confuses those who are new to programming.

In Python when you define a function, you list parameters on the same line as the def keyword within parentheses. These are like placeholders for the values (arguments) that the function works with when it’s called. In other words, arguments are passed to a function call and give values to function parameters.

Let’s take the following code as an example of a function:

def calculate_sum(a, b):
    result = a + b
    return result

number1 = 10
number2 = 15
print("The sum of the two numbers is " + str(calculate_sum(number1, number2)))

Which ones are parameters and which ones are arguments?

  • The parameters are a and b. They are present in the definition of the function on the same line as the def keyword. Those are the input values that the function expects.
  • The arguments are number1 and number2. You pass them to the function calculate_sum() when you call it.
  • The value of the argument number1 is assigned to parameter a and the value of the argument number2 is assigned to parameter b.

In other words, arguments provide concrete values to the function parameters. This allows the function to perform its intended purpose with the data provided.

Standard function arguments are also called positional arguments because they have to follow the exact same order of the parameters specified in the definition of a function.

Number of Function Parameters and Arguments

When using positional arguments the number of arguments passed to a function call has to match to number of parameters in the function definition.

Here is what happens if the numbers of arguments and parameters don’t match.

Passing more positional arguments than parameters

def calculate_sum(a, b):
    result = a + b
    return result

number1 = 10
number2 = 15
number3 = 20
calculate_sum(number1, number2, number3)

When passing more positional arguments than parameters, the TypeError raised by Python tells you that the function takes 2 positional arguments but 3 were provided.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: calculate_sum() takes 2 positional arguments but 3 were given

Passing less positional arguments than parameters

calculate_sum(number1)

When passing less arguments than parameters, the TypeError raised tells you that the function is missing one required positional argument with details about which argument is missing.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: calculate_sum() missing 1 required positional argument: 'b'

Now you know how to troubleshoot this TypeError if you see it when calling a function.

Changing the Order of the Arguments Passed to a Function

The order of arguments in a function call may change the output returned by the function.

For instance, let’s take the function below:

def calculate_sum(a, b):
    result = 2*a + b
    return result

number1 = 10
number2 = 15

When you call the function in the following way the value number1 is assigned to the parameter a and the value number2 is assigned to b.

Call this function using the Python shell to verify the result returned by the function:

>>> calculate_sum(number1, number2)
35

If you switch the order of the arguments, the value number2 is assigned to a and the value number1 is assigned to b.

>>> calculate_sum(number2, number1)
40

You can see how switching the order of the arguments results in a different value returned by the function.

However, there is an alternative to positional arguments. Python offers flexibility with keyword arguments that allow you to specify the names of the parameters when you call the function, enabling you to pass arguments in a different order.

The syntax you use when passing a keyword argument is the following:

parameter_name=argument_value

Let’s see how this works with the previous example of code:

>>> calculate_sum(a=number1, b=number2)
35
>>> calculate_sum(b=number2, a=number1)
35

As you can see, using keyword arguments, we make sure that the arguments are assigned to the correct parameters.

This feature is useful in functions that have many parameters, or when default values are involved. It improves code readability and reduces the chances of passing arguments in the wrong order.

Note: you can mix positional and keyword arguments when calling a function, but all keyword arguments must come after positional arguments.

Related article: If you are getting started with functions go through the Codefather tutorial that explains how to create a function in Python.

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